Dog importation and changes in heartworm prevalence in Colorado 2013–2017

BACKGROUND: Animal welfare organizations, in order to increase dog adoptions and reduce euthanasia of homeless dogs, are transporting large numbers of dogs from state-to-state. Only approximately 1/3 of animal welfare organizations reportedly test for heartworm infection, treat, or provide heartworm prevention prior to transporting dogs. The reported prevalence of heartworms in dogs in animal shelters or rescued following natural disasters ranges between 14.6–48.8%. This analysis was conducted to evaluate the correlation between dog importation and heartworm prevalence in dogs in Colorado. METHODS: Data were obtained from the Companion Animal Parasite Council (CAPC) interactive heartworm prevalence maps and the Colorado Department of Agriculture Pet Animal Care Facilities Program (PACFA) in order to analyze the relationship between dog importation and the changes in prevalence of heartworm (Dirofilaria immitis) in Colorado from 2013 to 2017. RESULTS: The prevalence of heartworms in Colorado dogs has increased 67.5% between 2013–2017. From 2014 to 2017, more than 114,000 dogs were imported into Colorado by over 130 animal shelters and rescue organizations, which could represent approximately 9.5% of the total estimated 2017 Colorado dog population of nearly 1.2 million dogs. Three large Colorado-based organizations responded to requests for details regarding the originating states from which they received dogs. The majority of these dogs were apparently shipped to Colorado from states with higher heartworm prevalence. New Mexico represented the source of the greatest number of relocated dogs, accounting for just over 30%. Nearly half (49%) of the dogs relocated by these three organizations came from either Texas or Oklahoma. CONCLUSIONS: Animal welfare organizations and veterinarians should increase the testing and prevention of heartworm infections in dogs both prior to, and following, transportation from areas with high heartworm prevalence. Repeated testing is recommended due to the 6-month pre-patent-period associated with D. immitis. Veterinarians and pet owners should increase vigilance with heartworm testing and prevention, even in areas with historically low heartworm prevalence. Movement of dogs from highly endemic areas may increase the risks of local transmission.