Transfusion-associated equine infectious anemia

A few years ago, I looked out my kitchen window one holiday morning and saw a newborn foal running outside of a fence line. The foal had been born to my neighbours' mare, a maiden mare, and they were out of town. The mare had rejected the foal and wasn't interested in any of my attempts to get them back together. She also had little colostrum (the first, antibody-rich milk that foals need to drink early in life to survive). To make a long story short, I ended up doing a field transfusion, collecting blood from another horse on the farm to give to the foal, to provide it with those much-needed antibodies. The donor horse was healthy and I didn't know of any disease issues in the area, so I was pretty confident that there wasn't a significant risk of disease transmission, but you never know. Ideally, equine blood donors are screened for infectious diseases, particularly equine infectious anemia (EIA), since EIA is a rare but nasty disease that can be spread by blood.

When I started to read a report the other day about a transfusion-associated EIA infection in a German foal, my first thoughts were "that's bad," followed by my ever-optimistic side thinking "well, maybe it was an emergency transfusion and it was a bad but unavoidable consequence" or "maybe it the donor was properly screened but was infected with the EIA virus after it's last test" (the latter situation is an ever-present risk when you are screening donors in advance (days, weeks or months) of collecting the blood for transfusion, since test results only tell you what their status was at the time of testing).

Unfortunately, it didn't take long to see that this wasn't an unfortunate or relatively unavoidable infection. Rather, I can only interpret this as stunning negligence.

Here's the story

  • On August 2, EIA was confirmed in a 3-month-old foal in North Rhine Westphalia. When the foal was two days old, it had a septic joint (and probably an overall deficiency in antibodies) and was treated with a plasma transfusion, which is a pretty standard procedure in such a case.
  • EIA antibodies were then detected in the donor.
  • Since 2009, 20 other horses had received plasma from this horse. Four have been confirmed as infected, and horses that live with these infected animals have been quarantined until test results are back. Positive horses are typically euthanized because they pose a lifelong risk of transmission of the virus to other horses.  The four positive horses in this case have been euthanized (and presumably the foal as well).

So, this wasn't some random emergency field transfusion, or a donor that got infected after testing. It appears that this donor has been used for years with no testing, despite the fact that it's well known that EIA transmission is a risk from blood transfusions and the virus is present (albeit rare) in Germany. While there are no standards of care for equine blood transfusions (as opposed to dogs), EIA testing is a standard recommendation in anything I've seen written about equine blood donor programs (click here for one example). Sometimes you get put into situations where testing can't be done in time for logistical reasons, but I can't see how anyone would not test horses that are to be used for a formal donation program or repeated transfusions. Failure to do low cost and easy EIA screening of that donor horse has resulted in the deaths of multiple horses, with the potential for even broader secondary transmission of this virus to additional animals.

Equine Infectious Anemia: Outbreak Out West

This week an article on discussed the current situation in the western provinces of Canada with regard to equine infectious anemia (EIA).  The prairies are seeing the highest number of EIA cases in years, with more than 70 horses affected on 22 different properties.

Also commonly called "swamp fever," EIA is caused by a retrovirus, similar to the human HIV.  There is currently no vaccine against virus, and once infected a horse carries the virus for the rest of its life.  Although EIA can cause severe clinical signs (including high fever, weakness, swelling of the lower limbs and along the ventral abdomen, and even sudden death) most horses that are diagnosed are not showing any signs of illness at the time, or may show milder, non-specific signs such as exercise intolerance and intermittent fever.  Affected horse of course also have varying degrees of anemia as the name suggests.  Episodes of more severe signs can occur even years after the initial infection, and during these episodes an infected animal poses the greatest threat to other horses because the viral load in the bloodstream is very high.  The virus cannot be transmitted directly between horses though - it is transmitted by blood, typically via blood-sucking insects like like deer flies, horse flies and stable flies (hence the association of the disease with swamps) or by reuse of needles for injections.  Fortunately EIA does not affect humans or any other animal species.  It has also not been shown to be transmitted by mosquitoes.

Because infection is life-long, in order to control the disease the only options for positive horses are euthanasia or life-long quarantine in a building with rigorous insect control to prevent spread.

The question is, why the sudden spike in the number of cases out west?  There are a few possibilities:

  • Possibility #1: There's been one, or a few, small local outbreaks that were initially caused by a very small number of positive horses that likely brought the virus back with them from somewhere to which they had traveled.  Hopefully this is the case, and testing has identified all the horses that were subsequently infected so that the virus won't spread further.  EIA testing is required prior to travel to many places and prior to participating in many competitions or shows.  Regular testing of animals that travel frequently helps to identify infected animals more quickly.
  • Possibility #2: For whatever reason, there are a bunch of horses being tested this year that have not been tested in the past, and they're coming up positive.  This would be much more concerning, because there's no way to tell how long a horse has been infected if it has not been tested in the past.  The longer an infected horse is around outside of a fly-proof quarantine, the greater the chance that an insect (or a needle) will transmit the virus to another horse.
  • Possibility #3: There are number of infected horses across the prairies that have not been tested, and the virus has been slowly spreading from these index cases and has finally reached part of the horse population that does get tested regularly.  This would be the worst case, as it would mean that there's a reservoir of infected horses that is still not being identified, and could continue to perpetuate the infection.

In the end, only time will tell which of these scenarios (or a combination of them) is playing out in western Canada.  Although a relatively low population density (of horses as well as people) in provinces like Saskatchewan helps to decrease the transmission pressure (simply by making it harder for an insect carrying the virus in its mouth parts from one horse to find another horse to bite), gatherings of horses for shows, trail rides and the like (particularly when EIA testing is not required) still create prime opportunities for transmission of the virus.

There are also a few things you can do to help decrease the chances of your horse contracting EIA:

  • Protect your horse from biting insects that transmit EIA (which will simultaneously help protect your horse from insects like mosquitoes that can transmit other viruses).  Use fly repellants, fly sheets, and avoid turnout during times of peak insect activity (dusk and dawn).
  • Never reuse needles, especially between different horses.  Also ensure that any other equipment that may be used on your horse (e.g. mouth gags) are always properly cleaned between animals and free from any blood contamination.
  • Avoid proximity to horses of unknown EIA status.  This can be tough to do if your horse goes to shows where EIA testing is not mandatory, but particularly with the current problems out west (or anywhere else that EIA may be circulating) testing for EIA prior to any event where there will be a gathering of horses should be strongly promoted.

Testing for EIA can be done with a simple blood test.  This used to be called a Coggins test, but now a more accurate ELISA-type test is used to test for the disease instead.

Image: Cross-section of the EIA retrovirus (source: USDA Animal and Plant Health Inspection Service)

Botulism outbreak kills 23 horses

Botulism has been in the news lately, with numerous outbreaks involving different species and some human food recalls. Botulism outbreaks are often pretty dramatic because of the number of individuals that can be involved, the severity of disease and the fact that it's often difficult to do much beyond damage control once the problem is recognized. Recently, there have been reports of widespread duck deaths along with a couple of different recalls and warnings in Ontario about potentially contaminated smoked salmon and improperly eviscerated salted fish.

On the equine front, there's also been a large botulism outbreak that is believed to have killed 23 horses in Maine (USA). The outbreak occurred over the last month and, as is typical, has been devastating because of the profound susceptibility of horses to botulinum toxin and the inability to do much to save the animals once it was realized that botulism was present.

In adult horses, botulism is caused by ingestion of food that's been contaminated with toxins produced by the bacterium Clostridium botulinum, as it grows. This relatively widespread bacterium doesn't normally grow and produce toxins in horse feed since it requires an oxygen-free environment and other specific conditions, but when these occur, the incredibly potent neurotoxin can be produced. Equine outbreaks are often associated with haylage or silage (which if improperly fermented allow for C. bolulinum to grow) or contamination of round bales (e.g. an smaller animal that died of botulism gets accidentally incorporated into the bale, where the toxins can persist and/or the bacterium can grow if the right environment is present deep within the bale). In this outbreak, silage is suspected to be the cause. The silage is being tested to confirm this suspicion.

You can never 100% prevent botulism, since strange sources are sometimes found, but avoiding high risk feeds (e.g. silage, haylage, moldy round bales), trying to ensure that dead animals do not get caught up in hay bales during the baling process and taking exceptional care when baling if botulism is present in wildlife in the area can help greatly. A vaccine is available but it only protects against certain types of botulism. If those types are the main types that cause disease in a given area,  vaccination can be useful, but good feeding practices are the most important preventive measure.

Image: Horses at a round bale feeder (source:

Botulism outbreak in horses

Botulism outbreaks in horses are usually bad news. Horses are very susceptible to botulism, and outbreaks in horses caused by contamination of food often end up killing multiple horses on a farm. The recent botulism outbreak in Reddington, IN is another reminder of how deadly it can be.

The outbreak involved a family that lost five horses to suspected botulism. "Suspected" because this disease can be hard to confirm sometimes, although it's usually possible to make a pretty solid presumptive diagnosis based on how the horses look and by ruling out the few other possible causes. The details are pretty sketchy. Apparently there are some other sick horses, but how sick they are and how many isn't clear.

