Four types of occult Dirofilaria immitis infection in dogs.

Occult heartworm infection, ie, infection with adult Dirofilaria immitis in the absence of circulating microfilariae, was detected in dogs with prepatent infection, unisexual heartworm infection, drug-induced sterility of adult heartworms, and an immune-mediated infection. In the immune-mediate occult infection, female heartworms contained microfilariae that were antigenically similar to circulating microfilariae but were in a privileged site inasmuch as the intrauterine microfilariae were not coated with circulating antimicrofilarial immunoglobulin G. Transplantation of these female worm into heart-worm-negative dogs resulted in microfilaremia.