Evaluation of the drug-resistance genotypes of Dirofilaria immitis infections in Ontario dogs (2015–2016)

For more than 3 decades, macrocyclic lactone (ML) heartworm preventives have been extremely effective at preventing Dirofilaria immitis infections in dogs. Reports of loss of efficacy (LOE) of the MLs in the early 2000s led to in-depth research which identified the presence of resistant strains of D. immitis in the Southeast United States. Detailed genetic analysis of such parasites identified genetic markers for resistance. In order to evaluate the prevalence of these markers in Ontario infections, microfilariae (MF) from 39 Ontario dogs, 22 from a ML-naïve population and 17 from a ML-exposed population, were collected in 2015–2016, analyzed and compared. The ML-naïve population comprised stray dogs from an area near Caledonia, Ontario where heartworm preventives have historically been rarely used. The ML-exposed population comprised client-owned dogs from veterinary practices across Ontario where preventives are commonly used. Overall, MF with resistant markers (two single nucleotide polymorphisms [SNPs]) were found in 3/39 dogs. However, in only one of those infections were both SNPs associated with ML resistance present. There was no significant difference in prevalence of these genetic markers for resistance between the ML-naïve population and the ML-exposed population of dogs (n = 22 genotyped and n = 17 genotyped, respectively). Despite the low prevalence of infections with ML-resistant genotypes, the fact that none of the dogs in this study had traveled outside Ontario suggests that the infections with ML-resistant genotypes were locally acquired.