Natural disasters and pet diseases
After the devastation caused by Hurricane Katrina, there were numerous efforts to rescue animals left homeless or ownerless. As part of this, some animals were shipped far away, including large numbers to the Northeastern US and Canada. It was a time-consuming and expensive endeavour by very well-meaning people, but was accompanied by much controversy. Beyond the concerns about resources spent importing animals into areas where there is no shortage of strays and limited resources to care for them, there were infectious disease concerns. These concerns were real because of the potentially high incidence of disease, high rate of carriage of certain infectious agents and movement of dogs to regions where these diseases are rare or absent. Anytime you get population migration (be it human or animal) in response to a natural disaster, you have the risk of those migrants spreading diseases.
A study published earlier this year in the Journal of the American Veterinary Medical Association (Levy et al 2011) characterized some of these concerns. The researchers tested blood samples from 414 dogs and 56 cats that were transferred from the Gulf Coast region within four months of Hurricane Katrina. They tested the samples for a variety of infectious agents.
Among the highlights in dogs:
- Overall, 74% of dogs had "evidence of previous or current vector-borne infections." That's a pretty high number. Current versus previous is important, since the animals are only an infectious disease risk if currently infected. At least some of the testing was aimed at detecting infectious organisms (indicative of active infection), not just antibodies (which may indicate active or previous infection), so there was clear evidence that many dogs had active and potentially transmissible infections.
- Bartonella, Borrelia (Lyme disease), Ehrlichia and Babesia spp DNA were found rarely, in less than 2% of dogs. These organisms are spread by certain types of insects, and a concern with moving infected dogs is that it might allow for introduction of the disease into a new population if there are insects capable of transmitting these infectious agents in the new area. Fortunately, these weren't common.
- Canine influenza antibodies were found in less than 2% of dogs. This is a more important finding since canine flu is quite transmissible. While the virus is present in various parts of the US, it's distribution is pretty patchy. A dog infected with canine flu that comes into contact with other dogs in a new area could easily be the source of a local or regional outbreak.
- Dirofilaria immitis (heartworm) antigen was detected in 49% of dogs. This is a major concern and was one of the big causes of controversy in some regions. Dogs that are infected with heartworm and untreated are sources of infection. If bitten by a mosquito, heartworm can be transmitted to the mosquito, and then to other dogs (and less commonly cats, and very rarely humans). Heartworm is quite uncommon in many regions, and there were concerns that the transfer of these dogs could result in local increases in disease. Recommendations were developed to reduce the risk of heartworm dissemination, but given the high rate reported here, it's likely they were not widely followed.
- 56% of dogs had antibodies against West Nile virus. This is an interesting finding but not really relevant from a disease transmission standpoint. It likely reflects heavy mosquito exposure. West Nile virus is pretty innocuous in dogs and cats, and infected dogs and cats cannot pass on the virus to mosquiotoes or other animals.
- Dogs infected with heartworm were more likely to be infected with West Nile virus compared to those not infected with heartworm - presumably an indication of mosquito exposure.
The concerns are summed up nicely in the paper's conclusions: "Cats and dogs rescued from the disaster region had evidence of multiple infectious diseases. The dispersal of potentially infectious animals to other regions of North America where some infections were not typically found could have contributed to new geographic ranges for these organisms or to underdiagnosis in affected animals because of a low index of suspicion in regions with low disease prevalence."
I'm not saying don't rescue dogs and cats during disasters. Personally, I have to question the wisdom of putting the time, effort and resources into shipping animals around the continent when pretty much every jurisdiction already has their fill of animals in need of care, but people have different opinions. What common sense and this study should tell us is that we need to think about the infectious disease implications of mass animal movement, particularly marginalized animals with questionable or unknown disease status. Any large scale movement of animals needs to be accompanied by careful assessment of possible risks, and measures to make sure animals are properly tested and treated so that they don't pose an undue risk to the regions where they end up.
Image: Hurricane Katrina on August 28, 2005 (NASA)(source: http://en.wikipedia.org)
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.
Heartworm in people
Heartworm is an important problem in dogs. It's a parasitic disease caused by Dirofilaria immitis and is spread by mosquitoes. It can cause serious, even fatal disease, and routine testing and preventive medication is an important thing for dogs in areas where D. immitis is present. Dogs (wild and domestic) are the natural host for this parasite, but other species can be accidentally infected, including people and cats. People become infected by being bitten by a mosquito that is carrying the parasite, having acquired it from an infected dog. Human infections seem to be quite uncommon and, interestingly, while this is a serious problem in dogs, it tends to be rather innocuous in people. In fact, the biggest problem with heartworm infection in people is the fact that it can be confused with other, more serious problems, leading to invasive testing.
After infecting someone, D. immitis works its way to the blood vessels in the lungs. This can result in a small area of inflamed tissue in the area. If a chest x-ray is taken, a "coin lesion" (a small, usually 1-3 cm spot) is often present. The parasite infection usually doesn't cause any problems in people, but lung cancer and tuberculosis can look the same on x-rays. Usually, open-chest surgery ends up being performed to get a biopsy of the area because of the concerns about cancer. In heartworm cases,the biopsy identifies the problem as D. immitis, which is much better than cancer, but the risks associated with having undergone such an invasive procedure are much greater than that of the parasitic infection itself.
Typically, treatment is not recommended in people because the infection rarely causes problems and people are "dead end" hosts, meaning they cannot pass on the infection. (Unlike in dogs, infected people don't have the parasite microfilaria in their blood, which is how the infection is passed on to mosquitoes and other animals).
Heartworm is a rare and rather innocuous problem in humans - it's nothing to lose sleep about.
Image: A diagram of a very severe case of heartworm in a dog, in which there are so many worms in the pulmonary arteries that there is "back-up" of the parasites into the right side of the heart, which is how the parasite got its common name.
Alternative heartworm treatments
Here's a recent question:
"We have a 'new' boxer age 2. The breeder believes in the raw food diet, and not many vaccinations or preventitive treatments. Recently the boxer has been shown to be heartworm positive, and she (the breeder) wants us to take a "holistic" approach to management. Are there any randomized trials to show any benefit to holistic treatment of heartworm?"
The quick answer is NO. There are no “holistic” treatments that have been shown to be effective in randomized trials, nor have any holistic treatments been shown to have any potential effect in in vitro studies. The only proven treatments are “conventional.” I consider it highly unethical to attempt other approaches because: 1) heartworm is a serious disease but one that can often be treated quite successfully and 2) untreated (or inadequately treated) dogs put other dogs at risk because they are sources of infection. Mosquitoes can transmit heartworm from infected dogs to other dogs in the area, and continue the cycle of infection. A serious and transmissible disease is not one for which unproven and likely ineffective treatments should be tried.
Heartworm is a potentially fatal disease that predominantly affects dogs, but can occasionally affect cats as well. It is most commonly caused by the parasite Dirofilaria immitis. It is spread by mosquitoes, which transmit the immature form of the parasite (microfilaria) which are found in the blood of infected animals. Upon being transmitted to a new host by a bite from an infected mosquito, the immature parasites eventually develop into adult worms. These worms lodge in the heart and the nearby blood vessels going to the lungs, and can cause a range of problems (e.g. lethargy, intolerance to exercise). Infection can be fatal - early (and effective) treatment is the key.
