She’s indoor only, we don’t REALLY need heartworm prevention, do we?! This is how the majority of our pet owners respond when asked about their feline friends’ status of protection against heartworm disease. Many cat owners don’t realize that, just like dogs, cats are susceptible to heartworm disease being transmitted by a single mosquito bite. A study conducted by North Carolina State University found that 27% of the cats diagnosed with heartworm disease were “inside-only” cats. For this reason, both indoor and outdoor cats are at risk and should receive heartworm preventative medicine.
Most heartworm infections in cats are comparatively light and consist of less than six adult worms. Although the number of worms may be small, because of the cats small size and severe inflammatory response to heartworms, any infection may be deadly in a cat.
Initial signs occur 3-4 months after infection with severe inflammation in the lungs and heart. This initial phase is often misdiagnosed as asthma/allergic bronchitis, but it is actually part of heartworm associated respiratory disease (HARD). Many cats will cough, wheeze, and breath with their mouths slightly open over the course of a few weeks. Once the body gets used to the infection, many cats tolerate the worms living in their lungs without apparent ill effects until the worms begin to die in 1-3 years, which begins the second phase of the disease.
The dying heartworms cause lung inflammation and thromboembolism (clogs in the arteries), which often leads to severe, acute lung injury. This type of reaction can occur even with a single-worm infection! About 20% of cats infected will die, 45% will have significant lung disease and live, and 35% of cats will have mild or inapparent disease
Heartworms may be detected via blood tests that look for worm particles (antigens), or looking for specific molecules that a cats body makes in response to heartworms (antibodies). A chest x-ray may also be helpful in determining how severely the lungs are being affected, and if there is supporting evidence for heartworm infection.
A safe, effective treatment is not available to kill the heartworms in cats. The medication that we are able to use in dogs (Immiticide) will often cause death of the feline patient, therefore its use is not recommended. Instead, treatment is focused on decreasing inflammation in the lungs with corticosteroids given as monthly injections, or preferably using an oral medication called prednisone. Bronchodilators (medication to open the small airways) may also be used.
Prevention is of course the recommendation of choice! There are several safe, 100% effective preventatives that may be applied topically (on the neck) or administered orally. There are also convenient products such as Revolution Plus which have broad protection against fleas and mites in addition to the heartworm prevention, so they provide full protection in a single application each month. A discussion with your veterinarian will help you to decide which type of product is right for you.