Can dogs spread Valley Fever?
I received this question yesterday, pertaining to a potential therapy dog.
Valley Fever, also known as coccidioidomycosis, is a fungal infection caused by Coccidioides immitis or Coccidioides posadasi. These fungi live in the soil and are most common in the southwestern US, northern Mexico, and parts of Central and South America. They are part of an unusual group of fungi called "dimorphic fungi," meaning they exist in two forms. One form in found in the environment (soil). This arthroconidial form is the infectious form. The other yeast-like form is present in the body tissues during infection, but is not (or at least is very minimally) transmissible.
Disease from Valley Fever is rare in healthy people. These fungi are mainly a concern in people with compromised immune systems. When illness occurs, flu-like disease, respiratory disease, rash and joint pain are the most common signs, but disseminated infection (i.e. infection throughout the body) is a much more serious form of the disease that can occur.
Valley Fever is similar in dogs, with most dogs have mild to inapparent disease, and most sick dogs having vague signs and respiratory disease. Cats often develop skin lesion. Disseminated disease can also occur.
While coccidioidomycosis can occur in both humans and animals, the risk of transmission between humans and animals is extremely low. The fact that it occurs in both humans and animals is because both humans and animals get exposed to the same sources, not because they spread it between each other.
However, there is a slight risk that shouldn't be ignored. There are two situations that are of concern.
- Bites: There is one report of a bite-associated infection in a veterinary technician. The risk of infection after a bite from an infected animal isn't known, but anyone bitten by an infected animal should seek medical advice. Presumably, nothing would be done initially but there could be close monitoring for disease so that it can be treated early if problems develop.
- Veterinary procedures: Infection has been reported in a person performing a necropsy (autopsy) on an infected horse. It was thought that infectious endospores were aerosolized when an infected area was cut with a saw as part of the procedure, and inhalation of the fungus lead to disease.
There's also a theoretical concern with handling bandage material from infected animals. While the active infection would be caused by the minimally infectious tissue form of the fungus, it's possible that infectious arthroconidia could develop in a bandage.
People with infected pets have little about which to be concerned. The main risk (which is also very low) is infection from a bite from an animal with disseminated disease. Basic bite avoidance should minimize this risk, however medical care should be sought following any bite and people at high risk of serious infection (e.g. people with compromised immune systems) should take particular care when interacting with infected animals. If a pet owner has to change a bandage on an infected animal, they should wear gloves, double bag and immediately dispose of the bandage, avoid contamination of the environment during bandage changing and thoroughly wash their hands after completing the task.
Image: The infectious arthroconidia of Coccidioides immitis (source: CDC Public Health Image Library #476).
TB in a bird...Human health risk?
A bird specialty store owner wrote me recently, concerned about potential tuberculosis (TB) exposure. A client's bird had been diagnosed with "human TB" and that person had spent a lot of time with the bird. The source of the TB hadn't been identified, and the store owner was worried about the risk that he/she had been exposed as well.
Is it really TB?
The first thing to consider in a case like this is whether TB was really present. "Human TB" is caused by Mycobacterium tuberculosis. Birds can be infected by M. tuberculosis, but are more often infected by Mycobacterium avium complex (MAC), a related group of bacteria. Based on what the store owner wrote here, it seems that M. tuberculosis was the cause of disease.
Can TB be spread from birds to people?
Probably, but the evidence is sparse. Tuberculosis can be spread from people to birds, and it's likely it can go the opposite direction. However, close and prolonged contact is typically required for TB transmission. Human-to-bird cases tend to be birds owned by people with active TB who have close mouth-to-mouth contact with their birds (e.g. mouth-beak feeding).
What is the likelihood of transmission?
Being in the same room as a bird with TB is probably pretty low risk (just like casual contact with a person carrying TB is low risk). The risk also depends to a degree on the type of disease the bird has. If it has respiratory tract disease it is probably more likely to be shedding the bacterium in its respiratory secretions, which poses a greater risk of transmission than other forms of the disease.
Is there cause for concern?
I guess there's always some degree of concern when it comes to TB, but I assume the likelihood of transmission of TB from the bird to the store owner is quite low. The source of TB wasn't known, but most likely the bird was infected by its owner, and if so, being around the bird's owner is probably as (or more) risky.
