Heartworms are 9-11” long worms that live in a cat’s heart or in the arteries going to the lungs (pulmonary arteries). Although they occur commonly in dogs, most people do not consider them a problem for the cat. However, recent studies of cats with heart and respiratory diseases have found an incidence of heartworms that is far greater than we previously thought.
Dogs vs. Cats
Dogs often have 30 or more worms present; however, many cats have only 2-4 adult worms in the heart. Adult heartworms live 3-4 years in the dog’s heart but only 1-2 years in circulation; cats usually do not produce any immature worms. Dogs have changes in the shape of the heart and pulmonary arteries as seen on radiographs (x-rays). Their pulmonary arteries become tortuous (curved) and enlarged. However, cats will usually have normal shaped hearts with pulmonary arteries that are blunted, presumably related to partial obstructions due to the presence of adult worms. Dogs usually have a persistent rise in eosinophils, a normal white blood cell that can be associated with parasitic infection. Cats have an eosinophil increase that occurs briefly within a few months of infection. By the time clinical signs are present, most cats have a normal eosinophil count.
Because heartworms are transmitted by mosquitoes, the disease is more common in areas of the country with large numbers of mosquitoes. Infection with feline leukemia virus or feline immunodeficiency virus does not seem to increase the risk that cats will get heartworms, even though they adversely affect the immune system.
The southeastern United States has a relatively high incidence of heartworm disease for both dogs and cats when compared to other parts of the country, although heartworm disease may occur anywhere that mosquitoes can survive. There is no apparent difference between infection rates between male and female cats, although male cats generally have greater numbers of worms than female cats. Although indoor cats might be at less risk for heartworm infection, more recent studies indicate that infection rates between indoor and outdoor cats are about the same. All ages are at risk for infection.
Heartworms are transmitted through mosquitoes. When an infected mosquito bites a cat, it deposits baby heartworms (larvae). The larvae migrate and mature for several months, ending up in the right side of the heart and the pulmonary arteries. They mature into adult heartworms about 6 months from the time they enter the cat. Shortly thereafter, they begin to release immature heartworms, known as microfilaria. Microfilaria live in the cat’s blood for about one month. They are ingested by mosquitoes feeding on the cat. (However, most mosquitoes acquire microfilaria by feeding on heartworm-infected dogs.) Because of their life cycle, it is necessary for a cat to be bitten by a mosquito to be infected with heartworms. Heartworms are not transmitted directly from one cat to another nor from a dog directly to a cat.
There are several methods used in diagnosing heartworms; unfortunately, none are 100% reliable so a combination of tests is often needed. The diagnostic sequence usually progresses as follows:
One of the difficult things about diagnosing heartworms is that there are no consistent clinical signs. The most common signs are coughing and rapid breathing. However, both can be caused by several other diseases. Other common clinical signs include weight loss and vomiting, also common in other diseases. Some cats seem to be normal, then die suddenly. This happens due to a reaction within the lungs to the young heartworms or when dead or live heartworms enter the pulmonary arteries and obstruct the flow of blood to the lungs.
1. There are two tests that are proving to be very helpful in diagnosing heartworms. The heartworm antibody test determines that the cat’s immune system has been exposed to heartworms. A positive test may indicate that an active infection is present. However, cats who have had heartworms but whose heartworms have died will also have antibodies for an unknown period of time. This test is very sensitive, so it is used first. However, if it is positive the next test is performed.
2. The next test is the heartworm antigen test. This detects the presence of the adult female heartworms. It is very specific, but not as sensitive. This means that a positive test indicates that heartworms are present, but a negative test does not mean that they are absent. Because the cat must have at least 2 adult female worms present to make this test positive, a negative test may mean that the cat may only have a small number of worms or that all the worms present are male.
In summary, a diagnosis of heartworms is confirmed if both the antibody and antigen tests are positive.
It should be noted that most veterinarians are able to perform an in-hospital test to detect heartworm antigen in dogs. However, the canine test is not as sensitive as the test for cats so using it will result in more false negative results.
3. Blood can be tested for the presence of microfilaria. However, less than 10% of cats with heartworms have microfilaria in their blood, and the microfilaria are only present for 1-4 weeks. Therefore, a negative test means little.
