Peer-reviewed veterinary case report
Heartworm drug resistance in Australian dogs studied with genetic
By Power, Rosemonde Isabella & Šlapeta, Jan·Published in International journal for parasitology. Drugs and drug resistance·2022·Sydney School of Veterinary Science, United Kingdom·View original on PubMed →
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Original publication title: Exploration of the sensitivity to macrocyclic lactones in the canine heartworm (Dirofilaria immitis) in Australia using phenotypic and genotypic approaches.
- Species:
- dog
Plain-English summary
A group of dogs in Queensland, Australia, tested positive for heartworm (Dirofilaria immitis) despite their owners believing they had been on effective heartworm prevention for a year. Out of 45 dogs surveyed, 16 still showed signs of infection after treatment with macrocyclic lactone (ML) drugs, which are commonly used to prevent heartworm. While this raised concerns about potential ML-resistance, genetic testing did not confirm any resistant strains in these dogs. The study suggests that more research is needed to determine if heartworm in Australia is becoming resistant to these treatments.
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Abstract
Canine heartworm disease is a potentially deadly cardiopulmonary disease caused by the mosquito-borne filarial nematode Dirofilaria immitis. In Australia, the administration of macrocyclic lactone (ML) drugs has successfully reduced the prevalence of D. immitis infection. However, the recent re-emergence of D. immitis in dogs in Queensland, Australia and the identification of ML-resistant isolates in the USA poses an important question of whether ML-resistance has emerged in this parasite in Australia. The aim of this study was to utilise phenotypic and genotypic approaches to examine the sensitivity to ML drugs in D. immitis in Australia. To do this, we surveyed 45 dogs from Queensland and New South Wales across 3 years (2019-2022) for the presence of D. immitis infection using an antigen test, quantitative Modified Knott's test, and qPCR targeting both D. immitis and the D. immitis symbiont Wolbachia. A phenotype observed by utilising sequential quantification of microfilariae for 23/45 dogs was coupled with genetic testing of filtered microfilariae for SNPs previously associated with ML-resistance in isolates from the USA. Sixteen (16/45) dogs tested positive for D. immitis infection despite reportedly receiving 'rigorous' heartworm prevention for 12 months prior to the study, according to the owners' assessment. The phenotype and genotypic assays in this study did not unequivocally demonstrate the presence of ML-resistant D. immitis in Australia. Although the failure of 16 dogs to reduce microfilaremia by >90% after ML treatment was considered a suspect phenotype of ML-resistance, no genotypic evidence was discovered using the genetic SNP analysis. The traditional quantitative Modified Knott's test can be substituted by qPCR targeting D. immitis or associated Wolbachia endosymbiont DNA for a more rapid measurement of microfilariae levels. More definitive phenotypic evidence of resistance is critically needed before the usefulness of SNPs for the detection of ML-resistance in Australia can be properly assessed.
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Search related cases →Original publication on PubMed: https://pubmed.ncbi.nlm.nih.gov/36417831/