Plague in New Mexico cat and dog
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Plague has been identified in a dog and cat from New Mexico. It’s not surprising, since plague is present in some wild animal populations in that region, but it’s still noteworthy because of the serious nature of the disease and the potential for transmission to humans.
Plague is a bacterial infection caused by Yersinia pestis. It’s carried mostly be certain types of rodents in different regions of the world, including parts of the southwestern US. It’s usually spread by fleas that bite an infected rodent and then bite a person or other animal, but it can also be spread by close contact with an infected animal. Cases in cats and dogs are uncommon, but occur in areas where plague is present in rodents, when pets have contact with infected fleas or close encounters with infected rodents (or rodent carcasses).
The latest two cases were in Santa Fe and Rio Arriba Counties in New Mexico. No details were provided about the form of plague (e.g. bubonic, pneumonic), the suspected source of infection or whether there is concern about human exposure. Finding plague in a pet is a concern for a few reasons. It indicates that plague is present in wildlife in the area, and people could be exposed from the same sources as the pets (i.e. fleas, contact with live or dead wildlife). Also, transmission of plague from pets to their caretakers can occur, particularly from cats with pneumonic plague (respiratory tract infection). Knowing that a person has had contact with a pet with plague is critical to making a prompt diagnosis. According to the World Health Organization, plague continues to infect more than 2000 people every year.
The New Mexico Department of Health has made the following recommendations:
- Avoid sick or dead rodents and rabbits, and their nests and burrows.
- Keep your pets from roaming and hunting and talk to your veterinarian about using an appropriate flea control product.
- Clean up areas near the house where rodents could live, such as woodpiles, brush piles, junk and abandoned vehicles.
- Sick pets should be examined promptly by a veterinarian.
- See your doctor about any unexplained illness involving a sudden and severe fever.
- Put hay, wood, and compost piles as far as possible from your home.
- Don’t leave your pet’s food and water where mice can get to it.
- Veterinarians and their staff are at higher risk and should take precautions when seeing suspect animal plague cases.
Photo: The vector of Yersina pestis: a flea (click image for source)
Dumb Easter decisions
Easter is one of those holidays when there are concerns about dumb pet purchases. Spur-of-the-moment purchases of inappropriate pets can lead to animal suffering and death, and risk of human infection. Easter's problems: baby chicks and rabbits.
Rabbits can make great pets. They're a long-term commitment, but they’re relatively low maintenance, a lot is known about how to raise them and they are generally low risk for transmission of infections to people. Chicks are a different story. Chicks are notorious Salmonella vectors and have been linked to numerous outbreaks. They are easily injured and often improperly raised. They also grow up (well, some of them do, at least) to be full sized poultry, something that most people don't really want.
A story from Vidalia, Georgia highlights some of the issues with Easter pets. In it, Tracy Gunn describes his need to buy a chick for his daughter - and not just any old chick, but a dyed chick, something that’s illegal in 36 US states, but not Georgia. Gunn states "I don't know what she's going to do with it." Sounds like a recipe for a few minutes of novelty, followed by a relatively short life for the chick. At least his daughter’s 17, and not in the high risk group for salmonellosis.
Alongside the cage full of multicoloured chicks was a collection of rabbits.
“The bunnies sell real good for Easter. We've been selling a lot of them about the last month. Can't keep enough of them.” said a store employee.
He followed that up with “They buy (rabbits) for their kids for Easter, then they take Easter pictures and stuff like that with them, I'm not sure about what happens to them afterward.”
That’s the problem. Kids get a few minutes of novelty enjoyment, but then a lot of those animals end up dead, released into the wild (not a good thing) or dropped off at an animal shelter, because people don't think about the "afterward" part before they buy.
Pet purchases need to be made with thought and foresight:
- Do I really want this pet?
- Am I committed to taking care of it for its entire life?
- Can I take care of it properly with my current living situation?
- Can I afford to take care of it properly?
- How do I take care of it?
- Are there any disease risks that I need to be concerned about?
