Animals acquire infection through the bite of an infected mosquito that deposits stage 3 larvae (L3) at the wound site. Within approximately 3 days after infection, the L3 molts to stage 4 larvae (L4) in the subcutaneous tissue. After weeks of migrating through tissue, larvae become sexually immature adults. Worms then enter the vascular system and reach the heart and pulmonary arteries. (See Figure 1.) Final maturation and mating occurs within the pulmonary vessels.3
Canine heartworm tests can detect infection with adult heartworms and immature larvae (microfilariae). Antigen tests detect proteins produced by adult female heartworms, whereas the modified Knott test is used to identify microfilariae in a blood sample. No test is currently available to detect adult male heartworms.