Delayed canine heartworm (Dirofilaria immitis) microfilarial reduction following Advocate™ for dogs (imidacloprid, moxidectin) treatment

Macrocyclic lactone (ML) anthelmintics are currently the only class of drugs available for canine heartworm prevention. Recent reports of Dirofilaria immitis infection occurring in dogs reportedly receiving ‘rigorous’ prevention in Queensland, Australia, coupled with the confirmation of ML-resistant isolates in the USA, has led to speculation about the potential emergence of ML-resistance in Australia. In this study, we describe two cases (Dog 1 and 2) of asymptomatic canine heartworm disease in Townsville, Australia, that were reportedly receiving ‘rigorous’ heartworm prevention according to the owners’ claims. We aimed to deploy currently available tools to assess the phenotypic and genotypic ML-resistance status of these two dogs. For phenotypic testing, we performed an in-vivo 7-day microfilariae suppression test using a dose of spot-on moxidectin (Advocate™ for Dogs, 100 g/L imidacloprid + 25 g/L moxidectin). This formulation is marketed as Advantage Multi® for Dogs in the USA, which claims a D. immitis microfilaricidal effect. For genetic testing, an Illumina amplicon metabarcoding approach was used to target single nucleotide polymorphisms (SNPs) previously associated with ML-resistance in D. immitis from the USA. Dog 1 and Dog 2 demonstrated <10 % and <40 % reductions in circulating microfilariae seven days after moxidectin treatment, respectively. These phenotypes were not corroborated by genetic SNP testing, as both dogs were classified as susceptible across all examined markers. To streamline testing of D. immitis SNPs, we developed a rhAmp™ SNP qPCR approach for rapidly genotyping suspect cases of ML-resistant infections at the two major loci (L15709_A and L30575). These findings illustrate a phenomenon shown in some heartworm cases outside the USA, whereby infected dogs are failing to see marked reductions in microfilaraemia after ML treatment but possess an ML-susceptible genotype.