- Are there any people in the household who are at high risk for disease caused by this type of animal?
If you can't answer these questions, don't buy or adopt an animal - of any kind.
More on vets and MRSA
There are a number of published studies regarding methicillin-resistant Staphylococcus aureus (MRSA) carriage by veterinarians, most reporting high rates compared to the general population. This is a concern because MRSA is an important cause of disease in both people and animals. Just having MRSA living in your nose doesn't mean you're going to get sick. Indeed, around 2-3% of normal, healthy people are likely carrying MRSA at this moment. However, if you are carrying MRSA, you are at increased risk of developing an infection under certain circumstances. In veterinarians MRSA carriage is also a concern because of the potential for transmission to patients (and potentially from those patients back to people).
A recent Australian study in the Australian Veterinary Journal (Jordan et al 2011) looked at MRSA carriage in different types of veterinarians. The rates were:
- 0.9% in industry and government veterinarians (who have limited contact with animals)
- 4.9% in small animals veterinarians
- 11.8% in veterinarians with horses as a major component of their caseload
- 21.5% in equine veterinarians
These results are similar to some of our earlier studies, with carriage rates in small animal veterinarians being higher than would be expected for the general population, and carriage rates in equine veterinarians being very high.
Why do veterinarians have high rates of MRSA carriage?
There's no definitive answer but there are some likely causes. Veterinarians have contact with large numbers of pets and horses, and we know these animals can carry MRSA. Even if the percentage of dogs, cats or horses carrying MRSA is very low, when you multiply that by the number of animals a veterinarian touches every week, you can see how contact with an MRSA-carrier is pretty likely. Veterinarians also tend to have close contact with sites where MRSA can be found, such as the nose. This makes the chance of having contact with the bacterium itself more likely. An additional issue the often sub-optimal use of routine infection control and hygiene practices (especially hand hygiene), which may also increase the risk of MRSA transmission. Put all these together, and it makes sense that veterinary personnel are at increased risk.
Why do carriage rates tend to be higher in equine veterinarians?
It could be because MRSA is more common in horses than small companion animals. Another plausible explanation is the fact that the horse's nose (the most likely site for MRSA to be living) is commonly touched during examination and restraint, and horses have pretty big noses to start. Additionally, good hygiene can take more effort on some farms, as sinks and often even hand sanitizer are not as readily available as they are in a clinic.
More information about MRSA in companion animals can be found on the Worms & Germs Resources page. More information about MRSA in horses can be found on our sister site, on the equIDblog Resources page.
This Worms & Germs blog entry was originally posted on equIDblog on 19-Apr-11.
Pediatrician says kids shouldn't have rats
A Montreal pediatrics resident has expressed concern about rat bite fever in kids. Dr. Karine Khatchadourian described three cases of this bacterial infection in a paper called "The rise of the rats: a growing paediatric issue," published last year in the journal Paediatrics and Child Health. The article didn't really present any evidence that this is a "growing" issue, but it is a disease of concern.
Rat bite fever is a bacterial infection most commonly (but not exclusively) associated with bites from rats. Healthy rats often carry the bacterium that causes the disease (Streptobacillis moniliformis in North America), and infection can occur when the bacterium is inoculated into the body by a bite, or when it's spread to mucous membranes like the mouth through direct mouth-mouth contact with pet rats (yes, some people kiss their rats).
The disease can be serious, and even fatal, if not properly diagnosed and treated. It's also a classic example of why physicians need to ask their patients about pet contact and why people need to take bites from pets seriously. Knowing that a rat is in the house, and particularly if a bite has occurred, is a key factor in helping make the diagnosis. If the physician doesn't ask the question, this critical piece of information may be missed, along with the diagnosis.
Being concerned about rat bite fever (and other zoonoses) is good, and ways to educate pet owners and physicians about such diseases are needed. However, extrapolating "rat bite fever is bad" to "rats are bad" is a stretch. The statement in the paper "Should we, as health care professionals, advocate to have rats banned from being sold in pets stores?" is over the top.
Every animal carries many microorganisms that can cause disease in people, given the right circumstances. Similarly, every person you meet is carrying something infectious. The key things to consider are:
- What is the likelihood of infection?