Botulism occurs in two ways. In adult horses, it almost invariable occurs after ingestion of the extremely potent botulinum toxin produced by the Clostridium botulinum bacterium. In foals, it usually occurs after ingestion of the bacterium, which then produces toxin in the foal's intestinal tract.

In adults, outbreaks are usually associated with contaminated feed.  There are some high-risk feeds like haylage and silage (see photo) that we usually focus on first, but sometimes botulism toxin can be found in hay or other common feeds. Haylage, silage and other fermented feeds become a problem with they are improperly fermented, allowing the Clostridium botulinum to grow and produce its toxins. Sometimes, contamination of feeds can occur when an animal that has died of botulism (and has the bacterium and its toxins in its body) gets incorporated into hay or other feedstuffs.

When an outbreak is suspected, a key step is removing any potentially contaminated feeds to reduce further exposure, although often it's too late by the time the disease is recognized. Antitoxin (which is pre-formed antibodies that help neutralize the botulinum toxin) can be given to exposed horses, but it's extremely expensive and does not reverse any damage that's already been done. That's why these outbreaks are often so disasterous, because when the diagnosis is made the only thing left to do may be damage control to try to save some of the less affected horses. That's tough because botulism has a very high mortality rate in horses.

The farm owners in Reddington are urging local horse owners to be on the lookout for botulism. It's reasonable, but rarely do we see multi-farm outbreaks from botulism. They also state that botulism doesn't affect cattle, which is wrong. Cattle are more resistant than horses, but they certainly can get botulism.

A botulism outbreak in horses poses little risk to people. People are susceptible to this horrible disease as well, but to get it someone would have to ingest the same contaminated feed that the horse did. There's no risk of transmission of botulism from an infected horse to a person or another animal.

California equine event biosecurity document

In response to the large and high-profile equine herpesvirus outbreak that occurred last summer, the California Department of Food and Agriculture has published a Biosecurity Toolkit for Equine Events. It's a nice, comprehensive document that goes over a wide range of things that can be done to reduce the risk of disease transmission at horse shows and similar events, and is one of the better resources available on the subject.

As I've said before, I don't use the term "biosecurity" for equine events since I think it's a fallacy. Biosecurity involves keeping infectious agents away from a group of animals. You can do that on a chicken farm, where you bring in a bunch of chicks at the same time, house them under tightly controlled conditions, keep people away, don't let them near any other birds, send them to slaughter all at once, and then disinfect the place before starting again. With horse events, we create absolutely beautiful conditions for infectious diseases to be introduced and transmitted. For example:

  • Many horses from many different areas with different health statuses are mixed together.
  • Vaccination requirements are often sparse to non-existent.
  • Horses have a high likelihood of direct and indirect contact with each other.
  • Healthy horses can carry a variety of infectious agents.
  • People often bring horses that they know are or have recently been sick, and there's little scrutiny of arrivals to detect any infectious horses.

So, for me, we instead deal with infection control when horses are involved, whereby we try to reduce (but know we can never eliminate) the risk of infectious diseases and outbreaks.

Is it just a matter of semantics? To a degree, yes, and maybe it's just the Professor side of me coming out. At the same time, I think it's important to consider the difference since we have to acknowledge the inherent risks that come with showing horses, think about the risks involved with different situations and come up with practical ways to reduce those risks as much as possible.

Good infection control practices for equine events, with measures taken by both organizers and attendees, are a good start.

Photo credit: John Goetzinger (click for source)

Rabid horses in Tennessee

Rabies is pretty rare in horses in North America, with only 37 reported cases in the US in 2010 and 1 in Canada in 2011 (the latest years for which data are available). So, finding two apparently unrelated cases of rabies in horses in the same area in the same month is pretty unusual and concerning. Yet, that's what's happened in Tennessee, where rabid horses were identified in both Rutherford and Marshall counties in January.

Little information is available about the cases, but both were identified as having the skunk rabies virus variant. That doesn't necessarily mean they were infected by skunks (since other species can be infected by this virus variant) but it is suggestive, and indicates that rabies must be active in the skunk population in that region.

Regardless of the source, these cases are a reminder of why rabies vaccination of horses is important, and why rabies vaccination is considered a "core" equine vaccine by the American Association of Equine Practitioners. Rabies is invariably fatal in horses and it's also a major public health risk. While I've been unable to find confirmed cases of horse-to-human rabies transmission, it's a possibility, and additionally, rabid horses have killed people because of their unpredictable and aggressive behaviour.

Vaccination is cheap insurance against rabies - it's never a 100% guarantee, but it's a whole lot better than without vaccination.

(click image for source)

Tropical Rhodococcus equi outbreak? has reported on an outbreak of Rhodococcus equi that has claimed the lives of seven ponies at an equestrian facility on the island on Mayotte (a French protectorate off the coast of Madagascar). Local agriculture officials report that two other horses are also affected, but recovering, and the facility has been quarantined

There are a few strange aspects of this report. Rhodococcus equi is an important cause of respiratory disease in foals, in which it can cause serious abscesses in the lungs. However, it's extremely rare in mature horses, and it seems that the horses affected in Mayotte were adults. It's not impossible, but an outbreak of Rhodococcus in adults would be incredibly surprising, indicating either something that made these horses remarkably susceptible to the bacterium, or the presence of a strain of Rhodococcus more able to cause disease in adults. A more likely explanation is that it's not actually Rhodococcus. There's no mention of what type of disease the horses had or for what other infectious agents tests were done. A Department of Agriculture official stated that the diagnosis was made by blood tests, but blood testing is pretty useless for diagnosis of Rhodococcus. So, I'd consider the diagnosis highly questionable without further information.

Whatever the cause, something that kills seven horses on a farm is remarkable and thorough testing is needed to determine exactly what's happening. In the unlikely event that this was caused by Rhodococcus, more work needs to be done to explain why the outbreak occurred and why it was so severe. If (as is likely) it wasn't Rodococcus, knowing the actual cause is important for controlling further spread and preventing problems in the future.

Fortunately, the remote nature of this location makes it rather unlikely that whatever's happening there will spread to another region soon.

Image: Location of Mayotte (click image for source)

Ontario equine biosecurity seminars

Equine Guelph has announced a series of equine biosecurity workshops across Ontario which will take place during the month of March 2012. The sessions are free to attend and will focus on how to apply basic and practical concepts of infection control to reduce the risk of disease exposure and outbreaks in horses of all types. The workshops are part of a broader Beat the Bugs biosecurity program being organized by Equine Guelph, which also includes a two-week e-session being offered in April 2012.

More information about the workshops (including dates and locations, and to register), e-session and the Beat the Bugs program can be found on the Equine Guelph website.

West Nile warning for BC horses

A Prince George, British Columbia veterinarian is warning other veterinarians and horse owners about West Nile in the province. Little information is available at this point, but the warning is in response to a diagnosis of West Nile infection in a horse from the area. The report calls it a "deadly disease" but it would be more appropriate to call it a "potentially deadly disease," since most horses that are exposed don't get sick, and many sick horses recover. I don't want to downplay the seriousness of West Nile, but it's important to keep it in perspective and make people panic.

The BC CDC has an ongoing West Nile surveillance program because of the obvious concern as this virus has worked its way across North America over the last decade. While it's taken it's time getting to BC, West Nile virus has been identified in the province, and only time will tell whether it becomes a serious health concern for horses or people. The latest update of the BC CDC surveillance data indicates one positive horse, but no positive humans (of 415 tested) or mosquito pools (2282 tested). The one equine case that was documented was from Central Okanagan. The horse had clinical signs consistent with infection, although the severity and outcome are not reported.

It's unclear to me whether this Prince George case is something that's happened just recently or whether the horse was sick. It's pretty late in the year for a mosquito-borne virus, but not impossible in some areas.

Does this report mean that horse owners in BC should be concerned?  Maybe. "Aware" might be a better term.

Horse owners and veterinarians always need to be aware of the infectious disease risks in their area, and areas to where a given horse may travel. Keeping apprised of ongoing West Nile virus surveillance can help determine the likelihood of exposure, but that doesn't mean you can wait until there's a case next door before you do anything. (Someone has to have the first case in an area, and you don't want that to be you.)

Whether or not to vaccinate against this virus depends on the likelihood of exposure and risk aversity. Available vaccines are rather safe and effective (not 100% on either account, like any vaccine, but quite good overall), and vaccination decisions should be made based on a well-reasoned discussion between veterinarian and owner, considering a variety of factors such as where the virus has been found and how much risk everyone is willing to take.

The news report has a quote recommending vaccination in the spring. That's the typical time people vaccinate against mosquito borne diseases, but that's not my recommendation. For me, the goal is to vaccinate so that peak immunity is present at the time when exposure is most likely. West Nile virus is classically a late summer/fall disease, based on mosquito types and their biting patterns. For that reason, I like to see horses vaccinated a little later in the year - closer to the high risk period. Again, it's important to know disease trends in each region to make the most informed decision.

