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: