In 1921, heartworm disease was first diagnosed in cats in Brazil. Then, in 1922, the first case in the United States occurred in Virginia. Now, the disease has been diagnosed in cats in 38 states and is recognized in Latin America, Asia and Europe. Yet for years, heartworm infection in cats was considered a rarity. Felines are not the normal hosts for heartworms, so the infection rate is lower. However, there are variations of the disease in dogs and cats, creating differences in diagnosing and treating the disease. There are no approved treatments for cats at this time, but there is an approved prevention method, Heartgard(TM) for Cats, and there are highly effective diagnostic tests specifically designed for cats, which test for antibodies as well as antigens.

One of these tests is the Heska(TM) Feline Heartworm Reference Lab Diagnostic Test. A serum or plasma sample from the cat is submitted to the Heska Reference Laboratory. Samples are analyzed every business day and results are provided by phone or fax within 24 hours of receipt of the sample. Different veterinarians use different companies to test for heartworm infection, but ask if your veterinarian uses a lab that checks both antibodies and antigens in order to get a reliable result.

How It All Starts

The heartworm (Dirofilaria immitis) is a nematode, or an elongated, cylindrical worm, mainly considered to be a parasite of the dog. However, cats can be infected. Both cats and dogs become infected through mosquito bites, which deposit the heartworm's larval stage under the animal's skin. Most cats have an immunological response which kills the larvae after they enters the cat's tissue. This accounts for the lower infection rate in cats. However, if the larva evades the defense system, it will proceed on to the heart where it will mature into an adult heartworm.

In a cat, the life span of an adult heartworm is one to two years, whereas in a dog, the heartworm can live from three to five years. Cats can tolerate a few adult heartworms for several months, but when the heartworms grow in size or when the number multiplies, clinical signs begin to appear. The cat is then considered to have heartworm disease.

Recognizing The Signs

Unfortunately, it is often difficult to recognize the signs of heartworm disease in a cat. The appearance of a cat with heartworms can differ greatly from that of a dog with heartworms. The physical symptoms of a cat infected are vague and often confused with other problems. There may be little change in the cat for months, but as the heartworms mature, increased pressure is placed on the heart. For many cats, this results in heart failure. Often, the first sign of the disease is the sudden death of the cat. This occurs when the worms are pumped out of the heart and into the lungs, blocking them. As a result, some of the signs to look for are coughing and difficulty in breathing. Inflammatory response in the lungs may be severe in cats. However, respiratory problems may be mistaken for asthma or a reaction to an allergen. Other signs of heartworms are weight loss, vomiting and fatigue.

Diagnosing the disease can be difficult for a number of reasons. Cats may have only two to four worms in the heart, whereas dogs may have 30. An antigen test searches for specific heartworm proteins, requiring at least four female heartworms to be present to receive a positive result. Because cats rarely have that many worms, much less adult females, they often test negative. Another test looks for microfilariae in the blood. These small larvae may remain in a dog's blood for years, but will rarely remain in a cat's blood for long. It is difficult to determine if an animal is infected without the presence of microfilariae, and a cat's test often returns negative. Another test checks for a rise in eosinophils, a normal white blood cell associated with parasitic infection. In cats, there is a brief rise in eosinophils, usually between four and seven months after infection, but not with any regularity. The levels soon return to normal before clinical signs ever appear.

When heartworms are suspected, but these other tests are negative, veterinarians may then perform radiographs of the heart and pulmonary arteries. An ultrasound may also be performed in order to attempt to see the heartworms in the cat.

In tests that seek out antibodies, such as the Heska(TM) Feline Heartworm Reference Lab Diagnostic Test, available through the Heska Corporation, a search is made for antibodies rather than antigens. These antibodies occur when heartworms are present. Contrary to the antigen test which requires the presence of up to four adult, female worms, the antibody test can detect infection with only a single male heartworm, or infection by a small number of adult or immature heartworms. Furthermore, the test is specific enough so that cross-reactivity to common gastrointestinal parasites does not occur.

If the result is positive, it means the cat was infected at least 50 to 60 days prior to testing. A positive result does not mean that the cat is currently infected though. It can also mean that the cat had been infected, but has since eliminated the parasite. A negative test can mean that the cat is not infected with heartworms, or that the cat was infected less than 60 days prior to the testing. A second testing within two to three months is usually recommended in that case, particularly if clinical signs exist.

If the test does return positive, an antigen test is then performed. Claude Piche, DVM, marketing director at Heska, explains that although the antigen test is often not sensitive enough for routine testing, it can help confirm the presence of adult female heartworms.

Risky Business

Unfortunately, the treatment options for dogs can be fatal for cats. The drugs used on dogs are toxic to most cats, so they are not a viable option. An arsenic-containing drug for the treatment of dogs has been used in cats, but there are still toxic reactions, often resulting in death. Currently, there are no products specifically designed to treat cats.

When adult heartworms die, the blood carries them to the pulmonary arteries and lungs where they often become lodged. Because cats' arteries are so small, just a few worms can completely block the arteries, causing immediate death. As a result, treating a cat for heartworm disease can be risky. One approach involves treating for signs of heart failure, putting the cat on a drug to prevent the development of further heartworms, and waiting for the adult worms to die of old age. The success rate is probably greater with this form of treatment, although there still exists the possibility of pulmonary artery obstruction or some other acute reaction to the death of the worms. The mortality rate following adulticide therapy has been reported to be between 20 and 30 percent.

Guarding The Heart

Because of the difficulty in treating heartworm disease in cats, prevention becomes more important. A heartworm preventative for cats has finally been approved. Heartgard(TM) for Cats, manufactured by Merck, is the only preventative specifically designed for cats. It is 100 percent effective in preventing heartworms, when given monthly. It eliminates the tissue stages of heartworms and also removes and controls adult and immature stages of hookworms. Heartgard is a meat-based chewable which most cats will eat whole or it can be mixed up in the food.

Heartgard for Cats is recommended in cats six weeks of age and older. Testing should be performed on the cat first, to check for existing heartworm infection, although cats already infected can be given the product to prevent further infections.

Even cats that remain indoors at all times are at a high risk of infection. Mosquitoes can get indoors, so special care must be taken. In a study performed at North Carolina State University, half of the cats infected with heartworms were indoor cats. It is still unknown as to why so many indoor cats become infected. Among the reasons suggested are the higher body temperature of pets which attracts mosquitoes, or the ability of mosquitoes to survive indoors longer than they can outdoors. In either case, the use of snug screens for doors and windows will help prevent mosquitoes from getting indoors.

Outside, eliminate any standing pools of water which are breeding grounds for mosquitoes. A more pleasant form of prevention is available through the erection of a birdhouse to attract purple martins. These birds eat thousands of insects each day, helping to reduce the risk of an infected mosquito passing on the larva. If the cat does go outside, try to avoid letting him out during late afternoon and evening when mosquitoes are most prevalent.

With care, heartworm disease can be prevented. Ask your vet what precautions he or she recommends for your geographical region. Feline heartworm does exist, but with care, hopefully your cat will live a long, healthy life.