So, horse owners in BC should be aware but not panic. A good discussion about vaccination and about general mosquito avoidance practices should be the first thing that happens.

Headshaking and equine herpesvirus

Headshaking is a frustrating problem in horses. It's often hard to identify a cause and treatments are frequently unrewarding. Many different possible causes of headshaking have been proposed, including equine herpesvirus type 1 (EHV-1) infection.

As is common with herpesviruses, EHV can lie dormant in the body, and it may be re-activated during times of stress. There's ample evidence that other herpesviruses can cause nerve pain with reactivation. In humans, re-activation of the varicella-zoster virus (the herpesvirus that causes chickenpox) causes shingles, which is a very painful disorder. Since dormant EHV-1 can be found in nerves in a horse's head, it has been suggested that pain caused by reactivation of dormant virus could be a trigger for headshaking.

A recent study published in the Journal of Veterinary Internal Medicine (Aleman et al 2011) investigates this theory. The researchers looked for the presence of EHV-1 in trigeminal ganglia (a group of nerve "nodes" in the head) in headshaking horses and healthy controls. While it was only a small study, there was no evidence indicating a role of EHV-1 in headshaking, since the virus was only detected in 1/8 headshakers compared to 0/11 controls.

This study doesn't absolutely rule out EHV-1 as a cause a headshaking, since it still could be one of many potential causes that is involved in only a minority of cases. However, this study suggests that EHV-1 is not a particularly common cause of headshaking, if it causes it at all.

Bat slaughter = Hendra virus control?

Bob Katter, an Australian Member of Parliament and leader of the Australian Party, has proposed culling flying foxes (fruit bats) as a way to control Hendra virus, which is spread by these large Australian bats. He's not the first person to make such a proposal, but it's a knee-jerk reaction that in reality doesn't make a lot of sense.

It's not completely clear whether Mr. Katter is proposing a plan to completely eradicate the flying foxes altogether, or to simply let people kill any such bats they find on their property, but neither approach is likely to be effective in terms of decreasing the risk of Hendra virus transmission.

If people kill flying foxes on their property, they'll just be replaced in short order by bats from neighbouring areas.

Trying to eradicate the entire species is a bad idea for a variety of reasons:

  • Tinkering with a complex ecosystem doesn't often turn out the way you want it to. Australians certainly know from past experiences that bad things can happen when new species are introduced (rabbits are just one example). The same might happen when a species is removed.
  • Eradication of the species is probably impossible or at least very difficult. I don't know much about the reproductive rate of flying foxes, but if the species can reproduce at a reasonable rate, they can probably replace the culled animals unless people are really aggressive and seek out all remote breeding sites. The limitations of culling have been clearly shown in rabies control, where it doesn't do much because culled dogs are quickly replaced by new dogs.
  • Eradicating flying foxes would be very expensive. What could that money do if put into research on vaccination, treatment, and other worthwhile ventures? What if efforts were focused on eliminating flying fox roosting sites in horse pastures? Overall, the impact would probably be much greater.

Why stop with flying foxes? Australia has lots of nasty critters, ranging from spiders to saltwater crocodiles. Should we kill all of those too? Dog bites kill more people than Hendra every year. Should we kill all dogs?

Hendra virus is not something to ignore. While infections in horses are rare, they are usually fatal and there's the risk of transmission to people. Human infections are very rare but often fatal. So, ways to reduce infection of horses as a means of reducing both human and horse disease are important, but the slaughter of flying foxes doesn't make a lot of sense.

Bad 2012 for Hendra virus in Australia

While you don't want to read too much into a single case, 2012 has started off in a bad way for Queensland horses. Hendra virus was identified in a Townsville area horses that died.  This zoonotic viral disease is  largely restricted to Queensland, Australia, but it has a high fatality rate in horses (and people). Hendra virus is spread by fruit bats and is an ever-present concern to Queensland horse owners and veterinarians, but a mid-summer infection is quite unusual (remember that it's currently mid-summer in Australia). Most cases tend to occur from July to September - this case is a reminder that seasonal trends are just that: trends, not absolute rules.

Fortunately, the attending veterinarian used proper precautions when handling the horse to limit the risk of zoonotic transmission of Hendra virus. However, there will presumably be an investigation to determine who had contact with the horse and their potential for exposure. There is currently no way to prevent or specifically treat Hendra virus infection. An experimental antiviral treatment has been tried in the past, but it's effectiveness if far from clear.

Last year was quite bad in terms of the number of Hendra cases that were detected in Australia. Let's hope this early 2012 case isn't a sign of things to come.

Horse rabies in Florida

Marion County (Florida) public health personnel recently issued a rabies alert after a horse in the area tested positive for the virus. It’s a standard alert, emphasizing avoiding contact with wildlife, reducing things that attract wildlife to houses (e.g. accessible pet food or garbage) and recommending vaccination. Interestingly, while this alert was prompted by a case of rabies in a horse, it only mentions vaccination of dogs, cats and ferrets. That may have been because it was an off-the-shelf alert, not really tailored to this situation, but it shows how horses can be overlooked when it comes to rabies.

Fortunately, rabies is a rare disease in horses. In 2010, there were 37 reported cases of equine rabies in the US and only one in Canada (two Canadian cases have been identified so far this year). That’s a very low rate, especially considering the number of horses out there, but it’s still more cases than there should be for a very serious yet highly preventable disease.

Unfortunately, rarity sometimes breeds complacency, so despite the fact that rabies is invariably fatal in horses and rabid horses pose a risk to people, vaccination of horses is often overlooked. While rabies is rare in horses, rabies vaccination shouldn’t be rare. Every horse in a rabies endemic region (or that might be traveling to such a region) should be vaccinated against rabies. It’s cheap insurance against a very dangerous and deadly disease.

Pigeon fever in Louisiana horses

Pigeon fever is an equine disease that doesn't have anything to do with pigeons. It's an infection caused by the bacterium Corynebacterium pseudotuberculosis which results in the formation of abscesses, usually along the chest (pectoral region) and lowest part of the abdomen. The name "pigeon fever" comes from the swelling in the chest region that vaguely resembles a pigeon-breast. A recent report describes and outbreak of pigeon fever involving at least 30 horses in Louisiana, bringing the estimated number of cases in the state in 2011 to over 100.

Pigeon fever is a regionally (and to a lesser degree seasonally) variable disease. It predominantly occurs in California, but over recent years it has expanded its range in the western US, and from this report, it's obvious that it has a good foothold in some other areas in the south east as well.

Corynebacterium pseudotuberculosis lives in the soil, and causes infections in horses when it gets inoculated under the skin via wounds and perhaps sometimes through fly bites. Once it gets into the tissues, it starts to grow and causes painful (and potentially large) abscesses that often need to be surgically incised in order to drain them.

Infection control practices on farms can help reduce transmission of the bacterium between horses and to reduce the risk of injuries. These include:

  • Quarantine of new arrivals and careful inspection for sign of infection.
  • Isolation of known infected horses.
  • Use of "contact precautions" when dealing with infected horses to prevent transmission of the bacterium via peoples' bodies or clothing. This involves the use of protective outwear (e.g. coveralls and boots that are only used for the infected horse(s)) and gloves.
  • Proper use of handwashing / hand sanitizer by people handling infected horses (or any horses, really, from a broader standpoint).
  • Prevention of cross-use of items like buckets between infected/quarantined horses and the general horse population.
  • Use of fly repellent, especially on horses with open wounds or draining abscesses.
  • Careful cleaning and disinfection of areas potentially contaminated by pus from draining abscesses.
  • Inspection of stalls, paddocks and fields for things that could cause wounds that might subsequently become infected.

Pigeon fever is a good example of why it's important to know disease patterns in your region (and those to where your horses travel). Being aware of the possibility of a specific disease is an important step in diagnosis, and knowing there is disease activity in any area in which your horse may have been is a key part of that. This disease is also an example of why we need ongoing disease surveillance and reporting, because if a disease makes it into new regions, veterinarians and horse owners need to know about that as soon as possible to allow for quicker diagnosis and use of control measures. Unfortunately, organized disease surveillance and communication is sorely lacking in horses.

Photo: A Jiennense Pouter Pigeon, which has a very pronounced breast compared to other breeds.  The swelling of a horse's pectoral region due to abscesses caused by C. pseudotuberculosis is the reason the disease is sometimes called "pigeon fever." (click image for source)

equIDblog has moved to Worms & Germs!

Unfortunately, because of funding challenges (translation... there was no more money), we were no longer able to maintain our equine infectious disease blog (equIDblog) as a separate site. However, based on the positive feedback we had and the amount of traffic on the original site, we still think it serves a role and are dedicated to maintaining equIDblog in some form. So, we are going to merge equIDblog with the Worms & Germs Blog. We have already moved the equIDblog Resources page, which can be accessed through the link in the title bar.  We will also gradually move all of the current archived blog posts from the site and place them under the equIDblog topic category which can be found in the left index bar.  Here we will maintain all of the site's current content and continue to provide information and commentary on equine infectious disease issues. Thanks to all of our loyal equIDblog readers for making the blog such a success, and we hope you'll continue to follow us here on the Worms & Germs Blog!

