After being a relatively rare problem in most regions over the past few years, West Nile virus (WNV) case numbers have boomed lately, with large outbreaks in some US states.
Forty-nine (49) confirmed or probable human cases have been reported in Ontario, the largest number in a decade. Considering we're just heading into the typical peak WNV season, it's quite concerning as the worst may be yet to come. At this time last year, there were only 24 reported cases.
Human cases have been reported in at least four other provinces: Alberta, Manitoba, Saskatchewan and Quebec.
Two equine cases of WNV have been reported, one in Saskatchewan and one in Quebec. It's hard to have a lot of confidence in this number because of the poor surveillance and reporting for this disease in animals in Canada, given that the CFIA has largely washed their hands of dealing with it. Infection with West Nile virus has been pretty much a non-entity in most regions over the past few years, at least in terms of diagnosed cases, and it remains to be seen whether equine cases will mirror the spike in human cases this year. Typically the trends are similar each year, so the next few weeks will tell us a lot.
The US is in the midst of its largest WNV outbreak ever. At least 1118 human cases have been reported so far in at least 37 states, with at least 41 deaths. Typically less than 300 cases are reported by this time of year. Texas has experienced a huge outbreak, accounting for about half of the US cases.
There hasn't (apparently) been a surge in equine cases, with less than 100 cases of WNV reported in horses as of August 18. Whether that's because of infrequent testing, biological or geographic factors resulting in less equine exposure or vaccination of horses (remember that there is no WNV vaccine for people) isn't clear.
Concern is being raised about risks to pets, but the true risk is very limited. While WNV infections have been reported in dogs and cats, these are extremely rare and dogs and cats are failry resistant to the virus.
Often, when a new infectious disease emerges, the first year or two are the boom years, after which things settle down. That was the pattern with WNV in most areas; however, this year in on track to meet or surpass the numbers from those early years.
Why is this happening? No one knows for sure. Changing weather patterns, by chance or through the larger spectre of global warming, are probably playing a major role. Warmer temperatures let mosquitoes mature faster and allow the virus to grow quicker in the mosquitoes. Milder winters help mosquitoes survive. Any factor that fosters more mosquito numbers and growth, particularly the subset of mosquitoes that bites both birds (the reservoir of the virus) and people, can increase the risk of human and animal exposure. Changes in rainfall, wetland management, climate and human proximity to mosquito breeding sites can all play a role.
If I was reincarnated as a mosquito, I'd want to live where I do now (convenient, eh?). I live in the country surrounded by areas of "protected wetland," which, in many cases, is a fancy word for swamp. I try to avoid mosquitoes, but getting bitten is a regular (daily) event. As I was getting swarmed last night, I was thinking that the mosquito-borne disease I'm really concerned about is Eastern equine encephalitis (EEE). While quite rare in Ontario, with only a handful or no cases in horses every year, it's a worry because it's almost always fatal. It also affects people, not via transmission from horses but from being bitten by mosquitoes that pick up the virus from birds. It's very rare in people, but it's highly fatal.
This is the time of year that we start seeing mosquito-borne infections in Ontario, and a Disease Alert from the province re-inforces concerns about EEE. The alert was issued in response to a case of EEE in a horse in New York state, not far from the Canadian border. The horse, from the Ogdensburg, NY area, showed signs of illness on July 23 and died the next day (a pretty typical progression for this disease). Since mosquitoes don't respect borders, cases in this neighbouring region suggest that infected mosquitoes might also be active in Ontario.
EEE isn't the only mosquito-borne virus that we worry about - West Nile virus being the other main issue around here - so mosquito control and avoidance are important. While you can never guarantee that you or your animal will not encounter a mosquito, various things can be done to reduce mosquito numbers (e.g. eliminating sites of standing water (which are mosquito breeding sites) wherever possible... swamps being a logical exception) and to reduce the risk of being bitten (e.g. avoiding high risk areas and times, long clothing, mosquito repellants). Vaccines are available for EEE and West Nile virus in horses (but not people), and the risk of these diseases should be considered when designing a horse's vaccination program.
