Heartworm life cycle in dogs

The life cycle of the heartworm (Dirofilaria immitis) is as follows: A mosquito ingests microfilariae (first-stage larvae L1) from an infected host animal. The L1 must molt twice within the mosquito in order to mature; therefore microfilariae passed to another dog by blood transfusion or across the placenta do not develop into adult worms. It takes approximately 2 to 2.5 weeks for infective larvae (now L3) to develop within the mosquito. Infective L3 larvae enter the new host when the mosquito takes a blood meal. The L3 larvae travel within the subcutis of the new host, molting into the L4 stage in about 9 to 12 days and into the L5 stage.

The young worms enter the vascular system approximately 100 days after infection, migrating preferentially to the peripheral pulmonary arteries of the caudal lung lobes. It takes at least 5 and usually more than 6 months before an infection becomes patent and gravid female worms release microfilariae. Thus a puppy younger than 6 months with circulating microfilariae most likely received them transplacentally and does not have patent heartworm disease.

Various species of mosquitoes throughout the world can transmit the infection. The disease is widespread in the US but is particularly prevalent along the eastern and gulf coasts and in the Mississipi River Valley. Heartworm disease has been found in animals in all 50 states, although few cases are encountered in other areas of the US and Canada. Heartworm transmission is limited by climatic conditions. For the L1 larvae to mature to the infective stage within a mosquito, the average daily temperature must be more than 64 degrees Farenheit for about 1 month. In most areas of the US heartworm transmission peaks in July and august.

More information on heartworm disease medication here.

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