Beat The Bugs: New Biosecurity Venture For Equine Guelph

Equine Guelph has received funding for Beat the Bugs, a new equine biosecurity venture. The program has been funded by the Agricultural Biosecurity Program, a Canadian federal-provincial-territorial initiative.

From ongoing cases of strangles to large outbreaks of EHV, it's abundantly clear that infection control is a critical area for the equine industry, and one that typically receives little attention in the absence of an outbreak. The new Beat the Bugs program will be launched in March 2012, and will facilitate development of farm-specific biosecurity programs through workshops and a two-week online program. The program is being developed by Equine Guelph with the cooperation of various academic and industry partners. More information can be found on the Equine Guelph website.

No Answer In Aussie Horse Deaths

Despite relatively intensive efforts, no cause has been identified for the deaths of 22 horses ona Kooralbyn farm in Queensland, Australia. Readily identifiable causes have been ruled out, leaving two main theories:

As mentioned in an earlier post, one possibility is botulism It can be very hard to diagnose botulism in horses, and the disease is often presumptively diagnosed by ruling out all other reasonable options.

The other possibility is tick paralysis, which unfortunately is equally difficult to diagnose definitively. Certain types of ticks can cause progressive and severe paralysis which may look similar to botulism in some ways. Scrub ticks (the name typically used for Ixodes holocyclus) were noted on one of the first affected horses, but there's no information about whether all of the other horses had ticks too, and if so whether they were ticks that can cause paralysis. Kooralbyn is in a region where these paralysis ticks can be found and spring (e.g. now in Australia) tends to be the peak time for paralysis cases, so it's a reasonable consideration.

It's frustrating but far from unusual not to be able to make a solid diagnosis in outbreaks. Beyond the frustration factor, it's also a concern because it makes it hard to take precautions to prevent it from happening again.

Image: Two Ixodes holocyclus ticks, one before and one after feeding (photo credit: Bjørn Christian Tørrissen, click for source)

Study Seeking "Superbugs" In Horses

A two-part study (Maddox et al. 2011) was recently published online in the Equine Veterinary Journal looking at antimicrobial resistant "superbugs" in horses in the UK.  The first part of the study had the simple objective of estimating the prevalence of both MRSA nasal colonization and fecal shedding of antimicrobial resistant E. coli in the UK horse population. The reason they were looking at E. coli is because this bacterium is part of the normal intestinal flora of most animals (including horses), so it's easy to find, and it is frequently exposed to antimicrobials whenever a horse is treated systemically (i.e. with either oral or injectable antibiotics), so researchers use it as an "indicator" for resistance that may develop in other bacteria as well.  E. coli doesn't typically cause gastrointestinal disease in horses like it can in people, nonetheless E. coli is a common cause of uterine, urinary and wound infections in horses, as well as septicemia in foals, and antimicrobial resistant infections in these situations can certainly be a big problem.

Out of 678 nasal swabs (taken from horses on over 500 premises), only 4 (0.6%) were positive for MRSA.  That's good to see, as it's very easy for MRSA to spread in a horse population "under the radar," because colonized horses do not have any outward signs that they are carrying the pathogen.  The overall low prevalence in the community setting is similar to previous community studies in the UK and other areas.  Out of 650 fecal samples, 452 (72.2%) were positive for an E. coli that was resistant to at least one antimicrobial.  That's not too surprising.  The bigger concern is that 233 (37.6% - over one third of all horses tested) samples contained multidrug-resistant E.coli (defined in this study as resistance to more than three antimicrobial classes) and 42 (6.3%) samples contained an E. coli that produced an extended-spectrum beta-lactamase (ESBL).  ESBLs are particularly problematic because they can be relatively easily transferred between bacteria, they confer resistance to a large number of commonly used antimicrobials in the beta-lactam class (which includes penicillins and cephalosporins), and are frequently associated with genes that confer resistance to other antimicrobial classes as well.  Bacteria that produce ESBLs are a significant problem in human medicine and have been reported to cause infection in horses as well.

The second part of the study used information collected from questionnaires filled out by the horse owners to try to determine risk factors that affected the odds of a particular horse shedding antimicrobial-resistant E. coli.  They used some pretty heavy-duty statistical analysis because they were looking at so many factors and different outcomes, and any time that happens you have to take the final numbers with a grain of salt. Nonetheless, the results can help point the way for future studies to help determine which factors may have the biggest impact on the risk. The authors found recent hospitalization and veterinary treatment for various conditions over the last six months were associated with higher odds of shedding multidrug-resistant strains of E. coli, and the type of farm/facility also affected the odds.  In addition, having a recently hospitalized horse on the premises (among other things) increased the odds of a horse shedding ESBL E.coli.

What does all this mean for the average horse owner?  The results really aren't new or startling.  We already know that antimicrobial resistance is a growing problem in equine medicine, as it is in veterinary and human medicine in general, and this is one more set of studies that provides evidence to that effect.  I have no doubt that if a similar study was performed in North America the same kinds of resistant bacteria would be detected, although the numbers may vary somewhat one way or another.  The second part of the study also re-emphasize the role that antimicrobial use plays in promoting development of and selection for resistant bacteria, and the potential for the treatment of certain horses to affect the microbes being carried and shed by the animals around them.  In the end, it comes down to being responsible about how we use antimicrobials in order to curb the development of resistance, so that these important drugs remain effective for treating serious infections in the future.  This applies equally to their use in people and animals of all kinds.

NDM-1 in Salmonella...ugh!

NDM-1 (New Delhi metalloproteinase 1) is a little bacterial gene that's attracted a lot of attention (and controversy, due to its name). NDM-1 can be picked up by certain types of bacteria, making them resistant to a whole lot of antibiotics. Some bacteria that carry NDM-1 are resistant to virtually every available antibiotic, which raises the spectre of the "untreatable infection."

Since it's discovery, NDM-1 has been found in multiple countries, often in people that were in India as tourists (or "medical tourists" who traveled to India for medical procedures they couldn't have done in their own countries), and in a few different types of bacteria. Recently, NDM-1 was found in an American upon his return from India, this time in Salmonella (Savard et al. 2011, Antimicrobial Agents and Chemotherapy).

The 61-year-old man was hospitalized in India in late December 2010 following a severe bleed in his brain. He was transferred back to the US on January 25, 2011. Upon arrival, he developed a fever and a multidrug-resistant bacterium, Klebsiella pneumoniae, was isolated from his breathing tube. This was concerning by itself, but later, Salmonella Senftenberg was isolated from the man's rectum. The strain was highly atimicrobial-resistant and was determined to carry the NDM-1 gene.

There have been complaints from people in India about the stigma associated with the "New Delhi" component of the name. In hindsight, many people wish it had been named differently because of this, but at least at the moment, it's undeniable that India is a (or the) hotbed of NDM-1. It's been found in various bacteria from water and seepage samples in New Delhi, but this is the first report in Salmonella. It's concerning because of the difficulty that would be encountered treating highly resistant Salmonella in infected people. Usually, antibiotics aren't needed when someone has salmonellosis, but when they are needed, it's important that they work. Highly drug resistant strains increase the chance of a bad outcome if ineffective antibiotics are used initially (before it's determined that the strain is resistant).

NDM-1 has not been reported in animals... yet. I assume it's inevitable that it will occur, since this gene appears quite able to move between bacterial species. If it increases in humans and in human-feces-contaminated sources like water, exposure of animals will certainly occur. If NDM-1 containing bacteria establish themselves in the intestinal tracts of healthy animals, it's going to be much harder to control.

Botulism Suspected In Aussie Outbreak

A couple of days ago, I reported about a deadly outbreak on a Queensland farm that had killed a significant number of horses. A definitive diagnosis has still not been made, but it is starting to look like botulism might be the cause. In my previous assessment, I said botulism was a potential cause of an outbreak like this, but that this particular disease has a pretty consistent clinical presentation so it was probably unlikely if no one was talking about it as a leading option. It looks like now they are, with the owner stating "I've been talking to vets all over Queensland and they're saying the symptoms are spot-on for botulism." It's unclear whether the first vet(s) didn't consider it or whether there wasn't much veterinary involvement at all initially. Regardless, botulism makes sense, particularly with the description of the type of illness that's now being provided.

Botulism testing is underway, but it's often hard to get a positive lab test for this disease in horses. Often, it's a presumptive diagnosis made based on the clinical appearance and lack of any other identifiable cause. Botulism outbreaks on horse farms can be devastating, as in this case. They are often associated with feeding improperly fermented silage or haylage, but there are a variety of ways horses can ingest feed (or water) contaminated with the toxins produced Clostridium botulinum.

The good thing that comes out of this, if botulism is diagnosed, is that there's not much risk to other horses in the region, or at least not any higher risk than is always present. Whenever a large number of horses die from an unknown cause, it's always a concern that a new disease might be involved, something that fortunately does not seem to be likely here.

