FELINE HEARTWORM INFECTION
FELINE HEARTWORM DISEASE
DO CATS GET HEARTWORM?
The answer to this question 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 presented was that within a given geographic area, the feline heartworm infection rate was approximately 10% of the 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 2-14% 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
Heartworm disease in the cat is caused by the inflammatory reaction
In the dog, heartworm disease is mostly about
Click here to see an animated depiction of how heartworm infection
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 coughing or vomiting chronically. 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 75-90 days after infection. The presence of even small worms is very 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 the cat is 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/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 THE CAT, NOT A HEART DISEASE
In the dog, 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 the cat, disease is not dependent on the presence of adult worms so this kind of testing has limited applications. Further there may be too few adult worms present to generate a positive antigen test.
In the dog, testing for microfilariae (off-spring of adult heartworms born in the host’s body) are also commonly performed. Unfortunately, in the cat microfilaria testing is virtually worthless.? First of all, infected cats usually do not have enough adult worms for the production of off-spring. There may be only a few adult worms present; single sex infection is common. Further, microfilariae, if any, are simply cleared too quickly to be found reliably. As mentioned, in cats heartworm disease in cats stems at least in part from migrating immature larvae. No adult worms (and thus no off-spring) are necessary for disease so microfilariae testing is not worthwhile in the cat.
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.
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 the cat 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 2-3 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.
Other helpful sites include:
Page last updated: 10/9/08