Feline Heartworm Infection

DO CATS GET HEARTWORMS?

The answer to this question is an unequivocal "yes" but the disease is vastly different from the canine form. It is very important to understand the differences and understand why feline infection is less common.

THE PARASITE AND ITS MIGRATION

The cat is not a natural host for the heartworm which means the migrating larval heartworm is not likely to find its way to the heart should it actually make its way into a cat's skin from a mosquito bite. Mosquitoes that carry heartworm definitely prefer to feed on dogs. Whereas a moderate heartworm infection in a dog would involve 25-50 adult heartworms, infected cats typically have less than six adult worms. Because the feline heart and blood vessels are so small, these few worms can wreak havoc. In a dog, six worms or fewer might not be considered worth treating. Whereas worms found in the canine heart can reach lengths up to 14 inches, the average length of worms found in feline hearts is only 5-8 inches. While an adult heartworm can expect to live 5 years in a dog, it will only live 2-3 years in a cat probably due to the cat's especially strong immune reaction.

SYMPTOMS OF DISEASE

The cat's immune system is extremely reactive against heartworms. For this reason, it is virtually impossible to detect microfilaria in an infected cat. (The cat's immune system removes them too quickly). Also, symptoms of infection tend to be more immune-related than heart failure related. Cats develop more of a lung disease, complete with respiratory stress, and chronic coughing or vomiting. Feline heartworm disease is often misdiagnosed as feline asthma. Sudden death may occur just as it may occur in infected dogs.

DIAGNOSTIC TESTING

Microfilaria (baby heartworms) testing is virtually worthless in the cat because the immune system of the cat suppresses heartworm reproduction. Antigen testing may not be accurate in the cat, as the worm burdens are typically very low. (Remember, it takes 1-3 female worms, depending on their age, for the test to trigger positive.) Antibody testing may be more sensitive but is not adequate alone. A negative antibody test is good evidence that the cat is not infected; however, a positive antibody test may indicate several things. It could indicate a mature infection. It could indicate the presence of immature worms in the body. It could also indicate a past infection. (Antibody levels will remain somewhat elevated after the heartworms have long since died of old age.) This means that a positive antibody test should be accompanied by some kind of signs of heart disease (either symptoms or radiographic or ultrasonographic evidence) or with a positive antigen test before making a diagnosis of active heartworm infection in a cat. Antibody testing can detect either antibodies against heartworm skin or antibodies against deeper heartworm body proteins. Antibodies against skin seem to be more indicative of current infection. Antibodies against deeper ("somatic") proteins, may indicate early infection (infection prior to the maturation of the adult worms) or an old infection. All three types of testing (Antigen, Cuticular Antibody and Somatic Antibody) may be needed to evaluate a cat's heartworm status.

TREATMENT

The same heartworm adulticide therapy used in dogs may be used in cats but it is extremely dangerous to do so. There may not be a choice, however, depending on the degree of illness from the heartworm disease. Approximately one third of cats receiving heartworm adulticide therapy will experience life-threatening embolic complications when the worms die suddenly. One month of cage confinement is typically recommended to control circulatory effort after adulticide treatment. In general, if the cat does not appear sick, the American Heartworm Society recommends attempting to wait out the worm's 2-3 year life span and simply monitor chest radiographs every 6 months or so. During this time it is recommended that the cat be placed on monthly preventative to prevent any re-infection. Since the major signs of disease in the cat are due to inflammation and immune stimulation, a medication such as prednisone(cortisone) can be used to control symptoms.

PREVENTION

Interceptor® (milbemycin) is a monthly chewable tablet for cats that prevents heartworms and protects against hookworms and roundworms.
Advantage Multi® for cats is a two ingredient topical monthly medication. One ingredient (imidacloprid) stays on the surface of the skin to kill fleas. The other ingredient (moxidectin) is absorbed into the bloodstream and prevents heartworms, earmites, hookworms and roundworms.