Local media are reporting an apparent case of West Nile virus infection in a horse in Northampton County, Pennsylvania. This is surprising since, while I know there are certainly mosquitoes emerging early with this mild weather, seeing active mosquito-borne infections at this time of year would be very unusual. That's particularly the case with West Nile virus, since it tends to be a late summer and fall disease based on the mosquito types that are predominant at that time of the year.
Information about this West Nile virus case is pretty sparse. The report simply says the horse was euthanized because it was "suffering from the virus." Knowing if and how it was actually diagnosed is important to determine whether it was truly an active infection or a false-alarm, like this winter's report of West Nile virus in British Columbia.
Regardless, it's still a good reminder that we are now heading into the time of year when we have to think about mosquito-borne diseases in various species (including people). Measures to reduce mosquito populations, such as eliminating standing water (see picture), and mosquito bite avoidance are always good, regardless of what diseases are currently being diagnosed.
Image source: www.saskatoonhealthregion.ca
Authorities in Hawaii are advising people in Oahu to eliminate standing water as a mosquito control measure. While it's always a good idea, it's of particular concern in this case bacause a rare type of mosquito, Aedes aegypti, was found around the Honolulu International Airport. This mosquito species is a highly effective vector for various infectious diseases, including dengue fever and yellow fever.
What does this have to do with companion animal disease? Well, nothing directly, but it's a good reminder of how infectious diseases can easily reach a distant area (even Hawaii) in a short period of time.
There are a plethora of mosquito-borne diseases out there, and presumably we don't even know about many of them. Mosquitoes don't fly very far, which helps contain these diseases to certain areas. However, mosquito-borne diseases can still spread over wide ranges if either the pathogen or the mosquitoes are hitch-hiking.
A common way for pathogens to travel is in various kinds of animals (especially birds) that can harbour the pathogen (usually a virus) and infect mosquitoes in distant areas.
Modern transportation can be an effective vehicle for pathogen-laden mosquitoes. Theoretically, all it takes is for a single infected mosquito to hop onto a plane and survive the flight to a new region. If the mosquito bites a susceptible host, it can cause a rare disease - that's of particular concern since it's unlikely that an exotic foreign disease in someone who has not left the country would be promptly diagnosed (and therefore promptly treated). Even worse, the disease could establish itself in the new region if a series of things happen:
- The mosquito has to bite something or someone.
- That something or someone has to be susceptible to the pathogen and that pathogen needs to grow inside the host's body to high enough levels that it can infect another mosquito.
- Another mosquito that can carry the pathogen must come along, bite the infected individual and acquire the pathogen.
- The new mosquito must then find another susceptible host to bite.
- The above needs to be repeated enough times that the pathogen establishes a foot hold in the area and starts causing disease.
Is this common? No.
Is it possible? Yes.
West Nile virus is an example of what can happen. This mosquito-borne virus came out of nowhere in North America in the early 2000's and caused widespread illness and death in humans, horses and various other species. Did it arrive via a mosquito on a plane? No one knows, but it's certainly a possibility.
We've had a really bizarre spring around here. At a time of year when we're usually bouncing back and forth between snow and rain, we seem to have skipped over spring and moved right into summer. It was 25 C (77F) today, the snow's long gone, frogs are deafening at night and, on the bad side, mosquitoes are swarming at dusk.
This has led to a lot of questions about the implications of the weather on the regional approach to heartworm prevention, since the typical approach is to start treatment in early June. Specifically, does the early spring mean that there's increased risk from this mosquito-borne parasite or that we need to start treating dogs earlier?
Dr. Andrew Peregrine, a veterinary parasitologist at the University of Guelph, has taken a break from fielding calls about this subject to provide some insight.
A big question is whether over-wintering infectious mosquitoes are a concern.