- How severe is the disease that may occur?
- What can be done to reduce the risk of infection?
- What is the cost-benefit, i.e. how do the potential risks compare to the potential benefits?
How can the risks be reduced?
- Rat owners need to be aware of the disease.
- Good handling practices are needed to reduce the risk of bites.
- Any bites that occur should be promptly cleaned and a physician contacted if there are concerns.
- Contact of rat saliva with broken skin or mucous membranes (e.g. kissing the rat) should be avoided.
- Physicians need to know whether their patients own pets, including rats, and know what diseases may be associated with those types of animals.
With this type of approach, the risk of infection can be reduced and the ability to properly and promptly diagnose the disease, in the odd case that it occurs, can be maximized.
I don't want to downplay rat bite fever. It certainly can cause illness, particularly in children under the age of 12. A recent paper reported a fatal case in a 14-month-old boy, however in that case the infection was associated with ferrets, not a rat.
Parents of small children need to think about the risks of zoonotic diseases, as well as injuries (e.g. bites) when deciding whether to get a pet, and what type of pet to get. If people like rats, take care of them properly and communicate well with their physician (and if their physician is aware of the issues), then the risks of serious disease are quite low.
Dr. Khatchadourian suggests that parents "should stick to cats and dogs, and steer clear of rats." However, that's no assurance that a zoonotic infection will not occur. There's no evidence indicating the risk of disease is less with those species. It doesn't even eliminate the risk of rat bite fever, since Streptobacillus moniliformis can be found in the mouths of dogs too.
Rather than banning rats from pet stores, we should focus on educating pet owners, veterinarians and physicians about zoonotic diseases.
Human rabies, Michigan, 2009
The latest edition of CDC's Morbidity and Mortality Weekly Reports describes a case of rabies in a Michigan man from 2009. While human rabies in most developed countries is very rare, this is yet another reminder of the ever-present risk of rabies exposure in many regions, and the ongoing need to be proactive to avoid this almost invariably fatal - but almost completely preventable - disease.
In the 2009 Michigan case, the man woke one day with a bat on his arm. Bats are classic rabies vectors, and you have to assume that any bat has rabies until proven otherwise. If you can't be sure that you weren't bitten or scratched (something that may be easier said than done, because bats bites can be very tiny), then you have to consider yourself exposed if you've had contact with a bat and the bat wasn't tested and rabies-negative.
Unfortunately, the Michigan man did not seek medical attention, and nine months later he started to develop signs of rabies. It started off with pain and progressive numbness in his left hand and arm, and pain in his neck and back. He developed weakness in his left hand and soon could not grip anything or raise it more than a few inches.
While he was being evaluated in hospital, he developed breathing difficulties and had to be placed on a ventilator. Various diseases were considered and numerous tests were run. After a little initial improvement, he began to deteriorate, with more profound neurological signs.
Five days after he was admitted to hospital, his wife was asked about possible animal bites, but she didn't know of any. A couple of days later, a relative recounted being told about the bat encounter, but there was little that could be done at that point, and the man died three days later. Rabies was eventually diagnosed.
Because of the potential risk of exposure, 11 family members that may have had contact with the man's saliva received post-exposure treatment.
Sadly, you can almost guarantee that rabies could have been prevented if he had reported the bite and received post-exposure treatment (even months later). Rabies education is critical so that people know the risks of exposure and know to get medical advice after any encounter with a wild animal.
Elephant TB: Ringling Brothers vs PETA
People for the Ethical Treatment of Animals (PETA) is battling Ringling Brothers and Barnum and Bailey Circus because of Karen, an elephant with tuberculosis (TB). The animal rights group is claiming that the elephant poses a public health risk because she has TB, and it appears that their appeal had initially convinced Baltimore city council to bar the animal from performing.
However, while Karen does have TB (along with about 12% of all Asian elephants in captivity in North America) she does not have active disease. This means she is not known to be shedding Mycobacterium tuberculosis, the cause of TB. Standard protocols are for captive elephants to be tested each year for TB shedding through culture of trunk washes, whereby the elephant essentially sucks some fluid into its trunk and blows it back into a collection bag. If TB is grown from that sample, the elephant has active disease and needs to be quarantined. Since Karen does not have active disease (and because close and prolonged contact are needed for transmission of TB), she is not believed to pose any public health risk.
