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What is Heartworm?

By S. Mithra
Updated Jun 04, 2024
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Heartworm is a parasite that infects and debilitates some animals, especially pets such as cats and dogs. The worm requires mosquitoes to complete its reproductive cycle, so it is most common in moist, warm areas like the Mississippi River Valley and Gulf of Mexico. Although entirely preventable, veterinarians can only treat it in the early stages, so it leads to many deaths per year.

Heartworm is classified as a parasitic roundworm. The most common species is Dirofilaria immitis. Worms spread their infection to animals, like wolves, coyotes, ferrets, and foxes, through mosquitoes, but not directly from one mammal to another. Like other parasites, they have a multi-stage reproductive cycle of dormancy and activity to improve their chances of survival.

Heartworm starts with tiny, egg-like young that circulate in the bloodstream of an infected animal. These microfilariae make themselves available to mosquitoes. When a mosquito drinks the mammal's blood, it ingests microfilariae, allowing them to develop into mature larvae inside the insect. This takes a few weeks, after which the mosquito inevitably deposits the heartworm larvae back into a mammal through another bite. The larvae grows and matures into a fertile, adult roundworm in the heart and lungs of the infested animal.

Dogs and cats slightly differ in their quality of infection and related symptoms. In dogs, the heartworm is more likely to migrate to the pulmonary vessels and the right chamber of the heart. Here, a worm can grow for many years, winding through the heart and eventually strangling it. In cats, the worms seem to prefer the lungs. After a few months, pets may develop symptoms.

A cat or dog with heartworm will be tired, easily exerted, listless, eventually exhibiting congested breathing, coughing, weight loss, coughing blood, shallow breathing, and the inability to walk long distances. Perhaps after a period of years, an animal can die from congestive heart failure caused by heartworm.

Just like flea infestations, heartworm can be prevented by monthly doses of medicine. These are available at your veterinarian's office in the form of topical gels, pills, or injections. In climates where the incidence of the parasite is higher, most vets recommend yearly tests to make sure your pet is still healthy, even if they are on preventative medication.

Heartworm can be diagnosed by a blood test. Once diagnosed, other methods of assessing how much damage has been caused to the liver, kidney, heart, and lungs are necessary to begin treatment. In the early stages, shots and surgery may rid an animal of a weak infestation. Some animals may be past the point of confidant treatment. Consult with your veterinarian to make sure your cat or dog is adequately protected.

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Discussion Comments
By serres47 — On Mar 01, 2008

My 7yr old German shepherd developed a gradual swelling in the left leg from mid thigh to toes. recently it started oozing clear fluid. I doubt if it's filariasis. Does dirofilariasis present this way with unilateral leg swelling?

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