If over-wintering mosquitoes aren't a concern, which is a leading consensus, then the mosquitoes we're seeing now are newly hatched.
- If that's the case, they have to feed on an infected animal and the parasite has to mature inside the mosquito before the mosquito poses a threat to dogs (and less commonly cats, and very rare humans).
- With the current temperatures, it should take at least three weeks for this to happen. Therefore, if a mosquito hatches in March and happens to feed on an infected animal, it would be into April before it's infectious. Heartworm preventive medication has 'reach-back' activity of at least 7-8 weeks, meaning that the medication will effectively prevent disease from exposure at least 7-8 weeks prior to treatment.
- So, if a mosquito promptly feeds on and infects a dog after it becomes infectious, heartworm medication given at the beginning of June would still be effective at preventing disease because of this reach-back period.
- In this scenario, there's no need to start treatment any earlier around here than the traditional June 1 start date.
There are some people who think that overwintering infectious mosquitoes are an issue, although evidence supporting that seems to be lacking. If overwintering infected mosquitoes are an issue, the mosquitoes that are around now might be infectious.
- In that case, June 1 might be too late in the event of an early exposure, and May 1 would be a more appropriate start.
Bottom line, there is no answer that's 100% certain since we don't have all the information that we need. June 1 is most likely perfectly fine. Anyone who's's ultra-concerned could start treatment on May 1. There's no real downside (apart from a little extra cost) of this extra treatment, but it's probably not necessary.
Although the weather in Southwestern Ontario seems quite confused lately regarding whether it wants to be winter or spring, at least we're still a few months off from having to worry about mosquitoes and the viruses they carry once again. Warmer parts of the world, however, are in the midst of their mosquito season, and some chickens are lending a hand to give people in the area a "heads up" about what's around.
The Health Department of Western Australia has detected Murray Valley encephalitis virus (MVEV) in chicken flocks in East Kimberley. The department has also tested and found the virus in its sentinel chickens in Wyndham and Kununurra. These sentinel birds play an important role as an early warning system when viruses like MVEV are circulating in the area. Just like West Nile virus, MVEV typically circulates between birds and the mosquitoes that like to feed on them, but problems occur when the same mosquitoes start to bite people (or other susceptible animals such as horses), particularly when there are a lot of mosquitoes, like when the weather is very wet or when there's been flooding. Although most people who are infected with MVEV or WNV fight off the virus with no difficulty, or may simply develop short-term, non-specific signs of illness like mild fever and malaise, in some people these viruses can cause severe infection of the brain (encephalitis) and may even be fatal.
Knowing that MVEV has been found in these "guardian" chickens lets people know (via warnings issued by the health department) to take extra precautions against mosquito bites, such as:
- Staying indoors during peak mosquito activity - dusk and dawn
- Wearing protective clothing including long-sleeves and long pants
- Applying insect repellent
In North America, you can pretty much substitute West Nile for Murray Valley in a case like this. Sentinel chickens have been used to provide early warnings of circulating WNV here, before cases are detected in people or horses. Another means of early detection that is also used is testing pools of mosquitoes directly.
It just goes to show you can still be an important part of the country's defenses, even if you're a little chicken :p
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.
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.
Yesterday, I wrote an equIDblog post about an outbreak of unexplained neurological disease in horses in the Murray River region of Australia. Today, a ProMed report indicates that Murray Valley encephalitis is now being considered as a possible cause of death in a man from the area.
Murray Valley encephalitis (MVE) is one of the possible causes of the equine neurological disease outbreak, and it's quite likely that if it caused disease in one species in the region, it did the same to another. This rare mosquito-borne disease hasn't been seen in decades in the region, but it's possible that high mosquito numbers following heavy rainfall and flooding have increased the risk of transmission.
While this virus poses a risk to both humans and horses, humans and horses pose no risk to each other. Both acquire the disease the same way - from mosquitoes - and neither can pass it on to the other. This is also true of other insect-borne viruses such as West Nile virus and Eastern Equine Encephalitis (EEE) virus.