Recently, I wrote about an outbreak of TB in people that worked with infected elephants at a sanctuary. That was a totally different situation from this one. The major differences, particularly with regard to the risk of transmission, are perhaps best explained by one of the authors of that report, Dr. William Schaffner, who said "If you're at a circus, you're at a great distance from the elephants. You do not have genuinely prolonged contact with them. You're there for 2 hours of the show. That sort of exposure should not put people at risk. I would let my grandchildren attend."
The dog in your bed probably won't kill you
A month or two ago, there was a lot of press about the risks of pets sleeping in beds. It was in response to an article in the journal Emerging Infectious Diseases that didn't put forth any new information, but summarized a few diseases that could potentially be transmitted by pets. Unfortunately, the relative risk of those diseases wasn't really explored, and some media reports latched onto diseases like the plague, transmission of which can occur between pets and humans but the likelihood of this in most areas is essentially nil.
Anyway, an article at Scienceline.org has taken a more balanced approach towards the subject. One sentence perhaps say it best: "Many of those scare headlines, however, missed the main point of Chomel’s work: For most people, the risks are minimal, and there are easy ways to go about preventing pet-to-owner disease sharing."
I won't go into details here, since you can read the article yourself, but a key component is that pet ownership is never no-risk, but is usually low-risk. Basic hygiene practices and common sense can reduce the risks further. The cost-benefit needs to be considered, and while we can never completely eliminate the "cost" aspect, the benefits of pet ownership certainly outweigh the costs in the vast majority of households.
Pet water frog warning
The US CDC is investigating a large, long-lasting and widely dispersed outbreak of salmonellosis that has been linked to contact with pet frogs, such as African dwarf frogs (see image). Between April 1, 2009 and April 5, 2011, 217 infections were identified in people in 41 states. A strain of Salmonella Typhimurium has been implicated.
Of the 217 infected people, 34% were hospitalized, which is quite a large proportion, but is probably due (at least in part) to the fact that stool samples aren't often cultured from people with milder disease (who don't go to the hospital). If you have severe diarrhea and are in hospital, you're more likely to be tested. As with most outbreaks, the 217 diagnosed cases presumably represent only the tip of the iceberg.
Of the people who got sick, 64% reported contact with frogs in the week before their illness began. Of these, 84% had contact with African dwarf frogs. This type of widespread outbreak with a single strain and a fairly clear link to a specific type of animal raises questions about whether there's a major breeder or supplier that is the source of the problem. Not surprisingly, the investigation identified a single water frog breeder in California as the source of frogs associated with this outbreak. Salmonella was identified from environmental samples at the breeder's facility. Testing is still apparently underway to confirm whether it's the outbreak strain, but it's pretty likely.
As with any other reptiles or amphibians, there are standard recommendations to avoid infections from aquatic (water) frogs:
- They should not be in households with high-risk people: children less than five years of age, the elderly, pregnant women and immunocompromised individuals.
- Care should be taken to prevent contamination of the house from aquarium/terrarium water.
- To avoid contamination, aquarium water should not be dumped down kitchen or bathroom sinks.
- Hands should be washed thoroughly after contact with aquarium water or the frogs themselves.
Photo: An African dwarf frog (Hymenochirus boettgeri) (photo credit: James Gathany).
Flesh eating disease from a turtle?
A UK hairdresser is recovering from necrotizing fasciitis (flesh-eating disease) that was linked to his pet turtle. The problem started when he cut his finger while cleaning out the turtle's tank. An infection developed, which isn't too surprising since a turtle terrarium is full of a variety of bacteria. However, instead of a mild, local infection, he developed an aggressive infection that started to spread up his arm. "His finger turned black and his arm became swollen and red." Amputation was discussed, which is not infrequently necessary in cases of such severe infection.
It doesn't sound like there were any cultures taken from the wound at the start, but after the infection didn't respond to the initial course of antibiotics, the man ended up in hospital in IV antibiotics. The infection progressed from his finger to his bloodstream and a bacterium, a Group G Streptococcus, was isolated from his blood.
