Peer-reviewed veterinary case report
Testing heartworm microfilaria movement to detect drug resistance
By Maclean, Mary J et al.·Published in Parasites & vectors·2017·Department of Infectious Diseases, United States·View original on PubMed →
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Original publication title: Does evaluation of in vitro microfilarial motility reflect the resistance status of Dirofilaria immitis isolates to macrocyclic lactones?
- Species:
- dog
Plain-English summary
Two dogs with suspected heartworm resistance were treated with Heartgard Plus (which contains ivermectin) every 30 days, but both developed active heartworm infections despite the preventive treatment. Researchers tested the microfilariae (immature heartworms) from these dogs and others known to be resistant to see if their movement could indicate resistance to the drug. Unfortunately, they found that measuring how the microfilariae moved in response to ivermectin did not reliably show which heartworm populations were resistant. This study highlights the presence of drug-resistant heartworms in the southern U.S. and suggests that current methods for detecting resistance may not be effective.
People also search for: dog heartworm treatment failure · heartworm prevention resistant dogs · signs of heartworm in dogs
Abstract
BACKGROUND: Several reports have confirmed that macrocyclic lactone-resistant isolates of Dirofilaria immitis are circulating in the United States; however, the prevalence and potential impact of drug resistance is unknown. We wished to assess computer-aided measurements of motility as a method for rapidly assessing the resistance status of parasite isolates. METHODS: Blood containing microfilariae (MF) from two clinical cases with a high suspicion of resistance was fed to mosquitoes and the resultant L3 injected into dogs that were then treated with six doses of Heartgard® Plus (ivermectin + pyrantel; Merial Limited) at 30-day intervals. In both cases patent heartworm infections resulted despite the preventive treatment. Microfilariae isolated from these dogs and other isolates of known resistance status were exposed to varying concentrations of ivermectin in vitro and their motility assessed 24 h later using computer-processed high-definition video imaging. RESULTS: We produced two isolates, Yazoo-2013 and Metairie-2014, which established patent infections despite Heartgard® Plus treatments. Measurements of the motility of MF of these and other isolates (Missouri, MP3 and JYD-27) following exposure to varying concentrations of ivermectin did not distinguish between susceptible and resistant heartworm populations. There was some evidence that the method of MF isolation had an influence on the motility and drug susceptibility of the MF. CONCLUSIONS: We confirmed that drug-resistant heartworms are circulating in the southern United States, but that motility measurements in the presence of ivermectin are not a reliable method for their detection. This implies that the drug does not kill the microfilariae via paralysis.
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Search related cases →Original publication on PubMed: https://pubmed.ncbi.nlm.nih.gov/29143656/