A diagnostic algorithm for evaluating cases of potential macrocyclic lactone–resistant heartworm

Andrew R. Moorhead, Christopher C. Evans, Ray M. Kaplan

The emergence of macrocyclic lactone resistance in canine heartworm poses a substantial threat to what is currently the only effective, FDA-approved available method of prevention. Further study of the biotypes is necessary to understand the mechanism of resistance and evaluate novel prevention options. Identifying cases of drug-resistant infection remains problematic, however, especially when poor compliance and insufficient testing are concerns. Furthermore, a definitive demonstration of resistance requires experimental infection and treatment, which is prohibitively costly. With the aim of identifying likely cases of macrocyclic lactone-resistant heartworm and preventing their continued spread, we describe an algorithm for determining the likelihood of drug resistance and appropriate treatment strategies for each case. This algorithm relies on the microfilarial suppression test (MFST), which has been used previously as an efficient and discrete measure of suspected resistance. By standardizing this method in a format that is readily available to practitioners, it could become possible to preliminarily survey the emergence and spread of resistance. Heartworm isolates identified through this method can be used in research to better understand macrocyclic lactone resistance so prevention strategies can be adapted.