A paper that will be published in the September edition of Emerging Infectious Diseases (Mead et al 2011) talks about the potential for dogs to act as indicators of Lyme disease activity and risk for people. The use of animals as sentinels for human disease is well established. Sometimes it’s because animals are more readily affected. Sometimes it’s because the disease is easier to diagnose in animals. Sometimes it’s because getting access to samples from animals is easier than from humans.
Because of the distribution of ticks that transmit the causative bacterium (Borrelia burgdorferi) and wildlife that act as the reservoir, the occurrence of Lyme disease is highly variable geographically. Knowing the amount of Lyme disease activity in a specific region is important for understanding the role of this disease in humans, and for implementing preventive measures.
Like people, dogs can get Lyme disease. Dogs are NOT sources of human infection, but since both dogs and people get Lyme disease the same way (from Ixodes ticks), infections in dogs can indicate the potential for infections in people (and vice versa). Since dogs may be more prone to being exposed to ticks, and since ticks are most likely to stay on dogs for the 24 hours or so that is required for ticks to transmit the bacterium, dogs may be more likely to be exposed to Borrelia spp. in endemic regions.
That’s the reasoning in the Mead paper which suggests that dogs, because of their potentially greater chance of exposure and tendency to produce a good immune response after exposure, might be good indicators of human Lyme disease risk. To examine this premise, the authors compared data about B. burgdorferi antibody levels in dogs to human infection data. (Note: These are two different things: In dogs, they looked at antibodies, which indicate exposure but not necessarily disease. In people, they looked at disease. It’s not inappropriate to compare the two, but you have to be aware of what they were comparing).
Overall, they showed (not surprisingly) that there was a relationship between antibody levels in dogs and Lyme disease in people. Some key findings were:
When the percentage of dogs with antibodies against B. burgdorferi was <1%, there were extremely low rates of disease in people in the area.
- This makes sense since it would indicate that there’s little or no transmission occurring in the area. Low levels of B. burgdorferi in dogs or people don’t necessarily indicate that Lyme disease is active in the area, since some cases could have been acquired during travel, and false positive results are possible with current testing.
The risk of disease is generally low to non-existent outside the highly Lyme-disease endemic areas: Northeast, mid-Atlantic and upper Midwest regions of the US.
- These are the areas where ticks capable of spreading the bacterium are common and where the bacterium is resident in wildlife, so that’s not surprising.
Where 5% of more dogs had B. burgdorferi antibodies in their blood, there was always an above-average incidence of Lyme disease in people in the state, with a lesser association at the county level.
- Again, this makes sense. If most dogs are exposed, more people are going to be exposed, and more people will develop disease.
In 15% of counties where dogs had a >5% rate of antibodies, people did not have above average disease rates. However, in half of them, the incidence of disease in people increased to above average in the following 3 years.
- This is quite interesting and perhaps the most important finding of this study. It suggests that monitoring rates in dogs may predict trends in people.
What does this all mean? Well, a lot of these results would be expected based on what we know about Lyme disease. However, the apparent close linkage between human disease rates and dog antibody rates, and particularly the potential that dog rates could predict human rates, is intriguing and could be useful. By routinely monitoring for antibodies in dogs, areas where Lyme disease might be on the rise or might be emerging in people could be identified, leading to more focused educational efforts directed at both the public and healthcare personnel. Getting the dog data (or at least getting good dog data) is perhaps the problem, since testing would need to be done on a subset of the dog population that’s not biased and is of adequate size to say something useful. There are ways to do this, but it takes time and money to do it right. Taking advantage of blood samples collected for heartworm testing is one possible approach, but careful thought needs to go into what could be done and whether it would be better than more intensive surveillance of humans or ticks.