Ivermectin is a drug widely utilized for its anthelminthic activity over a diverse range of animal parasites. It has proved to be particularly useful in the prophylaxis of infection by the heartworm (Dirofilaria immitis) in dogs and cats. Although its application in this respect has been as a filaricide in preventing early growth and maturation of naturally acquired larvae, it is known to have activity against young adults as well. Previous studies have shown that in vitro exposure to ivermectin induces somatic muscle paralysis in the nematode Haemonchus contortus, resulting in pharyngeal dysfunction and disruption of normal ingestion. Experiments were performed to determine the effect of in vivo exposure of adult canine heartworms to this drug.

Adult heartworms were harvested from groups of dogs treated monthly with ivermectin beginning four to five months after inoculation of infective larvae and from untreated control dogs. Live worms from both experimental and control dogs were fixed, embedded, and sectioned for conventional transmission electron microscopy.