Here's where more details would be useful. The news article simply says "...and the terrapin, called Cosmo, was identified as the culprit."
It doesn't say how Cosmo was implicated. To make a link, they'd have to find the same bacterium in the turtle's tank. Ideally, beyond just isolating the bug, they'd show that it was the same strain. It's possible this was done, but rarely do people go to that extent, so it's possible that the link was just presumptively made because the initial injury occurred in the tank. The problem with that is Group G strep can also be found in healthy people (10-25% in some studies). Therefore, while he set the scene for the infection in the tank, by breaking his skin, he could have become infected from bacteria already on or in his body. Additionally, other animal sources are possible, such as dogs (since one type of Group G strep is Streptococcus canis). If he cut his finger, then had contact with another animal, it could have been the source.
Most of the attention paid to turtles and infectious disease revolves around Salmonella, and that risk is real. However, turtles, like any other animal (or person), also carry a variety of other potentially harmful bacteria. These usually don't cause problems, but in certain situations, the risk of disease is higher. The skin is a wonderful barrier to infection, and any time it gets broken, there is a risk of disease.
In general, we recommend a few things when cleaning out an aquarium of any kind (be it for fish, reptiles or amphibians):
- Avoid having sharp objects in the aquarium. If any sharp or rough objects are present, take care to avoid contact with them while cleaning.
- Wear gloves, particularly if you have open wounds or if there are sharp/rough surfaces in the aquarium.
- Wash your hands thoroughly after having contact with aquarium water or contents. If you were wearing gloves, wash your hands after glove removal.
- Avoid contaminating other areas, especially kitchen sinks or counters, with aquarium water.
- If you cut yourself while cleaning out an aquarium, wash out the wound thoroughly as soon as possible.
- If you have a compromised immune system, try to avoid any contact with aquarium water or contents by getting someone else to clean out the tank.
Vets sued over MRSA
When I give presentations to veterinarians about infection control, I often talk about legal liability as one reason they need a good infection control program. I talk about the potential bad scenarios, such as someone getting an MRSA infection from an animal and then turning around and suing the vet. I usually say something like "I don't think it's happened yet, but you never want to be an index case". I may need to change that line now that a Texas veterinary clinic is being sued over a person's MRSA infection.
However, the lawsuit, filed April 4, isn't from an owner. Rather, it's from a veterinary assistant. The woman is suing the clinic, claiming they were negligent because they didn't warn her that she would be caring for an MRSA-infected animal. She claims that she contracted the infection on the job and that it "has physically impaired her for the rest of her life."
I know nothing about this case beyond what's in the link above, however it raises a few important issues and questions.
What does this mean for the veterinary clinic?
- I've been saying for years that the bar is being raised and clinics need a good infection control program, including training and education, to reduce infections of both pets and people.
- As awareness of zoonotic diseases and veterinary infection control increases, the potential for lawsuits may similarly increase.
What level of warning is required for veterinary employees?
- This varies with the type of person.
- A veterinarian shouldn't need to be informed that they will work with animals carrying zoonotic pathogens. If they didn't pick that up in vet school, they've got some other major issues.
- A veterinary technician should have a similar understanding of the risks and measures that should be undertaken to reduce those risks.
- Lay personnel are a different story. You can't assume a lay employee has any knowledge whatsoever about infectious diseases, zoonotic diseases or infection control.
- If there are minors in the clinic (e.g. co-op students, volunteers), you need to go even farther, and there should be written notification of parents of the risks, and measures that are taken to reduce the risks.
- In general, the less the veterinary education, the greater the need for clear and documented education about disease avoidance.
How do you prove this person acquired MRSA on the job?
- That's tough. Perhaps there was a clear link with a case. Even stronger would be showing that the human and animal MRSA strains were the same, but that's unlikely to have been done. Just because MRSA can be found in animals doesn't mean that MRSA infections all come from animals. Humans are thought to be the source of the vast majority of MRSA infections and pets, and while pets can potentially spread it back to people, this is ultimately a human disease. People pick up MRSA all the time in the general population, although the percentage of people who carry it at any given time is still low.
