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
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
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.