Sydney University Department of Medical Entomology Westmead Hospital
Dog Heartworm

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The dog heartworm, Dirofilaria immitis, is an increasing major veterinary health problem for dogs. Although heartworm in Australia has in the past been considered to be mainly a problem of tropical and subtropical coastal regions, in recent decades it has become increasingly prevalent in more southern areas.

Adult female worms in the dog's heart produce microscopic embryonic microfilariae which circulate in the peripheral blood. After ingestion, the microfilariae migrate from the mosquito's midgut, undergo development involving two moults, and after a period of approximately two weeks the infective third stage larvae migrate towards the mosquito's head. These larvae then transfer to the skin of the final host during feeding of the infected mosquito. Following their penetration into the final host, the third stage larvae migrate through body tissue where they develop into fourth stage larvae, then immature adults which finally enter the blood stream and reach the heart via the venous system. Adults mature in about 6 months and are subsequently found in the heart and major vessels. The adults mate, and the female produces millions of microfilariae to circulate in the peripheral blood to renew the cycle. The dog can be severely affected, with major vessels becoming blocked with chronic infections; death usually results in such cases from heart failure.

Mosquito vectors in all parts of Australia have not been adequately identified, although in southeastern Australia, Aedes notoscriptus and Culex annulirostris are prime suspects because they have been shown to be infected in field studies and are very good laboratory vectors. The former species is an excellent laboratory vector, has close contact with dogs in domestic and peri-domestic urban, and rural situations, and is thus suspected to be a very important vector. In certain circumstances Ae. vigilax, Ae. camptorhynchus, Cx. quinquefasciatus and An. annulipes, and possibly other species may need to be considered as local vectors.

Assoc. Prof. Richard C. Russell

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