Suspicious for heartworm preventive resistance? Put it to the test

At the American Heartworm Society (AHS) Triennial Heartworm Symposium in September 2016, Ray Kaplan, DVM, PhD, DEVPC, DACVM (parasitology), sat down to outline the plan he and Andrew Moorhead, DVM, PhD, both in the Department of Infectious Disease at University of Georgia's College of Veterinary Medicine, derived to help veterinarians diagnose a case of true heartworm resistance to preventives.

In essence, the protocol involves determining the amount of microfilariae in the blood. “If a dog presents with a history of compliance and a proper testing history and it's microfilaremic, right from the start that's going to be suspicious,” says Dr. Kaplan. Here's how to test your suspicion:

Obtain a blood sample, and perform a Knott's test to determine the number of microfilariae present per milliliter of blood.

Treat the dog with a microfilaricidal dose of a macrocyclic lactone.

Test another blood sample one week later to determine the number of microfilariae now present.

“We would expect to see greater than 75% reduction,” says Dr. Kaplan. If it's less than that, it's time to suspect resistance.

Hear all about the background of the test and more from Dr. Kaplan: