For this study, the research team wanted to understand the prevalence of exposure to Lyme, Anaplasma spp., Ehrlichia spp., and infection with heartworm in shelter dogs. To do this, they selected shelters in areas known to be endemic for each pathogen of interest. They were able to collect blood from at least 50 dogs per shelter taken in from the local area, over the four-year study period. They ended up with samples from nearly 4,000 dogs in shelters spread across 19 states.

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Alarming Findings

The study's findings are undeniably alarming. Overall, heartworm infection was the most common finding, with more than 1 in 10 dogs testing positive. The geographic range speaks to the growing presence of heartworm outside the notoriously hyperendemic areas: nearly one-third of shelter dogs tested in hyperendemic states like North Carolina and Louisiana were infected with heartworm, but so were about 5% of shelter dogs in Maryland and 10% in Missouri. The prevalence in these dogs was quite a bit higher than what is suggested by Companion Animal Parasite Council (CAPC) maps, where data shows less than 1% of dogs tested positive for heartworm in Maryland over the same period and less than 2% tested positive in Missouri.

Lyme, Anaplasma, and Ehrlichia exposure also had regional variation, but overall, 8% of all dogs were positive for Lyme antibodies and 10% had antibodies to either Anaplasma or Ehrlichia spp. which indicates they were bitten by infected ticks. In addition, almost 5% of the dogs had evidence of exposure to multiple pathogens, particularly for pathogens with shared geographic distribution (heartworm and Ehrlichia spp.) or a shared vector (Lyme and Anaplasma spp.). There were almost as many dogs positive for both Lyme and Ehrlichia, two pathogens that usually don't share the same tick vector and aren't typically considered to have major geographic overlap—these dogs were mostly concentrated in Maryland and Virginia, possibly highlighting the convergence of the geographic spread of Lyme to the south and Ehrlichia northwards.

Unique Risk Factors for Shelter Dogs

Shelter dogs face distinct risk factors compared to their owned counterparts, which may contribute to the high prevalence found in this study. Due to their backgrounds, many dogs arrive at a shelter without having received regular—if any—flea, tick, or heartworm preventives. According to recent research, owner relinquishment accounts for only a small percentage of overall shelter dog populations which means the medical or preventative history of most shelter dogs is unknown.

Additionally, heartworm, Ehrlichia spp., and Lyme can all infect dogs with no apparent signs for long periods of time, so even apparently healthy shelter dogs can be infected. Climate change has brought warming temperatures to regions where ticks and certain vector-borne diseases were previously uncommon. This expansion of the disease vectors' habitat particularly puts dogs living outdoors or without access to preventives at risk for exposures they may not have encountered in the past.

Challenges and Priorities in the Shelter Setting

Dogs often arrive at the shelter with limited or no medical history, making it challenging to assess their risk of exposure to vector-borne diseases. And since dogs infected with heartworm, Ehrlichia spp., or Lyme may remain asymptomatic, screening should not be limited to dogs with clinical signs. Rather, routine screening is necessary to identify these silent carriers. Particularly with current trends in transporting shelter dogs from communities with too many dogs to areas where interested adopters outnumber strays, care must be taken not to accelerate the spread of these vector-borne diseases outside their historically endemic areas.

Strategies for Veterinarians

Shelters often operate on tight budgets that limit the resources available for diagnostic tests, so it's incumbent upon veterinarians working with adopters of shelter dogs to understand the risks for vector-borne disease exposure and screen effectively for pathogens that shelters may not be funded to test for. By staying up-to-date on the prevalence of vector-borne diseases in your local area, and knowing the resources to look up disease prevalence in areas where dogs are transported from, veterinarians can set up routine testing protocols for their practices.

This is particularly important since we now know that the prevalence of exposure, and infection with, vector-borne diseases is higher in shelter dogs in any given region. By knowing the most effective protocols for screening for and treating these diseases, vets can not only prevent or treat disease in their patients but also contribute to veterinary public health by limiting the geographic spread of TBDs in shelter dogs transported around the country for adoption. At the time of adoption, owners should be counseled to start effective preventives for fleas, ticks, and heartworm for all dogs, lest new exposures occur at home. Educating new adopters on the implications of previous exposures to chronic infections like Lyme and heartworm is also crucial. The more informed the community, the more effectively these diseases can be managed.

Because of the increased prevalence of vector-borne disease exposure in shelters compared to in the home, there is a critical need for comprehensive vector-borne disease screening and preventive measures in dogs adopted from shelters. With this information, vets can combat the spread of vectors and the pathogens they carry when dogs are adopted, particularly if they are transported out of their local area for adoption, and contribute to reducing the burden of these diseases on these vulnerable animals—as well as in their humans, in the case of zoonotic diseases like Lyme—and simultaneously provide them with better health in their new homes.