Changes

==Life Cycle==

==Life Cycle==

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.  

''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 25cm¹. 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'', since microfilariae require a period of maturation to L2 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 month². 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.  

Once matured, L3 in the mosquito migrate to the labium of the vector, from which they erupt onto the host's skin as the mosquito feeds. Larvae then migrate into the bite wound and as most dogs are highly susceptible to heartworm disease, most L3 do establish infection. It takes 2-3 days for L3 to moult to L4, which remain in the subcutaneous tissues for up to two months before becoming young adults (L5) and migrating to the pulmonary arteries.  

Once matured, L3 in the mosquito migrate to the labium, from which they erupt onto the host's skin as the mosquito feeds. Larvae then migrate into the bite wound and, as most dogs are highly susceptible to heartworm disease, most L3 then establish infection. It takes 2-3 days for L3 to moult to L4, which remain in the subcutaneous tissues for up to two months before becoming young adults (L5) and migrating to the pulmonary arteries.  

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.

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 low¹. L5 in the pulmonary arteries also have a relatively short (2 year) survival time in cats.

==Pathogenesis==

==Pathogenesis==