A growing body of anecdotal reports and experimental evidence suggests that heartworm preventives (macrocyclic lactones) may not be completely efficacious in preventing heartworm infection in dogs. Reports of resistance have resulted in confusion about how best to prevent infection in veterinary patients.

The Companion Animal Parasite Council (CAPC) recognize this as an important and emerging problem. At the time of writing, reports of resistance appear to be confined to dogs from the south-central United States, where some animals have become infected while on preventives.

However, it’s clear that the vast majority of heartwormpositive dogs result from failure of compliance rather than product resistance. This means veterinarians should continue to recommend the use of heartworm preventives, where and when applicable, as the best way to prevent infections. It’s also important to continue emphasizing the need for heartworm diagnostic testing at recommended intervals and appropriate times (at least annually).

The CAPC will continue to monitor ongoing research and make updated recommendations on best preventive strategies as additional information becomes available. capcvet.org