Just Ask the Expert: Feline heartworm testing

Mar 01, 2009

Dr. Kazacos welcomes parasitology questions from veterinarians.

To ask your question, e-mail:

With the subject line: Parasitology Question

Q. Why should I test for heartworm infection in cats?


Kevin R. Kazacos, DVM, PhD
A. The most important reason is that heartworm testing can provide valuable information about a cat's possible infection with the parasite. For example, knowing that a cat has a heartworm infection might help you predict complications and select appropriate supportive therapy (e.g. oxygen, bronchodilators, and corticosteroids).1 In heartworm-infected cats, thoracic radiography and echocardiography are additional valuable diagnostic tools, especially for assessing disease severity, progression, or resolution.1 Cats are rarely microfilaremic, so microfilaria testing is not usually done in this species.

Heartworm antigen and antibody tests are most useful in cats with clinical signs. These tests have their limitations and require thoughtful interpretation, but when used together they increase the chances of making appropriate diagnostic decisions.

Feline heartworm infection can have a variety of clinical presentations, most notably feline respiratory disease (known as heartworm-associated respiratory disease or HARD) associated with L5 larvae entering the pulmonary vessels.2,3 In cats, a low percentage of these infections (0% to 25%) will progress to infections with adult heartworms.2 Usually only one or two heartworms will develop in cats, and about one-third of infections consist of heartworms of the same sex. If an animal develops an infection with adult female heartworms, it will become antigen-positive, providing useful information on current or recent adult infection. A detectable antigenemia is usually present by five-and-a-half to eight months after infection. The sex predilection of the antigen test is a limitation because it will not detect male heartworms. Also, the antigen test will not detect immature heartworms, so the test is highly specific (nearly 100%) but not particularly sensitive.2

A greater percentage of cats will become antibody-positive, but the timing of testing after infection will affect the results. Larvae of either sex can stimulate a detectable antibody response as early as two months after infection. Because of abortive infections in cats, which are probably common, background titers (indicating exposure) are also seen. Early experimental studies indicated a sensitivity and specificity of antibody testing as high as 98%; however, shelter surveys have revealed a lower sensitivity, in the range of 32% to 89%.2

An ancillary but important reason to test cats for heartworm infection is to gain information on the incidence of feline heartworm exposure and infection in your area. The incidence will generally be lower than that seen in dogs in the area and will vary based on local factors such as mosquito transmission and wildlife reservoirs for the parasite. With this information, you'll have a much better appreciation of how common heartworm infection is in your feline patient population. Feline heartworm infection continues to be under-appreciated in many areas. But because of the insidious and serious nature of feline heartworm disease, it is important that heartworm preventives be used in cats on a routine basis wherever the parasite is endemic.

Kevin R. Kazacos, DVM, PhD
Department of Comparative Pathobiology
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907

REFERENCES

1. Atkins C. Heartworm infection in cats. Clinician's Brief 2009;7(2):33-38.

2. 2007 guidelines for the diagnosis, treatment and prevention of heartworm (Dirofilaria immitis) infection in cats. American Heartworm Society, Batavia Ill. Available at: http://www.heartwormsociety.org/article.asp?id=47. Accessed Oct 23, 2008.

3. Blagburn BL, Dillon R. Feline heartworm disease: solving the puzzle. Vet Med 2007;3(suppl):7-14.