Heartworms in Cats

Heartworms have traditionally been considered a parasite of dogs; however, they have been occasionally found in cats for many years. Our concept of heartworms in cats has been that they were very uncommon and an “accidental” infection. However, newer data has changed this concept showing that heartworms are probably as common in cats as in dogs. The cat’s immune system is constantly trying to kill the larvae and the adults. It is largely successful so we do not see a large number of cats with adult heartworms. However, as the immature adults and the adult heartworms die, they cause a great deal of inflammation in the lungs before they are finally removed. Because over 90% of the juvenile heartworms are killed and thrown in to the lungs, inflammatory lung disease can be very significant.

The majority of disease caused by heartworms occurs in the blood vessels within the lungs and within the lung tissue itself because this is the primary location of the larvae and adults.  Most of the lung disease occurs when the juvenile adults and the adult heartworms die. This means that, in some cases, the body’s successful termination of the infection may cause more respiratory disease and even life-threatening situations than the presence of the adult heartworms.
Currently there is no reliable test for feline heartworms.
There is a safe and effective drug for killing heartworms in dogs. However, this drug is not safe for use in cats. Furthermore, if we kill a large number of juvenile adults or adults, they may cause severe, and even fatal, lung reactions. Therefore,“treatment” is based on the cat’s immune system’s ability to kill the larvae and adults. This is typically a slow process, which is actually an advantage to the cat. Although most cats kill their worms within about 2 years, some may take up to 4 years.

The second part of “treatment” is to relieve the inflammation caused by the worms with corticosteroids (“steroids”). They may be given orally and continuously at a low dose, or they may be injected when a breathing crisis arises.

The third step in “treatment” is to prevent further infections by using on of the heartworm prevention products.