Image: Poor tongue tone is one of the classic signs of botulism in horses, which results in difficulty eating and drinking (click image for source).

Queensland Horse Deaths Being Investigated

There's still no confirmed cause of death for 19 horses that died within a short period of time on a Kooralbyn, Queensland (Australia) farm. Sudden death and Queensland invariably conjures up thoughts of Hendra virus, considering the large number of Hendra cases that have occurred in the area this year, but that particular disease has already been ruled out.

Large numbers of horses dying on a farm around the same time obviously raises a lot of concern. Often, diagnosis is difficult and takes time. One problem is even deciding where to start. For an outbreak like this there are various considerations:

  • The usual (infectious) suspects: Common things occur commonly, and strange things are often caused by atypical presentations of common diseases. Hendra's been ruled out but other infectious causes can't be ignored.
  • Toxin-associated bacterial diseases: This mainly refers to botulism, which causes adult horses to get sick after eating feed contaminated with botulinum toxin. Poorly fermented feed or feed that has (accidentally) incorporated the remains of an animal that died of botulism can be the source. Botulism outbreaks are usually fairly characteristic clinically since animals with botulism have pretty consistent and remarkable clinical signs of illness. Presumably, it's low on the list here as I haven't seen it mentioned in any reports.
  • Toxins: A toxic cause is pretty high on the list, and can be hard to trace. Potential culprits include toxic plants or inadvertent access to toxic chemicals (I remember an outbreak of dead horses I was involved with a few years ago in which the horses died of pesticide exposure... on an organic beef farm!). Malicious poisoning always has to be considered as well.
  • New diseases: New diseases don't come along very often, but they can. Similarly, diseases that have been rare can sometimes peak out of nowhere, but something like this is much less likely than the categories above.
  • Miscellaneous: For example, if all the horses died inside, electric shock would have to be considered. I assume that's not the case here.

Diagnosis of the cause in an outbreak like this requires a lot of effort, including careful consideration of any signs of illness that were identified before death, good post mortem examinations (necropsies) of dead animals, thorough investigation of the farm looking for toxic plants and other toxic substances, investigation of other farms in the area to see if there are any unexplained horse deaths, investigation of any unexplained deaths of other animals in the area (e.g. looking for abnormal numbers of dead birds on the farm or in the area), testing of samples from horses for various toxins and testing tissues for possible infectious causes. Not easy or cheap, but hopefully effective, and hopefully will ultimately help save other horses from the same fate.

Equine Quarantine Recommendations

Quarantine is an important, effective and underused practice on horse farms. Quarantine can reduce the risk of introducing new infectious agents to animals on the farm or limit the spread of something that’s already starting to circulate through the herd.

A recent article from gives a good overview of why and how to implement quarantine. Importantly, it emphasizes that quarantine is not just for large, high-risk farms, and that any farm can and should have a quarantine plan. Not everyone can implement a quarantine protocol easily. We know and accept that, but the inability to implement a textbook quarantine program can't be used as an excuse to do nothing. Even a cursory quarantine program, with some basic practices to reduce direct and indirect contact of quarantined horses with other horses, can be very useful.

Most of the time, quarantine goes uneventfully and no problems are encountered. That sometimes leads to complacency, but it's the few cases where quarantine contains a problem that makes it all worthwhile. The implications of a single horse getting past quarantine can be huge, both for the individual farm and more broadly (e.g. Australian influenza outbreak). The article has some sound, practical advice that all horse owners should consider.

Image from Chemical Hazard Signs -

Who's Better At Biosecurity?

There are a lot of recommendations out there for establishing or improving sound infection control practices on horse farms, many of which can be found on this very blog.  Some refer to such practices as "biosecurity," but what's done on horse farms compared to the often very strict biosecurity protocols at facilities such as swine or poultry operations (e.g. all-in all-out management, closed barns, shower in) is very different, so we prefer to call it an infection control program, rather than "biosecurity."  Regardless, one of the biggest challenges with regard to infection control, in almost any setting, is getting people to comply with all the various policies and recommendations.  Unfortunately, it's not enough to just tell people what needs to be done (that'd be too easy!) - knowledge by itself usually won't change people's behaviour.  They also need to be motivated to change their behaviour, for example by the potential for a positive reward (e.g. they get to take their horses to shows) or the potential to avoid a negative outcome (e.g. their horses don't get sick).  Furthermore, individuals need to really believe they are capable (physically and mentally) of performing the required tasks - if they don't think they can do it or be effective at it, they're unlikely to try.  In reality, getting people to change their behaviour to adopt sound infection control practices can be quite complex.

A study soon to be published in Preventative Veterinary Medicine (Schemann et al. in press) looked at some of the factors that affect horse owners' biosecurity practices and perceptions.  The study was performed in Australia one year after the devastating equine influenza outbreak that occurred in 2007, using an online questionnaire to which 759 horse owners responded.  Each owner's biosecurity compliance was rated as low (30%), medium (20%) or high (50%) based on how often they reported using 16 different infection control measures.

Factors that were associated with low compliance or poor biosecurity practices included people who:

  • were younger in age
  • had two or more children
  • were not involved with horses commercially
  • had no long-term business impact from the 2007 equine flu outbreak
  • were not fearful of a future outbreak of equine flu in Australia
  • thought their current hygiene and access control practices were not very effective in protecting their horses

Now, studies based on surveys of this kind always need to be taken with a grain of salt, as the study population itself was difficult to define and the information was all self-reported by owners, which can lead to confounding and misclassification bias.  Nonetheless, the results are still interesting and on the whole are consistent with behaviour theory.  Those whose income and livelihood were not dependent on the horse industry, and those who were not fearful of another outbreak, would be less motivated to put the effort into infection control measures.  Furthermore, those who felt that what they were doing already wasn't really effective (for whatever reason) would be less motivated to try harder because they can't see the benefit.  It's quite possible (as the authors speculate) that having two or more children results in less compliance with infection control simply due to the time constraints associated with having kids.  Lack of time to properly perform infection control procedures is a major barrier to compliance, even in human hospitals, particularly with understaffing issues. The lower compliance among young people is also common to studies looking at protective behaviour in human health, possibly because young people have a sense of certain degree of invulnerability that results in riskier behaviour overall.

When it comes to infection control, the old adage "a chain is only as strong as its weakest link" is very important to remember.  Although 50% of the horse owners in this study reported having high biosecurity compliance, the 30% with low compliance could ruin all their efforts should another outbreak occur, by contributing to the transmission and propagation of the disease on their own farms as well as to others.  Hopefully this study will help the horse industry and government identify specific groups (i.e. young people, those not financially dependent on horses) at which educational and motivational campaigns can be targeted in the future.

Another EHV-1 Equine Hospital Quarantine

Once again, an equine hospital is under quarantine because of equine herpesvirus type 1 (EHV-1). There have been a number of such incidents this year, highlighting both the increasing concern with this important pathogen and perhaps more transparency and willingness on the part of hospitals to take aggressive infection control measures when it is detected.

The latest incident involves the University of Tennessee Equine Hospital. The entire situation is related to identification of EHV-1 infection in one horse that was admitted on September 15 and euthanized a few hours later because of severe and progressive neurological disease. The next day, the Tennessee state veterinarian implemented a seven-day quarantine, while the hospital voluntarily implemented a 14-day quarantine. Seven days is pretty short and if you're concerned enough that you think quarantine is needed - if it's going to be done at all, it should be done right (i.e. for longer than a week).

At last report, there was no evidence of transmission to other horses.  Presumably university personnel are closely watching horses in the hospital and are hopefully  in contact with people who had horses they after the EHV-1 horse was admitted but before quarantine was implemented.

The need for facility closure or quarantine is always something of debate. EHV-1 should be a containable problem with prompt recognition of affected horses, proper isolation facilities and compliance with infection control procedures. Identifying infectious horses is a key aspect, as they are not always screaming "I have EHV!" when you see them. If a horse with EHV isn't identified as a potentially infectious animal and isolated from the start, the risk of transmission goes up. In this case, it was stated that the horse was kept in a "separate area of the equine hospital." It's not clear whether this was in an isolation unit or not. If it was admitted directly to isolation and was handled with appropriate protocols, the risk of other horses being infected should be very low. Given the time frame involved (it was only in the hospital for a few hours), even if it was in the main hospital, the likelihood of transmission to other horses is probably still relatively low, but it's certainly possible.

From a disease control standpoint, it's much better to be overly aggressive at the start while you are sorting out what's going on rather than sitting back and hoping for the best. While this often results in negative publicity, it's better than ending up with an outbreak which results in even worse publicity, as well as more sick animals.

EPM Testing

Equine protozoal myeloencephalitis (EPM) is a frustrating disease. It's been referred to as one of the most overdiagnosed, underdiagnosed and misdiagnosed equine diseases - an apt description. In some areas, every horse (particularly every racehorse) that has any real or perceived abnormality (such as not running fast enough) gets treated, usually without any attempt to make a true diagnosis. Most of these animals don't have EPM, but some might, along with horses displaying a range of sometimes vague neurological signs. When an effort is made to really establish a diagnosis, unfortunately it's not straightforward, which leads to more confusion about the disease and how to manage it.

