In three to four months adult worms emerge. They can survive for about five years in a dog's heart and reach lengths of twelve inches. In cats, worms are smaller and live for only two or three years. The microfilaria are then swallowed by biting mosquitoes, and develop into infective larvae in the mosquito in ten to forty-eight days, depending on the climate. The disease may be advanced by the time an animal shows signs of infection.
Cats can harbor a heartworm infection at any age from nine months to seventeen years.
Some cats seem to be able to get rid of the infection spontaneously. The severity of the disease depends on the number of worms, their location, the duration of infection, and the dog's immune response. If there are only a few worms present, there may be no signs of heartworm disease. In cats, just a few worms can produce fatal illness, and sometimes infected cats die suddenly with no time for diagnosis or treatment. There may be no symptoms at all, or they may exhibit many of the same signs of infection as dogs.