- MRSA carriage rates have been shown to be higher in veterinarians than in the general public in a few different studies. I think it's clear that MRSA exposure is a risk of veterinary practice. However, proving that an individual infection came from a pet in a clinic is still difficult.
How can vet clinics reduce the risk of MRSA (and other) infections, as well as lawsuits?
- Have an infection control program in place.
- Make sure infection control practices and policies are written down.
- Make sure all employees are appropriately trained and document the training.
- Make sure people follow all of the required protocols.
Infection control isn't rocket science. At its heart, it's the application of some very basic practices. Infection control hasn't had a high profile in companion animal veterinary medicine in the past, but things are changing (albeit slowly). Available resources can help veterinary clinics implement a decent infection control program with minimal effort. A good resource is the document Infection Prevention and Control Best Practices for Small Animal Veterinary Clinics, which is available (free) for download on the Worms & Germs Resources page.
Resistant heartworm, cause for concern?
Heartworm is a parasitic infection (primarily of dogs) caused by Dirofilaria immitis, which is spread by mosquitoes. In areas where the parasite is present, the standard recommendation is for preventive treatment of dogs during the mosquito season, and annual testing to make sure they don't have the disease. Numerous types of heartworm medication are available, and there's been considerable debate about the potential for emergence of heartworm resistance.
In some areas, there have been increasing numbers of reports of apparent failures of heartworm preventive medication. These cases can sometimes be explained by factors such as poor compliance (e.g. the owner forgot to give the medication or did not give it properly), unnoticed vomiting/regurgitation of oral medication, or encountering an infected mosquito outside of the normal transmission period (i.e. in areas where it's a seasonal issue and medication is therefore not given year round). However, not all cases have clear explanations, and truly resistant heartworms have been identified.
Most of the concern has been focused around the inadvertent discovery of the "MP3" strain. For companies to be able to state that their product is effective against heartworm, they have to prove it in experimental studies using recent strains of the parasite. When the MP3 strain was used to test a potential new drug, they found out that it was actually resistant to the standard treatment.
A study in Veterinary Parasitology (Blagburn et al 2011) describes further testing that was done on this strain. The authors showed that only one of the four medications tested (imidacloprid/moxidectin) provided 100% protection in experimentally infected dogs with a single treatment.
At this point, there's limited information about clinical cases of resistant heartworm, and most of the anecdotal reports come from the central US. The MP3 strain itself was found in a dog from northeast Georgia in 2006. A big question is whether this is:
- a small, focal, regional issue
- a regional issue that's going to expand
- a wider but unidentified problem in many areas
There's no way to know for sure without surveillance, but it is cause for concern.
What does resistant heartworm mean to the average dog owner?
I think it means the days of being able to justify not testing dogs annually, even if it's certain that all heartworm medication has been given religiously since the last test, are over. Skipping annual testing was always a bit of a tenuous argument anyway, because of the potential for a dog to vomit or regurgitate oral medication, or to encounter an infected mosquito while not on medication, and it's even weaker now that resistance might be an issue.
Why is heartworm testing more important now?
It's important for two reasons. One is to make sure that heartworm is promptly diagnosed if it is present. This allows for earlier treatment before the parasites cause more damage. The other reason is to get information about whether resistant strains might be emerging in an area, which is helpful for the broader dog population. We have little information about the distribution of resistance, but if veterinarians start seeing heartworm cases in animals that have been properly treated with preventative medication, it's an indication that resistance might be developing in the area.
At this point, people shouldn't get too concerned about resistant heartworm, but we need to pay attention to the issue in case it increases or is in fact more widespread than we realize. Ongoing surveillance in different areas is needed to determine the scope of the problem, in terms of both the prevalence of resistant parasites and the impact on animal health. We need to be careful not to overreact, but at the same time we need to figure out what's going on. Pet owners need to have conversations with their veterinarians about the need for heartworm prevention in their pet, and the optimal approach to testing and prevention.
