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Dogs, foxes, wolves, coyotes, and cats.
Dirofilaria immitis is a parasite which feeds on its host organism's blood exclusively.
The nematode usually occupies the right chamber of the heart and the pulmonary arteries of dogs, foxes, wolves, coyotes, and cats.
(Lifecycle diagram from American Heartworm Society)
Dirofilaria immitis has an indirect life cycle.
The adult parasite sexually reproduces in its vertebrate host, and the offspring are transferred to the intermediate host, which is usually a mosquito or a flea.
The larva develops inside the intermediate host and molts twice.
When the intermediate host feeds, the larvae enter the new vertebrate host through the wound.
The parasite remains dormant in the vertebrate hosts muscle tissue for 85 to 120 days. After this time period, the parasites enter the host's blood stream, where they are carried to the heart.
Completion of the life cycle in the heart requires 7 to 9 months.
Olson, Wilford O. 1962. Animal Parasites-Their Biology and Life Cycles. Burgess
Publishing Company, Minneapolis pp.315
Noble and Noble. 1964. Parasitology-The Biology of Animal Parasites. Lea and Febiger Co., Philadelphia pp.310
Material from Heather Hathaway, 1995.