Heartworm disease prevention in dogs

Heartworm prophylaxis is indicated for all dogs living in endemic areas. Because sustained warm, moist conditions are important for heartworm disease transmission, the time of year when infection is possible is limited in most parts of the United States. Transmission is limited to only a few months in the most northern part of the US; year-round transmission is thought possible only in the far southern edge of the continental US. It appears that monthly heartworm preventive therapy is necessary only from June through October or November for dogs in most of the US and from April to November or December for dogs in in the southern one third of the US. Year-round monthly preventive therapy is probably prudent at the southern most edge.

Several macrolide drugs are currently available for preventing heartworm disease; the avermectins (ivermectin, selamectin) and the milbemycins (milbemycin oxime, moxidectin). Diethylcarbamazine (DEC) is also still available as a preventive agent. Preventive therapy can begin at 6 to 8 weeks of age. Before chemoprophylaxis is started for the first time, dogs old enough to have been previously infected should be tested for circulating antigen and (if DEC is to be used) microfilariae. Retesting for circulating antigen should be done periodically; usually every 2 to 3 years is adequate. The avermectins and milbecymins induce neuromuscular paralysis and death in nematode (and arthropod) parasites by interacting with membrane chloride channels. These agents have a wide margin of safety in mammals.

Periodic retesting is an important part of heartworm prophylaxis. After the first year of monthly prophylaxis, a heartworm antigen test should be done to confirm the dog’s negative status. If preventive therapy has been given as scheduled, retest intervals longer than 1 year may be sufficient. When DEC is used as preventive, yearly microfilariae testing is important before DEC is reinstituted. Supplemental antigen testing is also recommended.

We would love to hear your pet's story. Please add a comment.