Vin founder responds:

The query from Dr. Warren Kaplan regarding the safety of heartworm prophylaxis medications in heartworm-positive dogs deserves to be addressed. On the American Heartworm Society Web site (www.heartwormsociety.org/AHS%-20Guidelines-Canine.htm) it is clearly stated that: "The macrocyclic lactones may be administered to heartworm-infected dogs with few or no microfilariae. However, dogs with moderate to high microfilarial levels should be carefully monitored following administration of these drugs, as they are the most effective microfilaricides available."

Of the macrocyclic lactones, milbemycin (and likely moxidectin due to its close structural relation to milbemycin) is the compound with the most chance of causing a reaction in a microfilaria positive dog. The reason being that the dose used as a preventative is highly microfilaricidal.

Whereas for ivermectin the preventive dose is approximately an order of magnitude lower than the recommended microfilaricidal dose. This potential adverse reaction has been demonstrated experimentally and show to be preventable by pre-treatment with steroids (Sasaki Y, Kitagawa H, Ishihara K, Shibata M: Prevention of adverse reactions following milbemycin D administration to microfilaremic dogs infected with Dirofilaria immitis. Nippon Juigaku Zasshi 51[4]:711-5, 1989).