The first cases of feline heartworm disease were reported in the 1920s in Brazil and in the US. Both domestic and exotic felines can become infected. The diagnosis of heart disease in cats can easily be overlooked because of the vague clinical symptoms, unreliable diagnostic tests, and lack of acceptable treatments. The heartworm life cycle in cats produces a transient patency 7-8 months postinfection, with 1-3 adult heartworms living in the heart and pulmonary vessels. Feline heartworm disease has been shown by necropsy studies to be present in 0-19% of cats in US animal shelters, which is 5-20% of the infection rate seen in a parallel group of dogs. Heartworm antibody serologic surveys have been positive in 0.5-43 % of cats. In 2003, a heartworm survey of 630 shelter cats found that 5% had adult heartworms in the heart and lungs, whereas 5% were FeLV positive and 6% were FIV positive. While the FeLV- and FIV- infected and heartworm antigen positive cats were mostly males, cats positive for heartworm antibody were equally divided between males and females. Twenty-seven percent of cats naturally infected with heartworm disease lived only indoors, whereas only 8-19% of indoor cats were heartworm antibody positive.

In the initial stage of feline heartworm infection, there is inflammation from the arrival of immature worms in the pulmonary arteries. This produces an asthma-like pulmonary inflammatory response. Heartworm larvae can also migrate through the pulmonary parenchyma. Even if the heartworm infection is aborted by the cat, clinical signs and histological changes in the lungs may persist.

The second stage of heartworm infection is associated with worm death. The adult worms die and degenerate causing more pulmonary inflammation, thromboembolism, and anaphylaxis—the latter being the likely cause of sudden death. In cats positive for heartworm antibody but without adult worms, there can be marked pulmonary artery occlusive hypertrophy. Many infected cats are asymptomatic, although typical clinical signs include: dyspnea, 48%; coughing, 38%; vomiting, 19%; and sudden death 10%. On physical examination, nothing abnormal may be found, or there may be increased bronchovesicular sounds, neurologic signs, and about half of infected cats present in respiratory distress. Routine laboratory and other diagnostic testing including electrocardiograms are usually unremarkable, although there may be eosinophilia and basophilia. Microfilaremia is rarely seen in infected cats, and when present is transient (1-2 months).