Prevalence of heartworm in relocated, local and outreach clinic dogs: A Canadian sheltering perspective

This study reports heartworm (Dirofilaria immitis) prevalence in dogs tested by an animal shelter located in a low-prevalence region of Ontario, Canada. From 2015-2018, 4567 unique dogs were tested. The prevalence of heartworm was 3.9 %, with sub-prevalence of 0.3 % (2/662) for dogs surrendered within the Greater Toronto Area (both dogs were originally imported from the US); 6.6 % (130/1,981) for dogs relocated from beyond the Greater Toronto Area; 0% (0/1,668) for dogs tested at the shelter's public veterinary services clinic and 18.4 % (47/256) for owned dogs tested at outreach clinics at a First Nations community in south-eastern Ontario. More than half (54.7 %) of the heartworm-positive dogs originated from Canada. Most heartworm-positive dogs from the US (72/80; 90 %) were transferred from Texas and Georgia. Ninety-three heartworm-positive Canadian dogs were from Ontario, 4 from Manitoba and 1 from Quebec. Most (83/98, 84.7 %) were from four First Nations communities. The prevalence in homeless dogs from one First Nations community in south-western Ontario was 36.5 % (31/85). For 140 shelter dogs with at least one positive test result, there was 91 % concordance between shelter and reference laboratory antigen tests and poor agreement between antigen tests and microfilarial tests (approximately 50 %). Results of historical tests and post-relocation antigen tests were discordant in 28.2 % (shelter point-of-care tests) and 36.2 % (reference laboratory tests) of cases. This was most commonly due to negative historical results followed by positive results after relocation. Microfilarial filtration tests were positive for 77.1 % antigen-positive dogs from First Nations communities, compared with 37.1 % of dogs from other sources. Microfilarial counts were significantly higher for dogs from First Nations communities. This study demonstrated endemic and hyperendemic foci of heartworm within Canada, which were presumed to be associated with limited access to veterinary care. The results support recommendations to retest previously negative animals after relocation.