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Heartworm - Dirofilaria immitis

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The Parasite

Dirofilaria immitis, the dog heartworm, is a nematode in the Class Filarioidea. A large, whitish worm, the females are approximately 30cm long, the males 23cm long with a coiled tail. The adults are primarily found in the cardiopulmonary circulation, and after reproduction, the females produce small, vermiform embryos called microfilariae which may be found throughout the circulation.

The Life Cycle:

L1 Developing in Malpighian tubule in Mosquito, day 6 post-infection.
Circulating microfilariae are ingested by a mosquito of one of several genera during a bloodmeal. Several species of mosquito are capable of functioning as an intermediate host and vector. These prelarval stages migrate to the Malpighian tubules of the mosquito vector where development, involving two molts after the microfilariae differentiate into first stage larvae, will continue to the third stage larva (L3). These infective L3's migrate from the tubules to the lumen of the labial sheath in the vector's mouthparts.
Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27C (80F). Below a threshold temperature of 14c (57F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically.
During a later bloodmeal on an appropriate host, the L3 will exit the labium, enter the bite wound, and penetrate local connective tissues. Molting to the L4 ensues within seven days of infection. L4 stages undertake extensive migration through the subcutis, which continues for some 60-90 days until the final molt to the immature adult. The juvenile worms migrate to the right heart within a few days of their final molt, presumably carried 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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Copyright © 1997 - University of Pennsylvania School of Veterinary Medicine
Faculty: Drs. Colin Johnstone, David H. Knight, and James B. Lok
Comments to colinj@vet.upenn.edu
Student: Aimee Eisenberg V'99
Updated 6/26/97