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Heartworm Disease in Cats

Heartworm disease, so named because the adult worms live in the right side of the heart, is common in dogs, less so in cats. In fact, cats may be accidental hosts only, and certainly they are less perfect hosts for this parasite than dogs are.

Heartworm Life Cycle

A knowledge of the life cycle of this parasite (Dirofilaria immitis) is needed to understand how to prevent and treat it. Infection begins when L3 infective larvae in the mouthparts of a mosquito enter the cat’s skin at the site of a bite. The larvae burrow beneath the skin and undergo two molts that eventually lead to the development of small, immature worms. The first molt (L3 to L4) occurs 1 to 12 days after the cat is bitten by the mosquito. The larvae remain in the L4 stage for 50 to 68 days, and then molt into the L5 stage (immature worms).

Immature worms make their way into a peripheral vein and are carried to the right ventricle and the pulmonary arteries. In cats, the larvae may become disoriented and migrate into body cavities and the central nervous system. Approximately six months after entering the cat’s body, they mature into adults. Adults can grow from 4 to 12 inches (10 to 30 cm) long and live up to two to three years.

In dogs, mature heartworms produce larvae, called microfilaria, that circulate in the bloodstream. This is much less common in cats, possibly because the cat’s immune system removes the microfilaria or because low numbers of adult worms or same-sex worms actually prevent the production of microfilaria to begin with.

Heartworm Disease

Because of the small size of the cat’s heart, one or two worms may be enough to cause serious heart trouble or even sudden death. Signs of heartworm infestation include a cough made worse by exercise, lethargy, loss of weight and coat condition, and bloody sputum. At this point, it may appear that the cat has asthma or allergic bronchitis. The cat’s pulmonary artery response to heartworms is much more severe than is the dog’s.

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