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Preventing heartworms in dogs and cats

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Heartworms are a parasite of dogs and cats as well as wild canids and marine mammals. As the name would imply, these worms live in the heart but also appear in the pulmonary circulation from the pulmonary artery to the smaller vessels in the lungs. The scientific name for heartworms is Dirfilaria immitis. They are long, slender worms that can reach lengths of 10-11 inches.

Once believed to be restricted to dogs, heartworms in cats is far more common than generally appreciated, according to recent studies. The disease takes a different form in cats, and is difficult to diagnose definitively. Heartworm infection has also been reported in humans.

The adult worms live in the right ventricle and pulmonary arteries, where they breed and produce offspring known as microfilaria. These microfilaria are picked up by mosquitoes when they feed. These parasites are transmitted from animal to animal through the bite of mosquitoes. A large number of mosquitoes have been shown to be capable of transmitting heartworms by feeding on infected animals. After the worm develops to the next stage inside the mosquito, it enters another animal when the mosquito feeds again.

Heartworms were once limited in distribution in the United States to the Gulf Coast and Mississippi River basin. However, as a result of a mobile society and the fact that both mosquitoes and infected animals can move about easily, recent surveys have shown that heartworms now exist in all 50 states. In fact, heartworms occur in North and South America, much of Europe, Asia and Australia.

Heartworms in dogs

In dogs the disease is characterized by worms, sometimes found in large numbers – there can be as many as 250 worms living in the right ventricle, pulmonary arteries or occasionally in the vena cava. The result is interference with the circulation, but serious disease occurs when these worms die or are washed downstream, where they cause embolism or clot formation in the lungs. The clinical signs of infection range from no abnormalities in dogs with small numbers of worms to shortness of breath, heart failure and death.

Treatment involves the administration of an arsenic-like compound that kills the worms. Unfortunately, as they die, worms can cause severe problems. Because of this, treatment is a serious undertaking and requires extended periods of complete exercise restriction. Some patients may require surgical treatment.

Heartworms in cats

Cats do not develop the same disease as dogs, but present very real problems of diagnosis and treatment. Infection in cats typically involves between one and three worms. Heartworm disease in cats has been associated with a wide variety of symptoms from vomiting to coughing and even to sudden death. The worms do not mature in cats as they do in dogs, and so they do not reproduce. The worms instead frequently lodge in the circulation of the lungs.

Tests in dogs look for the presence of specific proteins that involve the adult worm. Since adult worms do not develop in cats, these tests are not as useful and can yield false results. Unfortunately, there is no recognized treatment for heartworms in cats, and we must rely on supportive and symptomatic treatments.

Finding solutions

Prevention is the answer when dealing with heartworms. A number of products are available from veterinarians that are highly effective in preventing infection in both dogs and cats. Animals should be tested prior to starting preventives to determine if they are already infected. In animals just starting preventive medications, it might be necessary to test again in six months.

Because exposure to mosquitoes is impossible to predict, the risk of infection exists most of the year, if not all year round. In addition to preventing heartworms, most of these products have the added advantage of preventing intestinal worms, which infect animals and potentially people.

It is recommended that all dogs and cats be given a combination of heartworm and intestinal worm control products monthly all year round.

For further information, refer to the Companion Animal Parasite Council and the American Heartworm Society. Most importantly, see your veterinarian regularly.

Credit: Reviewed by Amy I. Attas, V.M.D.
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