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.

这项研究报告了心丝虫（Dirofilaria immitis）由位于加拿大安大略省低流行区的动物收容所测试的狗中的流行率。从2015年至2018年，测试了4567条独特的狗。worm虫的患病率为3.9％，在大多伦多地区投降的狗（均为原产于美国的狗）的患病率为0.3％（2/662）；从大多伦多地区以外地区迁出的狗为6.6％（130 / 1,981）；在收容所的公共兽医服务诊所接受测试的狗为0％（0 / 1,668），在安大略省东南部的第一民族社区的外展诊所接受测试的拥有狗为18.4％（47/256）。超过一半（54.7％）的丝虫阳性狗来自加拿大。来自美国的大多数丝虫阳性狗（72/80； 90％）是从德克萨斯州和乔治亚州转移来的。

安大略省有93例丝虫阳性狗，曼尼托巴省有4头，魁北克省有1头。大多数（83 / 98，84.7％）来自四个原住民社区。来自安大略省西南部一个原住民社区的流浪狗患病率是36.5％（31/85）。

对于140个至少具有一项阳性测试结果的庇护犬，庇护所和参考实验室抗原测试之间的一致性为91％，而抗原测试与微丝蛋白测试之间的一致性较差（约50％）。病例的历史检测结果和移位后抗原检测结果不一致，分别为28.2％（临时医疗点检测）和36.2％（参考实验室检测）。这最常见的原因是历史记录为负数，搬迁后为正数。来自第一民族社区的77.1％抗原阳性狗的微丝过滤试验呈阳性，而来自其他来源的狗的微丝过滤试验为37.1％。来自第一民族社区的狗的微丝计数显着更高。

这项研究证明了加拿大境内心丝虫的地方性和高流行性病灶，据推测与获得兽医护理的机会有限有关。结果支持重新安置后重新检测先前阴性动物的建议。