Heartworm Disease in Cats

Heartworms, scientifically known as Dirofilaria immitis, are worms that can grow up to 12 inches in length and live in the heart and lungs of affected animals. They are transmitted by mosquitoes and infect both wild and domesticated animals. The previous article focused on dogs, the most common household pet that is affected by this disease. However, cats are also a susceptible species, and it is important to point out the differences in the disease process between the species.

The transmission of heartworm is the same as with dogs. A mosquito will bite an infected animal (such as a dog, fox, coyote, or wolf) and pick up the baby heartworms swimming in that animal’s blood stream. These baby worms, known as microfilaria, will grow inside the mosquito over 10-14 days and mature into a larval stage that can infect another susceptible animal when the mosquito bites that animal. From there, the heartworms will make their way to the vessels in the lungs and heart and continue to mature over the next 6-7 months. Once the worms have matured into adults, they can live for years and continue to reproduce, creating more microfilaria for mosquitoes to pick up and take to another at risk animal.

Cats are known as “atypical hosts” for heartworms. This means that while a mosquito can bite a cat and inject heartworm larvae into the bloodstream, it is more difficult for the heartworms to grow into adulthood and survive. Cats with heartworm will typically only have a few adult worms that will live to 2-3 years, whereas in dogs the worms can live 5-7 years and there can be 30-300 worms living in them! Therefore, most cats will only harbor juvenile worms. While this is better than having numerous adult worms living in their body for many years, this also makes it more difficult to test for worms as the test relies on a specific protein from an adult female worm. This means that cats can have worms living in them but test negative. It is also important to note that while cats don’t tend to have many adult worms, the immature worms can still do significant damage to the cat’s body, mainly in the lungs.

The clinical signs of feline heartworm disease range from subtle and vague such as coughing, vomiting, decreased appetite, to more dramatic such as fainting, respiratory distress, and even sudden death. Another important difference between cats and dogs is that if a cat is found to have heartworms, there is no approved treatment for killing adult worms. This makes using prevention the only safe way to avoid heartworm infections in cats.

There are multiple factors to consider when judging the risk of heartworm to your pet. While the incidence of heartworm disease is different in different areas of the United States, it does not mean a pet is immune just because of the state or area in which they live. When thousands of homeless/stray pets are relocated from different parts of the country due to environmental disasters, they can expose unprotected wildlife and pets who will then become carriers of heartworm. Those carriers will transmit the disease each year during the period of “mosquito bearing” weather in that area. It is important to add that multiple studies have also revealed heartworm disease in indoor only cats. The American Heartworm Society recommends having your pet tested for heartworm every 12 months and giving your pet heartworm prevention 12 months a year (year-round).

Lastly, perhaps a “mixture” of dogs and cats, ferrets are also susceptible to heartworm infections. Ferrets are more vulnerable than cats and just one worm can cause significant damage due to the size of a ferret’s heart. It is very difficult to test for the presence of heartworms in ferrets, and like cats, there is no treatment. Therefore, prevention is imperative.

For more information, visit https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics