The Companion Animal Parasite Council (CAPC) is an independent council of veterinarians, veterinary parasitologists, and other animal health care professionals established to create guidelines for the optimal control of internal and external parasites that threaten the health of pets and people. Among these guidelines are recommendations pertaining to the prevention and treatment of heartworm disease.
Recently, CAPC changed its guidelines relating to the treatment of existing heartworm infections. Specifically, the council has altered its recommendations to advise against using the “slow-kill” method of heartworm treatment. This alteration comes as a result of definitive evidence presented at the American Association of Veterinary Parasitologists Conference at the end of July indicating the existence of resistance in some heartworm populations in the Mississippi River Valley region to commonly used preventive medications, such as ivermectin, selamectin, milbemycin oxime and moxidectin.
Previously, the “slow-kill” method, which involves the monthly administration of commonly used heartworm preventive medications in an effort to slowly kill off the existing population of heartworms in an infected dog, was offered as an alternative method of heartworm treatment. The preferred treatment involves adulticide therapy (treatment to kill off the heartworm adults) using the medication melarsomine. Melarsomine kills off the heartworms much more quickly than does the “slow-kill” method, which uses macrocyclic lactones and can take up to two years to kill all of the infecting heartworms.
This is the recommendation quoted directly from the CAPC canine heartworm guidelines:
Adulticidal therapy using long-term macrocyclic lactone administration – the “slow kill” method – is not recommended especially in light of resistance. It has been demonstrated that repeated macrocyclic lactone administration over a period of time to infected dogs increases the proportion of circulating microfilariae that possess resistance markers (i.e., application of long-term drug pressure will select for survival of drug-resistant microfilariae).
As many as 20% of dogs infected with heartworms will continue to have circulating microfilariae for at least a year or longer when receiving monthly treatment with macrocyclic lactones.
The “slow-kill” method should be avoided. If it is the only medically acceptable option, microfilariae should be eliminated prior to exposure to preventive doses of macrocyclic lactones.
The concern is, of course, that continued usage of this treatment option will select for more resistant populations of heartworms, eventually making prevention of the disease much more difficult in healthy dogs.
The American Heartworm Society takes the same position, saying “slow-kill methods using continuous monthly administration of prophylactic doses of any macrocyclic lactone are NOT RECOMMENDED.”
This change does not affect the current recommendations relating to heartworm prevention. Parasitologists and veterinarians urge dog owners to continue administering monthly heartworm preventive medications year round. Yearly heartworm testing is also advocated for all dogs.