Heartworm -

Dirofilaria immitis


Life Cycle


Dirofilaria immitis
adults are primarily found in the right ventricle and pulmonary arteries of dogs. After reproduction, the females produce small, vermiform embryos called microfilariae. They can cross the capillary beds and so are found throughout the vascular circulation.

Preparasitic phase

Circulating microfilariae are ingested by a female mosquito  while taking a bloodmeal from an infected host (A). These prelarval stages migrate to the Malpighian tubules of the mosquito vector where development through to third stage larvae takes place (B). These infective L3's migrate from the tubules to the lumen of the labial sheath in the mosquito's mouthparts.

Parasitic stage

During a later bloodmeal on an appropriate host (C) - primarily dogs but also cats and ferrets, the L3's  will exit the labium, enter the bite wound, and penetrate local connective tissues. Molting to the next stage (L4) occurs within seven days of infection (D). L4 stages undertake extensive migration through the subcutis, which continues for some 60-90 days until the final molt to the immature adult (E). The juvenile worms migrate to the right heart within a few days of their final molt (F), carried there by the venous circulation.

Final maturation and mating occur in the pulmonary arteries, and the adult worms live in the right heart and pulmonary arteries, where they may survive for up to seven years. Production of microfilariae by inseminated female worms begins approximately six and a half months (192 days) after infection. Microfilariae are then released into the circulation, for a mosquito to ingest during a subsequent blood meal.


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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000