The Classic Comprehensive Handbook of Cat Care: HEARTWORMS
Heartworms (Dirofilaria immitis) are roundworm parasites ranging from 6 to 12 inches long (15 to 30 cm) that are found in the hearts, pulmonary arteries, and venae cavae of infected dogs. They can cause serious, even life-threatening disease in dogs, and they may also cause health problems in cats, although much more rarely. These worms are transmitted by mosquitoes that feed on infected animals and ingest immature forms (larvae) of the heartworm along with their blood meal. After a period in which the larvae mature the mosquito transmits the larvae that cause infection to a new host when it takes another blood meal.
There are areas all over the world where heartworm infection is likely to occur. In the United States, infection in dogs is particularly common along the Atlantic and Gulf Coasts, in the Mississippi Valley, and in Hawaii.
However, heartworm infection is also found in the Midwest, Pacific far west, and Alaska. If you live in an endemic area and your cat spends a great deal of time outdoors, he or she is at risk of heartworm infection.
Cats are more resistant to heartworm infection than dogs. When infection does occur there are fewer worms, they do not grow as large, and they usually die spontaneously in about two years. Since cats are not the normal host for the heartworm, larvae may often travel to other areas of the body such as the brain, spinal cord, and tissues under the skin instead of the heart. Signs of heartworms in infected cats may not be visible at all, or they may be very nonspecific such as loss of appetite, lethargy, and weight loss. On the other hand, the course of illness can be rapid, resulting in a sudden, unexpected death due to obstruction of the arteries of the lung (pulmonary embolization). More chronic cases may be marked by sporadic coughing or difficulty breathing and thereby be confused with feline asthma. Sometimes the main sign of infection is bouts of vomiting; in areas of high heartworm incidence, infection must always be considered as a possible cause of unexplained vomiting by cats who may have been exposed to infected mosquitoes.
Adult heartworms in cats do not readily produce the young larval forms that are usually found in the blood of infected dogs and that are most often used to diagnose infection in them. This fact coupled with the fact that heartworm infection in cats can mimic many other diseases often makes diagnosis difficult. A veterinarian who suspects heartworm infection in your cat can perform blood tests, chest radiographs (X-ray pictures), heart and blood vessel evaluation, and other specialized tests as needed to determine the presence and severity of heartworm-induced disease. When diagnosis is confirmed, he or she can give you the best advice as to whether treatment is advisable, since the drugs used for treatment may be associated with serious, even life-threatening, side effects themselves.
Prevent heartworm infection in your cat by keeping your pet indoors during mosquito season. Consult your veterinarian about prevention in high-risk areas.