The Animal Health Diagnostic Center at Cornell University have released a document on EPM testing and diagnosis. This document discusses when and why to test, along with important information about the available, recommended tests. It's a nice, comprehensive overview of the subject and worth a look for equine veterinarians as well as horse owners wondering whether their horse may have this enigmatic disease.

Equine Infectious Neurological Disease Update: Ontario

The latest update on equine infecious neurological diseases in Ontario (Eastern equine encephalitis (EEE), West Nile virus (WNV), rabies and neuropathic equine herpevirus type 1 (EHV-1)) is available from the Ontario Ministry of Agriculture, Food and Rural Affairs.

There aren't a lot of surprises, and it's good to see the numbers of cases have remained relatively low. Most notably, there have been three EEE and five WNV cases confirmed, from different regions of the province. This shows that these diseases are still occurring in Ontario horses, albeit at a very low rate.

You always have to consider the limitations of surveillance data like this. To make the list, a horse has to get infected, get sick enough for someone to notice, a veterinarian has to be called and proper samples have to be taken for diagnostic testing. There's certainly no guarantee that this happens in all instances, and it's reasonable to assume that a few more cases of these diseases have occurred in Ontario this year.

In Ontario, August and September tend to be the months of highest activity for EEE and WNV, and as we move into cooler weather (and decreased mosquito activity) the risk of EEE and WNV will start to plummet. I wouldn't be surprised if the numbers increase slightly by the time the final tally is made, but there are no indications that we have major disease activity at the moment.

Surveillance data such as this, including total numbers of cases in the province and an indication of areas where case occur, are important for horse owners and veterinarians to consider when determining their vaccination programs.

Equine Biosecurity Risk Calculator

After an extensive development and review process, an Equine Biosecurity Risk Calculator (click here) is now available online on the Equine Guelph website. This joint venture of Equine Guelph and Colorado State University, with support from the AAEP Foundation and Vetoquinol, is designed to offer practical advice on equine infectious disease risk and control.

While no one can really give an exact number regarding "risk" of infectious diseases on any particular equine facility, it is possible to identify areas in need of improvement. That's what this "calculator" is designed to do. Horse owners can complete the online questionnaire and get a general assessment of risk, plus detailed information about areas of concern. The calculator is free to use and no identifying information is collected.

Every horse owner should try this and see what areas are identified that could be improved. While facilities are quite variable in their application of infection control practices, I've yet to encounter a "perfect" facility - undoubtedly everyone will get at least a few ideas to consider to help reduce the risk of infectious disease in their animals.

MRSA from foal to girl

A case report highlighted by and presented at the ASM/ESCMID MRSA conference in Washington DC last week described a horse-associated methicillin-resistant Staphylococcus aureus (MRSA) infection in Dutch girl.

The girl, a 16-year-old with a severe neuromuscular disease who was wheelchair-bound and on a ventilator, developed an infection following an insect bite. When the infection didn't respond to initial treatment, a sample was taken for culture and MRSA was identified. The girl didn't have any known risk factors for MRSA infection but had had close contact with a foal. The Friesian foal had been at a veterinary hospital prior to the girl's infection. It had a wound infection that was successfully treated with antibiotics, but no culture was taken at the time. The foal was considered a possible source of the MRSA, particularly since the strain that was recovered was ST398, which is widely found in livestock and which is regularly seen in horses in the Netherlands. After the girl's infection was identified, the foal was tested and was also found to be carrying MRSA. The girl's infection was successfully treated and the foal eliminated MRSA carriage without treatment (which is expected in horses because long-term carriage of MRSA seems to be rare to non-existent in this species).

The source of the infection could not be definitively proven, but given the fact that the horse was at a facility that regularly sees MRSA cases, that the strain involved is typically associated with livestock, and that the girl had no other livestock contact, it's a reasonable to assume it came from the foal.

We've known for a few years that MRSA is an issue in horses, and that it can be passed between horses and people - in both directions. Equine veterinarians and horse owners have abnormally high MRSA carriage rates. MRSA carriers are people who have MRSA living in or on them (most often in the nose) without any signs of infection. Most carriers never have a problem, but disease can develop in some situations. The incidence of human MRSA infections transmitted from horses is low, although it's almost certain that many horse-associated MRSA infections are not reported because the link with horses isn't made or people don't mention the horse contact. article is incorrect in stating that this is only the third case of horse-to-human MRSA infection, since we've already published two such reports, one of which included multiple cases. Regardless, it's an uncommon problem but it is probably also under-recognized. Horse owners shouldn't panic about MRSA, but they should realize that MRSA is circulating in the horse population and that by nature of their frequent and close contact with horses, they are at higher risk for MRSA carriage, and likely also infection.

More information about MRSA in horses can be found on the Worms & Germs Resources - Horses page.

(click image for source)

Safe Sex For Horses

The recent (or, I guess, not so recent, since it seems to have been lingering in the background for a while) contagious equine metritis (CEM) outbreak in the US was a good demonstration of the potential impact venereally transmitted diseases can have on the horse industry. Concerns about such things are greatest in breeds that only allow live cover, since an individual stallion may be exposed to a large number of mares every year, and with the mobile nature of the horse population, it's possible for one infected horse to disseminate an infection widely across or between countries.

While CEM is probably the highest profile venereal disease in horses, it's not the only one. Other pathogens like equine arteritis virus and equine herpesvirus type-3 (equine coital exanthema virus) are also of concern, along with various opportunistic bacteria.

Proper management and infection control practices can greatly reduce, but not eliminate, the risk of sexually transmitted infections in horses. Unfortunately, such practices aren't always used. Sometimes it's because of lack of consideration or laziness. Sometimes it's because of lack of awareness. To help improve awareness and make it easier for people to take appropriate precautions, the American Association of Equine Practitioners (AAEP) has recently released Biosecurity Guidelines for Control of Venereally Transmitted Diseases. It's an excellent resource for veterinarians and horse owners, and should be part of the infection control program for anyone breeding horses.

Economic Impact Of Aussie Horse Flu

A huge equine influenza virus outbreak occurred in Australia in 2007 - a classic example of what can happen when a virus gets into an area where it's never been before. There were huge numbers of affected horses and a massive disruption to the industry due to quarantines and other control measures.

A special edition of the Australian Veterinary Journal (July 2011) includes a series of papers covering different aspects of this outbreak. In one paper (Smyth et al) the authors look at the economic consequences and tried to determine the financial costs of the outbreak. Such estimates are always tough to make and can never be 100% accurate, but they can give a general idea of the scope and impact of an outbreak. Not surprisingly, the costs were pretty astounding.

Australian Government

A series of measures were implemented to assist individuals and organization that were impacted. The total cost of those packages was over $263 million AusD.

State/Territory Governments

New South Wales and Queensland were most seriously affected, but all states and territories were impacted. These governments provided support in addition to the federal funds. For example, Queensland allocated over $27 million to various efforts, while New South Wales contributed more than $46 million.

Racing and Wagering Western Australia

This is the government body that regulates racing in Western Australia. The outbreak cost this agency around $500 000, a figure that does not include lost employee time and approximately $15 million in lost wagering revenue. Some of this was recovered through insurance, but it's now unlikely that they will be able to get further insurance to cover outbreaks.

Harness Racing Industry

It's always hard to figure out the true costs to an industry after a major disaster because the trickle down effect goes so far, affecting people who provide support and services (e.g. hay suppliers) to various businesses that are affected directly because people in those groups don't have money to spend. The total identifiable costs were calculated to be over $23 million, about half of which was to owners and trainers. The authors acknowledge the true costs were probably much higher.


A large inquiry was commissioned after the outbreak. This cost over $5 million.

Animal Health Australia

This group coordinated the emergency response and had to divert tremendous personnel time and resources. This included the vaccination program that distributed 670 000 doses of vaccine.

Households and businesses

Overall, it was estimated that horse associations lost $281 million, horse businesses $65 million and households $34 million.

Horse deaths

The value of horses that were reported to have died was close to $1 million, despite the fact that equine flu is uncommonly fatal. This number doesn't include intangible costs associated with losing a horse. However, reported deaths may be a minority and it was estimated that true horse death costs may have been $44 million. (However, I suspect the death rate estimate used for this value is high.)

Veterinary treatment

Estimated costs...$35.7 million.

Do the exact numbers matter? No. They simply show that an infectious disease outbreak can cost a lot. In many areas, horses receive little government attention because they are not food animals, despite the fact that the highly mobile horse population is probably at much higher risk of importing a new disease, and despite the fact that the economic impact of the industry is huge (and often overlooked by governments and groups that fund agricultural research).

If nothing else, this should serve as a reminder to government and industry groups that attention needs to be paid to infection control and emergency planning. While groups are often reluctant to put  much or any time, effort and funds into these areas, the amount of money that would be spent is inconsequential compared to the potential impact of even a small outbreak.

