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Heartworm in Dogs

Heartworm Caused by Mosquitoes

Heartworm disease is caused by a parasite called Dirofilaria immitis, which lives in the right side of the heart and the adjacent blood vessels. Its presence in these blood vessels causes cardiovascular weakness, compromised lung incapacity, and eventual death. Heartworm disease occurs primarily in dogs but can occur in cats and other animals on rare occasions.

Heartworm is transmitted from dog to dog (and cat to cat) by mosquitoes. Over 70 species of mosquitoes have already been implicated. Transmission of the parasite occurs as follows: when a mosquito draws blood from a dog or cat infected with heartworm, it takes with it a number of small immature worms called microfilariae. Once inside the mosquito, the microfilariae develop into larvae. Later, when the mosquito bites a new victim, the larvae are injected and that dog or cat becomes infected.

It takes about six and a half to seven months for the larvae to mature and start producing thousands of new microfilariae inside the circulatory system. The adult worms end up occupying the right chamber of the heart and the pulmonary arteries, while the microscopic microfilariae circulate throughout the bloodstream.

All these worms within the blood vessels produce an increased workload on the heart, along with restricted blood flow to the lungs, kidneys, and liver, eventually causing multiple organ failure. At first, pets may exhibit a chronic cough and reduced exercise tolerance, followed by sudden collapse and death.

Once infected, one pet can easily become a “carrier” or reservoir of infection for an entire neighbourhood. Sometimes, a dog or cat may have heartworm disease but show no symptoms. By the time symptoms do occur, the disease is well advanced.

Prevention is preferred to treatment. While there are effective treatments available, most veterinarians prefer to promote prevention of heartworm disease. Oral and topical medications that are administered monthly and have shown to be highly effective in preventing heartworm disease are available from your veterinarian.

Reprinted with permission from

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