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Understanding Heartworm Disease in Cats

Source: PetWave, Updated on October 25, 2016
Heartworm Disease Guide:

The Disease

Heartworm disease is a devastating disease that will eventually cause death if it is not treated. Heartworm disease affects a cat's heart, and this disease is transmitted through mosquitoes. The heartworm itself must go through stages of development, and once established in a cat they can live up to seven years. Heartworm disease has become a serious threat to pet owners in areas where mosquitoes are found, and heartworm prevention is highly advised to all pet owners who live in these areas.

Heartworm disease is caused by a parasite, Dirofilaria immitis, which lives in the arteries of the lungs and in the right side of the heart. Heartworm disease in cats is less frequent than in dogs. Cats are susceptible, but appear to be more resistant than dogs. Cats may exhibit non-specific clinical signs, such as vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss.

The average time from when the cats are infected through the bite of a mosquito until the presence of adult worms is about 8 months. Outdoor cats are at the greatest risk on infection; however, a high percentage of indoor cats (as defined by their owners) have been infected. The distribution of feline heartworm infection in the US is geographically similar to canine infection, but it occurs in fewer numbers than it does in dogs.

The Heartworm Parasite

Infective larvae are transmitted into the host’s bloodstream by the bite of an infected mosquito. The mosquito transmits the larvae, the larvae grow, develop and migrate in the body over a period of several months to become sexually mature male and female worms. These mature adult heartworms live in the arteries of the lungs and right side of the heart and can be up to 7-14 inches long. As mature adults, the worms mate and females release tiny immature worms called microfilariae into the blood stream.

Where is Heartworm Disease Found?

Heartworm disease occurs worldwide in tropical, subtropical, and some temperate regions. Until the late sixties, the disease was restricted to southern and eastern coastal regions of the United States. Now, however, cases have been reported from dogs native to all 50 states (Figure 1).

For most of North America, the danger of infection is greatest during the summer when temperatures are favorable for mosquito breeding. In the southern U.S., especially the Gulf Coast and Florida, where mosquitoes are present year-round, the threat of heartworm disease is constant.

Distribution of Dog Heartworm in the United States

Figure 1. Distribution of Heartworm in the United States.

CREDITS: American Heartworm Society (

Transmission of Heartworm Disease in Cats

The transmission of heartworm begins with a cat that is already infected with the disease. The mature female heartworm, already thriving in a cat’s heart, will release heartworm babies, known as microfilaria, usually around 6-7 months after the cat has become infected. The microfilaria circulates through the cat’s blood, and when a mosquito feeds off of a cat with microfilaria in its blood stream, the mosquito becomes a potential carrier of heartworm.

Inside the mosquito, the microfilaria takes two weeks or longer to develop into the larval stage. If the microfilaria are not taken up by a mosquito, they will never mature into larva. Once they are in a larval stage, they can be passed into an animal when the mosquito feeds. Not all animals are susceptible to heartworm, but cats and dogs are.

Over a period of 2-3 months, the microfilaria develops into mature heartworms. At 6-7 months after the initial infection period, the adult heartworms are established in the cat’s heart and able to produce microfilaria. The cycle is repeated when a mosquito feeds of off the newly infected cat.

If you suspect that your cat may have heartworm, have it tested immediately. If you live in areas where your cat may be at risk for heartworm, keep your pet on a heartworm preventative and have your pet tested for heartworm once a year.

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