Image source:

Ontario Equine Influenza Activity

Like human influenza, equine influenza can be a seasonal disease with periods of particularly high activity. It's possible that we are in a period of high flu activity in horses in Ontario, based on cases we've seen and anecdotal information from the general population. We've seen a couple of confirmed cases of influenza over the past week (which is rather unusual), and unconfirmed rumours suggest that there may be many influenza cases in horses in the province, particularly at racetracks. Given the non-reportable nature of influenza, the often tight-lipped nature of horse owners when it comes to disease, and the typical lack of diagnostic testing to confirm flu in horses with flu-like disease, it's hard to say what's really happening.

Regardless, people should be on the lookout for influenza. Horses that develop a fever and/or signs of respiratory disease should be evaluated and isolated unless a non-infectious cause is identified. Facilities with affected horses should implement effective infection control practices to reduce the spread both on the farm and to other farms. People should discuss the risk of influenza exposure in their horses and talk to their veterinarian about whether influenza vaccination is indicated. Veterinarians can also use this situation as a reminder to owners of the ever-present risk of various infectious diseases, and as a lead-in to an assessment of the regular farm infection control plan (and if the farm doesn't have one, get started on it).

EIA Outbreak In Arkansas

A large equine infectious anemia (EIA) outbreak has claimed the lives of 40 Arkansas horses. The outbreak has hopefully been contained to one farm in Johnson County. Two horses on the farm died from this viral disease, while 38 others were euthanized, presumably because they tested positive. Approximately 40 other horses are on the farm, and are presumably negative (at this time).

So far, the outbreak seems to be limited to this single farm. There is one neighbouring farm, which is being investigated to see if the insect-borne disease has spread. The affected farm does not show horses or 'sell any horses to any extent,' so hopefully the odds of the outbreak being contained are good.

A big question remains unanswered and is critical for determining whether this outbreak will be controlled: Where did the virus come from? Viruses don't just spontaneous pop out of nowhere (although sometimes it seems that way at first) - there had to be source of infection, and that source could have spread it elsewhere. With the large number of positive horses on this farm, it's concerning that the virus could have been introduced a while ago, and the longer the time frame, the harder it is to both track and control.

MRSA In Aussie Horses

In the early 2000s, we took a lot of bad publicity in Ontario (particularly at the Ontario Veterinary College) because of MRSA in horses. While MRSA had been found in horses before and there were anecdotal reports of MRSA infections and outbreaks in different areas, the fact that we intensively investigated the issue and published a lot of our findings made it look like we were the hotbed of MRSA internationally. We suspected at the time that MRSA was widespread in horses and that the limited reports were because few people were looking or publishing their observations. That suspicion has been supported by reports over the past few years of MRSA in horses in many countries, and it appears that MRSA is present in horses around the world.

A recent study from Australia (Axon et al, Australian Veterinary Journal 2011) provides more support for this conclusion. In the study, horses that were admitted to a veterinary hospital's intensive care unit over a 30 day period in 2008 were tested for MRSA carriage by culturing swabs from their noses. MRSA was isolated from 3.7% of horses, which is similar to the prevalence here based on data we've gathered over the years.

For the second part of the study, the authors looked at medical records from horses at the hospital from 2004-2009 and collected data on MRSA infections. During that time, MRSA was isolated from 75 horses.

  • That number (75) surprises me a little, since it's much higher than what we see here. Even though we see approximately 2% of horses carrying MRSA when they arrive at the hospital, we have a very low MRSA infection rate in our patient population. A few of those 75 horses probably didn't really have MRSA infections, since nine horses only had positive nasal samples which is more likely to be from subclinical colonization rather than infection of the nasal passages. A few others had MRSA isolated from catheter sites, and it's hard to say whether those are truly infection or just contamination of the skin. So, the number of true infections might be lower, but it's still a significant issue. It would be interesting to know how many of those horses came in with MRSA infections versus how many picked up MRSA in hospital.

Wound infections were most common, accounting for 43 (57%) of the cases. Five horses were euthanized because of the MRSA infection, all of which had joint infections that did not respond to intensive treatment.

One farm accounted for 18 MRSA-positive results in the second part of the study, as well as two positive horses in the surveillance part of the study. This farm would seem to have a pretty big MRSA problem, which we've seen occasionally on a few biohazardous breeding farms that we've found over the years.  MRSA can be controlled on farms like that but it takes effort. We've had some farms address the issue properly and eliminate MRSA, while others essentially ignored the problem and continued to have widespread MRSA for years. 

Not surprisingly, most of the MRSA isolates in the Australian study belonged to sequence type 8 (ST8), the group of MRSA that we find in horses here in Ontario and internationally. This is a recognized human strain that seems to have become adapted to horses. It's also found in a disproportionately high percentage of horse owners and horse vets, likely indicating movement of teh strain between horses and people.

Overall, the results of this study are not surprising, but are very useful in that they support the notion that MRSA is present in horses around the world, and the situation with MRSA in horses is probably quite similar in many different countries. 

More information about MRSA in horses can be found in on the Worms & Germs Resources - Horses  page.

Rabies In New Mexico Horse

Rabies is pretty rare in horses, but there have been a few reports this summer.  Though rare, rabies is still a major concern because it's invariably fatal in horses, and almost always fatal in people.

Rabies was recently diagnosed in a horse in Eddy County, New Mexico. It started showing undefined signs of rabies and was euthanized two days later. Several people who worked with the horse are undergoing post-exposure treatment.

While rabies can potentially be spread from horses to people, I'm not aware of any confirmed cases of such transmission. Since rabies usually kills people, even a plausible risk is cause for concern. Additionally, and perhaps more importantly, rabid horses can be very dangerous, and multiple people have been killed by aggressive rabid horses. 

Rabies should be a core component of a horse's vaccination program in any part of the world where rabies is present in the wildlife population. Unfortunately, that's not always understood. One area resident stated she'd never heard of rabies in horses, and while she has her dogs and cats vaccinated, she hasn't vaccinated her livestock. No vaccine is 100% preventative, but rabies vaccination is a cheap and very effective way to reduce the risk of this fatal disease significantly.

21 Horses Dead In Beirut Racetrack Outbreak

A somewhat confusing report in Lebanon's The Daily Star describes an outbreak of disease at Beirut's Hippodrome (racetrack) that has resulted in the deaths of 21 horses; 2 from disease and 19 that were euthanized as part of the outbreak response.

It's not specifically stated in the report, but the commentary about glanders, a highly infections disease caused by the bacterium Burkholderia mallei, suggests that glanders has been diagnosed or is strongly suspected. The two affected horses had been imported from Syria, which has been the source of other glanders outbreaks, and the disease is present in multiple countries in the region, so it's a likely cause. The reason for euthanasia of the 19 "healthy" (presumably exposed) horses is unclear. Hopefully it was done because they were identified as infected, however the report suggests that euthanasia preceded diagnosis.

Little information was provided about the response, beyond culling, with only a statement that "all relevant procedures had been taken." Additionally, a quarantine and testing requirement for horses entering Lebanon was announced by the mayor (presumably, a federal body has mandated this since I doubt the mayor of Beirut can set national importation policy). Measures for controlling glanders outbreaks are fairly well described and involve extensive investigation of horse contacts and movement, screening of potentially exposed horses, euthanasia of positive horses, quarantine of potentially exposed farms and use of stringent infection control practices. Hopefully a proper response is being undertaken here.

Beirut's mayor stated that there was "no possibility that the disease could be spread to humans," but that isn't consistent with glanders since this is a potentially serious disease in people as well. Two grooms that worked with the first two horses were tested and so far are negative and healthy. The likelihood of someone getting infected from an infected horse isn't great, but it's a very nasty disease so care must be taken and investigation of human contacts is important.

The Agriculture Ministry will apparently release its "all-clear" report in 6 months (assuming no more cases crop up). Let's hope that things truly are "all clear" in 6 months, with a subsequent report (presumably after official confirmation that the outbreak is over) thereafter. More details about the extent of testing and the results would be nice in the short term, to get a better understanding whether this is truly just a case of a couple of infected horses being imported with no transmission outside of the initial group, but political issues often limit the information flow in glanders outbreaks.

Image: Beirut Hippodrome (click for source)

More Mosquito-Borne Mayhem

The mosquito-borne diseases eastern equine encephalitis (EEE) and West Nile (WNV) continue to rear their ugly heads in the northeast as we get further into the late summer season during which they are most common.

On August 13 there was an unconfirmed report of a case of West Nile in a horse at Woodbine Racetrack, just north of Toronto.  No additional details have been forthcoming regarding the severity of the infection or the status of the horse, if WNV infection has in fact been diagnosed.  Nonetheless, the Ontario HBPA is urging horse owners to ensure the vaccination status of their animals for West Nile is up-to-date.  Unfortunately, if horses are not already vaccinated at this point, even vaccinating them immediately may still leave them susceptible to virus for the next few weeks until they are able to fully respond to the vaccine.  This news follows close on the heels of news reports regarding increased numbers of WNV-positive mosquito pools in various regions north of Toronto, and thus is not altogether surprising.

