Feline Heartworm Infection
By Wendy C. Brooks DVM, DABVP
Feline Heartworm Disease
Thanks to the American Heartworm Society for these pictures. Images courtesy of Dr. Ray Dillon and Dr. Byron L. Blagburn, Auburn University College of Veterinary Medicine
Do Cats get Heartworm?
The answer is an unequivocal yes but the feline situation is vastly different from the canine situation. While it is true that the feline infection is not as common as the canine infection, the feline infection has recently been found to be a much more widespread problem than previously believed. In the past, a common statistic was that within a given geographic area, the feline heartworm infection rate was approximately 10% of the canine infection rate. Recent research indicates this is not so; in heartworm endemic areas, the incidence of feline heartworm infection rivals or surpasses that of feline leukemia virus or feline immunodeficiency virus. An incidence of 2% to 14% of all cats has been reported for endemic areas, making heartworm a concern for any cat living where there are mosquitoes.
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 complete its life cycle. The migrating worm uses molecular sign posts to tell it how to get to its host’s pulmonary arteries. The worm is prepared to read canine directions and may get lost in the feline body, ending up who knows where. Most of the larvae that actually make it to the pulmonary artery die soon afterwards due to the massive immune attack from the feline body. Very few larval heartworms survive to adulthood in cats.
- Whereas a moderate heartworm infection in a dog would involve 25 to 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. In a cat, a single worm could easily represent a lethal infection.
- 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 to 8 inches long.
- While an adult heartworm can expect to live 5 years in a dog, it will only live 2 to 3 years in a cat, probably due to the cat's strong immune reaction.
Heartworm disease in cats is caused by the inflammatory reaction generated by the worm’s presence.
In dogs, heartworm disease is mostly about the obstruction of blood flow from the physical size of the worms.
Symptoms of Disease
The cat's immune system is extremely reactive against heartworms. For this reason, it is virtually impossible to detect microfilariae 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 distress, 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.
In cats there are two phases where the disease can exert symptoms. The first is when immature worms reach the lung and pulmonary arteries, as early as 75 to 90 days after infection. Even small worms are inflammatory and disruptive to the circulation. Cells of inflammation infiltrate the lung and interfere with the cat’s ability to breathe. The second phase where problems can occur is when the worm dies. Since cats are not the natural host for this parasite, most immature worms that make it to the lung are killed. The presence of the dead worm is extremely inflammatory. (Imagine your body trying to remove or digest the dead body of another animal inside your lung and circulation!)
The effects of this kind of widespread inflammation can reach far beyond the lung and circulatory system. The kidney can be affected as well as the gastrointestinal tract and even the nervous system.
Heartworm disease is primarily a lung disease in cats, not a heart disease.
In dogs, diagnosis is usually not complicated. A blood sample is tested for proteins that can only be found in the body of the adult female heartworm. In cats, disease is not dependent on adult worms so this kind of testing has limited applications. Furthermore, there may be too few adult worms to generate a positive antigen test.
In dogs, testing for microfilariae (off-spring of adult heartworms born in the host’s body) is also commonly performed. Unfortunately, in cats microfilaria testing is virtually worthless. First of all, infected cats usually do not have enough adult worms to produce off-spring. There may be only a few adult worms and single sex infection is common. Microfilariae, if any, are simply cleared too quickly to be found reliably. As mentioned, in a cat heartworm disease stems at least in part from migrating immature larvae. No adult worms (and thus no off-spring) are necessary for disease so testing for microfilariae is not worthwhile in a cat.
Antibody testing may be more sensitive but is not adequate by itself. A negative antibody test is good evidence that a cat is not infected; however, a positive antibody test may indicate several things. It could indicate a mature infection. It could indicate 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.
So if no single test is reliable, what are we supposed to do for testing? The American Heartworm Society currently recommends using both an antigen test and an antibody test for screening apparently healthy cats. If a cat is sick and heartworm disease is suspected, both these tests are recommended, plus chest radiographs and/or echocardiography to assess heart and lung disease.
Since the major signs of disease in cats are due to inflammation and immune stimulation, a medication such as prednisone can be used to control symptoms. In general, if the cat does not appear sick, the American Heartworm Society recommends attempting to wait out the worm's 2 to 3 year life span and simply monitor chest radiographs every 6 months or so.
The same heartworm adulticide therapy used in dogs is best not used in cats as it is extremely dangerous to do so and is considered a last resort. 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 (generally an unacceptable statistic). One month of cage confinement is typically recommended to control circulatory effort after adulticide treatment.
The good news is that feline heartworm infection is 100% preventable and there are currently four products on the market that are reliably effective.
The dose of ivermectin (active ingredient of Heartgard) needed to prevent heartworm infection in cats is about 4 times higher than that in dogs. Heartgard was the first FDA-approved heartworm prevention medication available for cats. It is a monthly flavored chewable available by prescription. The American Heartworm Society recommends testing prior to administration.
Interceptor® also makes a monthly chewable for cats with the same active ingredient (milbemycin oxime) as Interceptor for dogs. Interceptor for cats also protects against hookworms and roundworms.
Revolution® entered the anti-parasite scene in 1999. This product covers fleas, roundworms, hookworms, and ear mites in addition to preventing heartworm in cats. Uniquely, this product is applied topically rather than orally. Read more information direct from the manufacturer of Revolution.
Advantage Multi® is a new product from Bayer which combines imidacloprid for flea control and moxidectin for heartworm preventive in one product. Read more information direct from the manufacturer of Advantage Multi®.