The National Center for Veterinary Parasitology (NCVP), the American Heartworm Society (AHS) and the Companion Animal Parasite Council (CAPC) have updated their heartworm testing recommendations for dogs. The specialists in these organizations recommend annual screening with an antigen test as well as a microfilaria test on all dogs. This includes dogs who are on heartworm preventative year round. Here’s why:
Treatment for a dog with heartworm disease involves killing the adult worms that live in the heart and then killing off the immature microfilaria that circulate in the bloodstream. Traditionally, the approved adultacide melarsomine was used first followed by moxidectin or a similar drug to kill the microfilaria. The American Heartworm Association has the protocol outlined on their website. Because of complications with thromboemboli when the adult worms are dying or the fact that some dogs cannot receive the adulticide therapy, some veterinarians turned to a ‘slow-kill’ method. The ‘slow-kill’ involves long term therapy with one of the macrocyclic lactone preventatives such as ivermectin or milbemycin combined with the antibiotic doxycycline that suppresses production of microfilaria.
Unfortunately, there are drawbacks with using the ‘slow-kill’ therapy. First, the infected dogs produce antibodies against the heartworm antigen that interferes with antigen testing. One study showed fifty percent of the dogs tested will have false negative results to antigen testing. Second, these dogs continue to harbor heartworms that mosquitos may transmit to other animals. It is also feared that the slow-kill method has caused heartworms to become resistant to the preventatives.
For further information on heartworm disease and other animal parasites may be found at http://www.capcvet.org/capc-recommendations/canine-heartworm/ and https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
-Little, Susan. Symbiosis in parasitology: Heartworm recommendations updated. VETERINAY MEDICINE, Sept. 2014 Vol 109:9, pp. 290-292.