Leptospirosis vaccination in dogs
Leptospirosis is disease caused by several types of Leptospira bacteria. It's often called a re-emerging disease in dogs, because the incidence has been increasing over the past couple of decades in many areas. While the overall number of infections is limited, when it does occur lepto can cause serious disease, including kidney failure. It's also a zoonotic disease: lepto can be transmitted from sick dogs to the people who handle them. Infected animals shed the bacteria in their urine, and when the urine comes in contact with broken skin or mucous membranes (mouth, eyes, nose), transmission of infection can result.
Canine vaccines against lepto are available. In the past, lepto vaccination hasn't been all that popular because the vaccines tended to have a higher rate of side effects compared to most vaccines, and the types of lepto that the vaccines targeted were often not the most important types causing disease. This has changed more recently as newer vaccines have become available. These vaccines seem to have low rates of adverse effects and provide protection against the four types of lepto that tend to be the most important. These vaccines have also been shown in research studies to protect against disease and to reduce shedding of the lepto bacterium in urine (thereby decreasing the risk of transmission to other animals or people).
Lepto is not among the "core" vaccines that groups say all dogs should have. That's because the risk of lepto exposure varies greatly between regions and dogs. The key thing to consider when deciding whether or not to vaccinate is what each individual dog's risk of exposure is. The main reservoir of lepto is wildlife such as rats and raccoons. If infected wildlife are in the area, they may be peeing out lepto and contaminating the environment, particularly any type of standing water (lepto loves to live in warm, wet environments).
In the past, much of the concern has been focused on dogs that go out and swim in streams or ponds. The "poster child" for lepto has typically been the Labrador Retriever that goes out gallivanting in the bush, but that may be changing too. When you consider where wildlife such as rats and raccoons live and how high the wildlife infection rates can be, remember that these pests can be present in urban areas in incredibly large numbers. Living in a city does not make a dog safe from exposure to lepto. In fact some urban areas, with large numbers of wildlife crowded into high-traffic areas like parks, are probably higher risk than rural areas.
Deciding on vaccinating requires an understanding of how common lepto is in the area and whether there is a risk of exposure for the individual animal. This is a disease for which a veterinarian's understanding of disease patterns in an area (including any specific areas that be particularly high risk) and the types of possible exposure of the dog play a big role. There's no "one size fits all" recommendation for lepto vaccination.
Treat the pet, not the lab result
The title of this post describes a very basic concept, but one that is sometimes forgotten or hard to follow. The key point is that the goal of treating a sick pet is to make the pet well. Getting well and getting "normal" laboratory data aren't necessarily the same thing.
A question that comes up a lot with MRSA, MRSP and other bacterial infections is "should my pet be tested after treatment to see if the bug is gone?" The answer is usually "no."
One reason to not re-test is just what I said above. The goal of treatment is to make the patient healthy. That doesn't necessarily mean that MRSA, MRSP or whatever bug is causing the problem needs to disappear. In fact, the bacterium that causes a given infection often remains (in smaller numbers) in or on the body somewhere. If it's a skin infection, the bacterium may still be present on the skin where the infection was. However, if the body is handling it well, then it's not necessarily a problem. We have to remember that every animal (and person) has multiple types of bacteria in or on it that can cause disease given the right circumstances. We're never going to eliminate them all. In fact, trying to get rid of all potentially bad bacteria would probably result in bigger problems.
Another concept that I emphasize a lot is only do a test if you have a plan for using the result. If you don't have a clear reason to do the test, and if the result won't impact your decisions or provide information you need in the future, then why do it? While a negative culture might be nice to see, it's not necessarily a guarantee that a particular bug is gone. Furthermore, a positive culture doesn't lead to actions that are any different from those that would be taken if the culture is negative in most situations because, as mentioned above, we're looking for clinical cure (getting better) rather than microbiological cure (getting rid of the bug). Rarely would we extend treatment or do anything different in response to a positive culture in a healthy animal.
The bottom line is post-treatment cultures are rarely needed. There may be some circumstances where testing after treatment is useful and where the results would lead to a defined plan of action, but these are few and far between.
Image: A Mueller-Hinton agar culture plate being used to test the antibiotic susceptibility of a bacterial isolate according to the Kirby-Bauer method. (source: CDC Public Health Image Library #10785)
