Changes

==Life Cycle==

==Life Cycle==

Dirofilaria immitis adults reaach maturity and sexually reproduce in the right ventricle and the pulmonary arteries. After mating, female worms release larvae known as microfilariae into the circulation. When a mosquito takes a blood meal from the infected dog or cat, microfilariae are ingested. Mosquitos are true intermediate hosts for Dirofilaria immitis, and microfilariae require a variable period of maturation in the vector depending upon environmental conditions. For example, maturation at 30°C takes around 8 days, but when temperatures are down to 18°C, this takes around one month^ferasin. Below 14°, development is halted and resumes when temperatures rise. In cooler climates, this means that transmission of heartworm disease to new canine or feline hosts can only occur in warmer months.

Dirofilaria immitis adults reach maturity and sexually reproduce in the pulmonary arteries and right ventricle. Adult males are around 15cm in length, and females are around 25cm^merck. After mating, female worms release larvae known as microfilariae (or L1) into the circulation. When a mosquito takes a blood meal from the infected dog or cat, microfilariae are ingested. Mosquitos are true intermediate hosts for Dirofilaria immitis, and microfilariae require a period of maturation to L2 and then L3 in the vector. The duration of this development depends upon environmental conditions. For example, maturation at 30°C takes around 8 days, but when temperatures are down to 18°C, this takes around one month^ferasin. Below 14°C, development is halted and resumes when temperatures rise. In cooler climates, this means that transmission of heartworm disease to new canine or feline hosts can only occur in warmer months.  

Mosquito vector species acquire the first stage larvae (microfilariae) while feeding on an infected host. Development of microfilariae to the second larval stage (L2) and to the infective third stage (L3) occurs within the mosquito in ~1-4 wk, depending on environmental temperatures. This development phase requires the shortest time when the ambient temperature is >86°F (30°C). When mature, the infective larvae migrate to the labium of the mosquito. As the mosquito feeds, the infective larvae erupt through the tip of the labium with a small amount of hemolymph onto the host’s skin. The larvae migrate into the bite wound, beginning the mammalian portion of their life cycle. A typical Aedes  mosquito is only capable of surviving the developmental phase of small numbers of HW larvae, usually <10 larvae per mosquito.

Cats differ from dogs in that they are more resistant to infection with Dirofilaria immitis. A lower percentage of exposed cats develop adult infections, and when this does occur the burden is usually low^merck. L5 in the pulmonary arteries also have a relatively short (2 year) survival time in cats. however, migration into different organs has been described in this species.

In canids and other susceptible hosts, infective larvae (L3) molt into a fourth stage (L4) in 2-3 days. After remaining in the subcutaneous tissue for close to 2 mo, they molt into young adults (L5) that migrate through host tissue, arriving in the pulmonary arteries ~50 days later. Adult worms (males ~15 cm in length, females ~25 cm) develop primarily in the pulmonary arteries of the caudal lung lobes over the next 2-3 mo. They reside primarily in the pulmonary arteries but can move into the right ventricle when the worm burden is high. Microfilariae are produced by gravid females ~6-7 mo postinfection.

Most dogs are highly susceptible to HW infection, and the majority of infective larvae (L3) develop into adults. Ferrets are susceptible hosts, and cats are somewhat resistant. A lower percentage of exposed cats develop adult infections and the burden is often only 1-3 worms. Further evidence of relative resistance in cats is the short survival time of many L5 in the pulmonary arteries; adult worms probably survive no longer than 2 yr. Aberrant migration into different organs, including the CNS, has been described in cats.

==Pathogenesis==

==Pathogenesis==