Heartworm disease is a widely discussed topic in veterinary clinics throughout Indiana. It is one of the main diseases about which dog owners are informed, and measures are taken to prevent its occurrence. Heartworm disease is not addressed as often when speaking to cat owners, even though feline heartworm disease has been increasing in incidence over the past ten years.

Heartworm disease requires the mosquito vector in order to develop. The adult worm, located in the pulmonary artery, right atrium, and/or right ventricle releases microfilariae that circulate through the blood stream. These microscopic larvae can be ingested by the mosquito during a feeding. Within the mosquito, the larvae undergo further development to the L3 stage, and are re-released back into the circulation of another animal during a subsequent feeding by the mosquito. These L3 larvae migrate and develop into L4 and, finally, L5 stages to become mature heartworms.

In general, the prevalence of feline heartworm disease is about 5-10% of that in dogs in a given area. The rate of infection is due to the mosquito species in the area, most notably the Culex spp. and Aedes spp., as well as the mosquito’s preference for feeding on dogs versus cats. In addition, cats appear to be more resistant to infection by the causative agent of heartworm disease, Dirofilaria immitis. Cats typically have only a short, transient phase of microfilaremia and low worm burdens, in contrast to dogs that typically have higher adult worm burdens and a much longer period of microfilaremia. Reports also show that cats are more susceptible to aberrant migration of L4 larvae which have been found in the central nervous system and cutaneous tissues. The presence of a bacterium, Wolbachia, which is consistently found in heartworms, may have a role in the immune response against D. immitis in both dogs and cats, especially after death of an adult worm or during release of microfilariae.

The typical clinical presentation of heartworm disease in the cat is quite different than what one would expect in the dog. While dogs typically show a progression of pulmonary and cardiac clinical signs, cats often present in an acute dyspneic crisis and some may acutely collapse and die. Signs in cats are primarily associated with the initial migration of immature worms through the pulmonary arteries as well as with the death of an adult worm. In the interim, there are often no clinical signs. Most heartworm disease lesions in the cat are found in the lungs at necropsy. They include muscular hypertrophy of the pulmonary arteries and arterioles, diffuse infiltration of large numbers of inflammatory cells within the intima of the pulmonary arteries, interstitial fibrosis, and increased macrophages within the alveoli. It is not uncommon to find these pulmonary changes in cats even when no worms are found at necropsy. In areas of endemic feline heartworm disease, only 4-5% of cats that tested serologically positive for heartworm infection actually had worms present at necropsy. Cats appear to have more dramatic hypersensitivity reactions to initial infection of larvae and, therefore, may have permanent pulmonary changes even if they have been able to reject a full-blown heartworm infection.

It can be difficult to diagnose heartworm disease in the cat as the serological tests often used in dogs need to be interpreted differently in the case of a feline infection. The antigen tests detect a protein found in the reproductive tract of the adult female worm. Sensitivity for antigen tests tends to be lower in the cat due to the lower heartworm burden and the frequent single-sex or even single worm infections. However, the antigen tests do have a high specificity, so few false positives occur. The interpretation of the feline heartworm antibody can also be problematic. The antibody tests may be positive simply due to antibody response to previously circulating microfilariae or a previous adult worm infection. Therefore, false positives are more likely with the antibody test. To increase both sensitivity and specificity, it is recommended to combine both the heartworm antigen and feline heartworm antibody tests, along with thoracic radiography and, potentially, echocardiography.

Heartworm disease in the cat is typically self-limiting, and therefore is often treated symptomatically with corticosteroids, oxygen therapy, and furosemide. Heparin or aspirin are used as anti-thrombotic agents because heartworm-positive cats present with pulmonary thromboembolism. Surgical removal of worms is also a possibility, but a life-threatening, acute anaphylactic reaction is highly probable if a worm is damaged during the procedure.

Monthly heartworm preventative is commonly prescribed for dogs in Indiana, but it is also recommended for cats living in endemic heartworm regions. Indoor cats are just as likely to be infected with heartworm disease as outdoor cats. There does not appear to be a predilection for gender or age, and infection can occur despite a healthy immune system.

Recent literature has shown that heartworm disease continues to be an important topic in both canine and feline medicine, and it should be considered when cats present with acute dyspnea or even sudden death.