The life cycle of the heartworm, dirofilaria immitis, in dogs takes roughly 9 months to complete. The end result is a parasitic infection in the heart of the dog that can be fatal. While heartworm infection is possible in cats, it is rare. Other species susceptible include foxes and raccoons. The normal life cycle involves an infected dog as a starting point, a mosquito as an intermediary and finally another dog as a host.
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Heartworm Life Cycle Starting Point
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The heartworm life cycle starts with a mosquito bite to a dog with a heartworm infection that has matured and has migrated to the heart of the animal. The mosquito picks up a microfilaria, a minute larvae stage of the heartworm, from the blood of the infected dog. The microfilaria larvae goes through three transformations, resulting in what is referred to as the third stage infective larva, before moving to the mouthpart of the mosquito.
Transmittal to the Dog
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The third stage infective larva leaves the mouthpart of the mosquito when it has stung the dog for its blood meal, but the larva does not enter the dog directly. The larva leaves the mosquito to rest on the skin of the dog. After the mosquito leaves the dog the larva enters the dogs body through the open wound of the mosquito bite. The larva remains just under the skin of the dog for as long as four months.
Moving Through the Dog
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As the larva matures it moves from the flesh just below the skin into the blood stream of the dog. The larva circulates within the blood stream until it reaches the dog's heart. The larva attaches itself to the heart muscle.
Maturing Heartworm
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Once the infective larva reaches the heart it matures over a period of about five months. The worms continue to grow and ultimately cause damage to the heart muscle that can be fatal to the dog. The adult heartworms also produce microfilaria which circulate in the blood stream of the dog. At this point, a mosquito bite to the dog can start the entire life cycle over again.
Treating Heartworm
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Treatment of heartworm in dogs is largely preventative. No symptoms occur until the adult worms have damaged the heart causing weakness and ultimately heart failure. Blood tests are available to determine if microfilaria are present in the blood test. This can be followed by a medication, in pill form, that kills the larva as it circulates in the dog's bloodstream before settling into the heart. These medications are sometimes administered as a preventative even if blood tests are not performed.
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