The first case of West Nile in a human in New Jersey was recently diagnosed in a man from Mercer County.  Again, no additional details about the severity of the infection or the man's condition are available, but the public is once again being urged to protect themselves against mosquitoes by wearing long sleeves and pants, using insect repellant, and eliminating standing water in which mosquitoes may breed on their property.  Elsewhere the death on August 14 of a four-year-old girl in New York from infection with EEE has been reported.  She is the fifth person in New York state to die from the disease in 40 years.  The girl first began showing signs of infection earlier this month, but the diagnosis of EEE infection was only reached last week.  EEE has a high mortality rate in humans as well as horses.  Just as infection in animals can act as sentinel indicators for disease risk in humans, these human cases indicate that WNV and EEE are active in these respective areas, and humans and horses alike are at risk of infection.  Mosquito avoidance can help protect both, and in addition timely vaccination of horses can help decrease the risk of disease.

West Nile Warnings

Following on the heels of the beginning of the start eastern equine encephalitis (EEE) season, warnings are going out about another mosquito-borne disease in parts of North America, West Nile virus (WNV) encephalitis. While mosquitoes have been swarming around here for months, WNV only becomes a real concern in southwestern Ontario starting in mid-to-late August. This time of year, the virus starts increasing in the mosquito population, and the types of mosquitoes that bite both birds and mammals (and therefore act as a bridge between the bird reservoirs and susceptible mammals like horses and people) become more common and more active.

Reminders about WNV have been issued in Ontario over the past few days because of increasing numbers of mosquito pools testing positive for the virus. Mosquito pools are groups of mosquitoes that are caught and tested together to see if the virus is present. The more pools that are positive, the more mosquitoes that are positive and the greater the WNV activity in the area.

People are being reminded to take measures to reduce mosquito breeding grounds on their property, which mainly means getting rid of standing water. Avoiding mosquito bites through basic measures, such as staying away from mosquito-infested areas (particularly at dawn and dusk), wearing light coloured clothing with long sleeves and long pants, using insect repellent containing DEET and making sure windows have intact screens, is also emphasized.

Recommendations are similar for horse owners; reduce mosquito breeding sites and reduce mosquito exposure of your horses (although this can be easier said than done). Vaccinating horses against WNV should also be considered. The dramatic decline of WNV infection in both people and horses after it first emerged back in 2001/2002 was quite impressive, and WNV hasn't ended up being the major problem it could have been. In 2011, there was only 1 confirmed case in a horse in Ontario. Presumably, there were more undiagnosed cases but this shows how the disease has leveled off to be an uncommon but still present issue.

Despite disease due to WNV being rare in Ontario (remember that the virus itself is not rare because the mosquito pools are still testing positive), WNV shouldn't be ignored because it still has the potential to cause severe illness in horses and people. When deciding whether or not to vaccinate your horse, consider carefully the risk of mosquito exposure, WNV activity in your local area, WNV history in horses and people in the area and your own level of risk aversion.

Bug of the Month: Equine Infectious Anemia Virus

Equine infectious anemia (EIA) is a rare disease but one that a lot of time, effort and money are put into avoiding. Most people know about this disease by way of the most common test (formerly) used to diagnose it: the Coggin's test (see image).

Equine infectious anemia is caused by a virus of the same name ( equine infectiious anemia virus, EIAV). EIAV is a lentivirus which, like all other lentiviruses, causes persistent infection. Unlike most other lentiviruses that cause slow, gradual preogression of disease, EIAV infection usually causes a sudden onset of disease (acute phase) followed by recurrent disease. After the initial (acute) phase, horses can appear normal, which means they can be a silent reservoir of the disease. That's why routine testing for EIAV is required in many circumstances: to detect silent carriers so they cannot continue to transmit infection.

Signs of acute EIA vary, but usually include fever, lethargy and decreased appetite. Anemia (decreased red blood cell count) and thrombocytopenia (decreased platelet count) can be detected. Anemia is more common and pronounced with recurrent infections. Intermittent illness often develops after the first acute episode. Affected horses may experience short (3-5 day) periods of fever, lethargy and decreased appetite. The severity of the anemia often correlates with the severity and frequency of these disease episodes. In some horses, these episodes are very common, long and severe, and these horses often have severe weight loss and anemia. However, most infected horses stop developing obvious signs of disease after a year and seem perfectly healthy.  This may be good for the one horse, but it's bad for other horses in the area to which the virus may be transmitted.

EIAV is a bloodborne virus that can be transmitted by blood-feeding insects, especially tabanid flies (horseflies, deerflies). Stable flies can also transmit EIAV but do so less effectively. Contaminated medical supplies such as reused needles and syringes can also transmit EIAV.

Fortunately, EIA is now rare in most regions and positive tests are quite uncommon. Routine testing for EIA is usually required for shows, sales, transportation and other situations where horses are mixed, in order to detect and remove carriers. Unfortunately, identification of a horse as a carrier is not good for the horse or owner (or other horses in the area) - EIA is not treatable, and horses that have positive tests ("reactors") are quarantined, as are all other horses that are housed within 200 yards. Horses living close to reactors are tested (usually 30 and 60 days after removal of the reactor) and only released from quarantine after getting negative test results at least 60 days after the last reactor was removed. Reactors are usually euthanized. If prompt euthanasia is not chosen, reactors are usually prominently branded or tattooed. They must be kept under quarantine for the rest of their lives, and at least 200 yards from other horses. (200 yards is used because the flies that transmit the disease don't usually travel that far.)

The best way to reduce the risk of EIA is ensuring that all horses are regularly tested. New horses coming onto a farm must be tested BEFORE arrival or after arrival but while in quarantine at least 200 yards away from other horses. Needles and other items that might be contaminated with blood should never be re-used, both due to the EIA risk and to avoid other potential problems.

Overall, EIA is a very rare disease, but the severe implications of a positive test mean that we need to be vigilant.

Image from

Eastern Equine Encephalitis - Not Just For Horses

Over 50 horses have died from Eastern Equine Encephalitis in Florida this year. The disease, caused by a virus of the same name, affects the brain, resulting in a broad range of clinical signs from behaviour changes to blindness to irregular gait. The disease is also sometimes called “sleeping sickness” because some horses may become severely depressed, with low head carriage and droopy eyes, ears and lips. Almost all horses that develop neurological signs from this infection die. Only 35 cases were reported in Florida in 2006 and 2007 combined.

There are actually three related equine encephalitis viruses – Eastern, Western and Venezuelan – which are called EEE, WEE and VEE for short. VEE is found in South and Central America and Mexico, and occasionally in the southern United States, but has never been reported as far north as Canada (VEE is a reportable disease in Canada). It is unique among the three diseases as the only one in which an infected horse will carry enough virus in its bloodstream to infect a mosquito, which could then pass the virus on to another animal. The EEE and WEE viruses, just like the West Nile virus, do not reach high enough levels in the bloodstream of horses to do this. The mosquitoes usually pick up the viruses from passerine birds, which do not become ill from the viruses (unlike West Nile virus in birds from the family Corvidae).

People can also be infected by EEE, WEE and VEE. About 10 fatal cases of EEE in people are reported in the United States every year. But horses cannot transmit EEE or WEE to humans, even if they’re bitten by the same mosquito. A higher number of cases in horses, however, may mean a higher number of mosquitoes that are carrying the virus. There is no vaccine for these viruses for humans, but there are vaccines available for EEE, WEE and VEE for horses.

In the end, EEE is just one more good reason to make sure you wear mosquito repellent when you’re enjoying the great outdoors during the summer. Visit the Health Canada website for safety tips on using personal insect repellents. EEE is very uncommon in Ontario, but horses that live in or travel to the southern United States should be vaccinated. Talk to your veterinarian about whether or not your horse should be vaccinated. Remember that fly control is also important for our equine companions (and also helps protect them against West Nile!).

Equine Infectious Disease Information Sheets

Click on any of the highlighted links below for more information about these horse-related infectious disease topics. Topics that are not highlighted are in development and coming soon. New information will be added as it becomes available, so be sure to check this page regularly for the latest updates.

Bacteria Viruses Parasites Other
Clostridium difficile Rabies Bots Pleuropneumonia
Clostridial Myonecrosis Eastern Equine Encephalitis Equine Protozoal Myeloencephalitis (EPM) Neonatal Diarrhea
Strangles (Streptococcus equi) Equine Herpesvirus Cyathostomes
(Small Strongyles)
Needlestick Injuries
Methicillin-resistant Staph aureus (MRSA) Equine Influenza Large Strongyles Colostrum
Lawsonia West Nile Virus Tapeworms  
Rhodococcus equi Equine Infectious Anemia - Summary Pinworms  
Tetanus Equine Infectious Anemia - Full-length    
Botulism -
Botulism -
"Shaker Foals"

All information sheets found on this page can be freely downloaded, printed and distributed. The authors only request that this website (, previously is acknowledged as the source.  The downloadable files on this page can be opened with Adobe® Reader®.  To get the latest version of Adobe® Reader® for free, click here.