4. Cats suspected of heartworms can be tested for their level of eosinophils. Eosinophils are normal white blood cells that occur in increased numbers when certain parasites are present. They are elevated in the presence of heartworms, but this elevation only occurs for a few months. In addition, cats with intestinal parasites (“worms”) and allergies also commonly have increased eosinophil counts.
Radiographs (x-rays) permit us to view the size and shape of the heart. They also allow us to measure the diameter of the pulmonary arteries. Many cats with heartworms have an increase in the size of the pulmonary arteries; they may suddenly come to an apparent stop (blunted) on their way to the lungs due to worms obstructing them. However, many cats with heartworms have no abnormal findings on their radiographs, especially early in the infection.
An angiogram is an x-ray study in which contrast material (dye) is injected into the heart or veins and seen as it goes through the pulmonary arteries. This illuminates the arteries so they can be seen better. There is some risk to this procedure so it is not used often.
An ultrasound machine produces an image of internal organs and structures without the use of radiation. It is a testing procedure that is becoming more and more common in veterinary practices. With it, one is able to view the internal structures of the heart and the pulmonary arteries. In some cats, the actual heartworms can be seen; this finding confirms the presence of heartworms. However, in many cats the worms are not seen.
Dogs that have heartworms are treated with a drug this is safer than the originally used drug and is very effective. However, there is no such drug for cats. Another problem is that when the heartworms die they pass through the pulmonary arteries to the lungs. This can result in sudden death. Thus, we have a dilemma when a cat is diagnosed with heartworms. One of two choices must be made:
1. Treat with the drug designed for dogs. However, this is a drug that is unproved for cats and is not recommended as the chance for complications and side effects, including death, is high.
2. Treat the symptoms of heartworm disease and hope the cat outlives the worms. Since heartworms live in a cat for about two years, several months of treatment is needed. When cats are in a crisis, they are treated with oxygen, corticosteroids (“cortisone”) to relieve the reaction occurring in the pulmonary arteries and lungs, and, if needed, drugs to remove fluid from the lungs (diuretics). When they are stable, they are treated continuously or periodically with corticosteroids. However, the threat of an acute crisis or sudden death always exists.
Logically, we would think that indoor-only cats would be very unlikely to get heartworms. However, studies of heartworm infected cats consistently show that about one-third of them live indoors all of the time. As strange as this seems, there is a good explanation. The cat is not the natural host for heartworms. Because of this, the cat’s immune system is able to kill the larval stages most of the time. Each time a cat is exposed and successfully kills heartworm larvae, some immunity is develops. This is a type of natural vaccination. However, indoor-only cats do not get this constant exposure so their natural immunity is not as good. When a mosquito gets into the house and bites the cat, this cat is actually more likely to develop heartworms than most outdoor cats.
It is strongly recommended that dogs take drugs to prevent heartworms. It is well accepted that even dogs in cold climates should be on heartworm prevention at least part of the year. The monthly chewable tablet, HeartGard* and the monthly topical products, Revolution*, Advantage MultiTM, are good insurance against a disastrous disease. Therefore, prevention of heartworms is safe and easy. The reasons that heartworm prevention should be considered for your cat are:
1. Diagnosing heartworms is not as easy in cats as in dogs. A simple and reliable in-hospital blood test is not yet available, and the tests that are most reliable must be sent to an outside laboratory. Often, radiographs or ultrasound studies are needed to confirm the diagnosis. Many cats are diagnosed with an autopsy following sudden death.
2. Heartworms are not nearly as common in cats as they are in dogs. However, they are probably more common than we realize. As we look more aggressively for heartworms in cats with better and better tests, we expect to find that the incidence is greater than we thought in the past.
3. There is not a good treatment for heartworm-infected cats. Effective drugs are not available, and cats that seem to be doing well may die suddenly. Treating heartworm infections in cats is risky, and not treating these cats is just as risky. If they are cured of the disease, it takes about two years.
4. Cats given heartworm prevention drugs have not shown signs of toxicity. Since they only have to be given once each month and since they are formulated so that cats will eat them readily, administration is not a problem (in most cats). There is a wide margin of safety, even in kittens as young as 6 weeks of age.
5. Exposure to mosquitoes is all that is required for transmission. Cats do not have to be exposed to cats or dogs infected with heartworms.