This is a good example of why human and animal disease surveillance need to be linked, and why governments need to put resources into testing of animals beyond food animals. Rapidly identifying a disease in horses or other animal species can help determine whether there is any risk to humans, and hopefully lead to preventive measures being taken earlier.
Prevention of this disease is focused predominantly on mosquito avoidance. It's impossible to completely prevent mosquito exposure, but some basic practices can help reduce the risk. Click here for some practical tips on protecting yourself and your horses from mosquitoes.
Image: Location of the Murray River in Australia (click for source)
This Worms & Germs blog entry was originally posted on equIDblog on 13-Mar-11.
A horse in Ontario was recently diagnosed with Eastern Equine Encephalitis (EEE), a serious neurological disease caused by a virus of the same name, which is transmitted by mosquitoes. The horse was from the North Durham region. The last reported cases of EEE in Ontario were in 2004. A few weeks ago, the Worms & Germs Blog talked about a large number of cases of EEE that have been reported in Florida this year.
Here are some of the key points to remember about EEE:
- Like West Nile, EEE is a seasonal disease. It is more common in warmer areas, especially some regions of the southeastern US. It is rare in cooler climates, but occasionally EEE is found in horses in Ontario.
- EEE is usually fatal in horses, and there is no effective treatment.
- EEE can also occur in people, and can be fatal in some cases.
- Infected horses cannot transmit the EEE virus to people, but if a horse gets EEE from the mosquitoes in the area, then people could also potentially be exposed to the virus by mosquitoes.
- A vaccine for EEE is available for horses, but most horses in Ontario are not vaccinated for EEE because it is so rare. Nonetheless, vaccination can be considered because the disease is so devastating when it occurs.
- As for West Nile virus, avoiding mosquitoes - for both horses and people - is an important preventative measure for EEE.
For more information, see the Worms & Germs Blog post "Eastern Equine Encephalitis – Not Just For Horses", or the CDC's website on arboviral encephalitides.
In Ontario, and many other regions, mid-August is the beginning of the high risk period for West Nile virus infection in people and animals such as horses. The Ontario Veterinary College has published an informational video on YouTube. This video has information about measures you can take to reduce the risk of West Nile virus exposure and disease, for both people and horses. Click on the image to watch the video. More information about West Nile virus is also available in the blog post entitled West Nile virus in dogs and cats.
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!).
One thing that has become very clear around my house the past few days is that mosquito season has arrived. Along with the annoyance, buzzing and itching, mosquitoes are of concern because they can transmit various viruses, including West Nile virus. While West Nile season tends to be later in the summer and fall in most regions, West Nile virus awareness and mosquito bite avoidance should be considered whenever mosquitoes are around.
Most of the attention about West Nile virus has been paid to humans, horses and birds, because of widespread illness and death in those species. Dogs and cats can certainly be exposed to West Nile virus if they are bitten by an infected mosquito; just like people and other animals. However, there are very few reports of West Nile virus infection in dogs, and even fewer in cats. Most dogs and cats that are infected do not develop signs of disease and simply develop antibodies against the virus.
Even if dogs or cats develop West Nile virus infection, there is no risk of transmission to other animals or human. This is because there is never a large enough amount of virus in the blood of these animals for a mosquito to pick it up and transmit it to another animal or person. The virus also can't be transmitted by direct contact with an infected dog or cat. West Nile virus is not found in saliva, so it is also unlikely to be transmitted by an animal (instead of an insect) bite.
There is no reason to be concerned about transmission of West Nile virus from your pets. If you live an area where West Nile virus is present, you should take precautions to avoid mosquito bites, such as avoiding mosquito-dense areas, wearing insect repellent with DEET, wearing long pants, long sleeves and socks when outside, avoiding peak mosquito hours (dusk and dawn) and eliminating any areas of standing water where mosquitoes can breed.