ABSTRACT
OVERVIEW Heartworm disease (HWD) is caused by the nematode Dirofilaria immitis. Various species of mosquitoes throughout the world serve as an obligate intermediate host and transmit the parasite. Heartworm transmission is limited by climatic conditions. For the 1st stage larvae (L1) to mature to the infective stage within a mosquito, the average daily temperature must be more than 18° C (64° F) for about a month1. In temperate latitudes of the Northern Hemisphere, heartworm transmission peaks in July and August; in subtropical regions, year-round transmission can occur. Microfilariae develop into infective stage larvae within two weeks at 27° C (80° F)1. The heartworm life cycle continues when a mosquito ingests microfilariae (L1) during a blood meal from an infected host animal. The L1 develop into the infective 3rd stage larvae (L3) within the mosquito over a 2-2.5 week period. The mosquito transmits infective larvae to the new host during a subsequent blood meal. The L3 travel subcutaneously within the new host, molting first into the 4th stage larvae (L4) in about 9-12 days, and then into the 5th stage larvae (L5). The young worms (L5) enter the vascular system about 100 days after infection; they migrate preferentially to the peripheral pulmonary arteries of the caudal lung lobes. At least 5 (usually more than 6) months pass before an infection becomes patent and gravid female worms release microfilariae. Microfilariae passed to another dog by blood transfusion or across the placenta do not develop into adult worms because the parasite life cycle requires the mosquito as intermediate host.
