Many cat owners are unaware that their kitties can be infected with heartworms. Even indoor cats.
Heartworms are a parasite that looks like a piece of spaghetti when full grown. They are spread by mosquitoes. When a mosquito takes a blood meal from your cat, it passes along a tiny larval stage of heartworms. As the larval stages mature, they migrate through the tissues on their way to the cat's heart. This migration, along with the cat's immune reaction, is what causes so much damage in most cats. Unlike dogs, cats are not the primary host, and they develop a very strong reaction trying to rid their system of these invaders. This battle frequently takes place in the lungs, resulting in a syndrome called HARD (Heartworm Respiratory Disease). Affected cats will present with a history of coughing, trouble breathing, lethargy, decreased appetite. Those larval stages that survive past this point will continue their migration to the heart. An adult heartworm may live several years before it dies, and when it does, another reaction may occur.
Adding to the problem is the difficulty in diagnosing a cat with heartworms. In dogs, there are simple blood tests that measure antigens (proteins from the worms) or the presence of larval stages in the blood. These tests are sufficient for dogs because they are the natural host and often have large numbers of worms present. In contrast, cats may only have one worm present, providing a much lower concentration of the antigen to detect. Cats rarely have larval stages in their bloodstream. We frequently perform both tests, along with chest x-rays and/or ultrasound when we are looking for heartworms in cats.
Clinical signs of heartworm disease is cats may range from no clinical signs, vomiting, coughing, trouble breathing, to sudden death. In addition, there is no treatment to remove heartworms from cats like there is for dogs. We may provide supportive care in the form of anti-inflammatory medications like steroids, but we otherwise have to wait for the heartworms to die on their own.
Fortunately, there are preventatives to help protect our cats from this deadly disease. There are monthly chewable tablets (Heartgard, Interceptor) and monthly topicals (Advantage Multi, Revolution). These formulations also have the benefit of providing intestinal parasite protection, and some also help prevent flea infestation.
The American Heartworm Society (https://www.heartwormsociety.org/heartworms-in-cats) has some good information regarding heartworm disease in cats. Please check it out and ask your vet how to protect your cat!