This web page was produced as an assignment for an undergraduate course at Davidson College.
Adult heartworms (D. immitis) live in the pulmonary artery and right chambers of the heart. The lifespan of the worm is about 5 years. During this time, thousands of immature heartworms, known as microfilariae, are produced. Microfilariae live in the blood stream and small vessels of the circulatory system. They cannot achieve maturity within the host, though, and must spend some of their lifecycle in a mosquito.
Female mosquitos act as a vector for this pathogen and ingest immature microfilariae during bloodmeals. An overview of the lifecycle of the worm follows.
Juvenile larvae are ingested into the mosquito during a bloodfeeding.
Within 24 to 36 hours, the microfilariae travel from the intestinal region of the mosquito to the Malpighian tubules.
A molting occurs, and the microfilariae mature into a second juvenile stage.
Nine days later, the immature heartworms travel to the abdominal hemocoel to complete a second molt. This allows maturation to third stage, juvenile larvae.
At this point, the young heartworms are about 900 micrometers in length and are able to travel to the mouth of the mosquito.
A bloodmeal transfers the third stage larvae from the mouth of the mosquito to the host animal, in this case, a dog.
Complete maturation occurs within the host:
After about 9-12 days, juvenile third stage larvae living within the skin and muscle tissues complete a third molt. They remain in these areas of the host for 80 days as fourth stage juveniles.
By this time, the worms are about 20 - 25 millimeters and begin to enter the heart of the host animal.
Final maturation occurs in the heart and the worms then begin to procreate, infesting the bloodstream of the host animal with microfilariae that will mature within a mosquito after a blood meal. (Dept. Nematology UC Davis, 2005)