Diagnosing feline heartworm disease (Sponsored by IDEXX)

Feline heartworm is more prevalent than once believed, and cats likely face more damage from juvenile heartworms than from adult worms. Within two to three months, juvenile worms can cause serious pulmonary disease in cats, known as heartworm-associated respiratory disease (HARD). Compounding this dangerous threat is the fact that neither antibody nor antigen tests can accurately detect HARD.

Antibody tests can detect infection as early as two months post-exposure but cannot differentiate between cats in which the infection is active and cats in which the infection has cleared. A study from Auburn University also showed that antibody tests fail to detect approximately 50% of HARD infections. Antigen tests can only detect adult worm infection, which is a small part of the total feline heartworm disease picture. The cats that test heartworm antigen-positive represent just the tip of the iceberg. Experts estimate that for every cat with a positive heartworm-antigen test result, there are as many as 10 more cats afflicted with HARD, and even more are at risk of exposure to feline heartworm disease. This indicates that many more cats may suffer from heartworm disease than previously estimated.

AS NEW RESEARCH ON FELINE HEARTWORM DISEASE SURFACES, SO DO QUESTIONS OF PREVALENCE, DIAGNOSIS, AND PREVENTION. HERE ARE ANSWERS TO SOME OF THE COMMON QUESTIONS.

1. What does the current literature tell us about the rate of heartworm infection in cats compared with that in dogs?

Historically, the prevalence of feline heartworm disease has been estimated to be 10% of the canine rate in any given area. This ratio, however, compares adult worm infections in dogs and cats, not juvenile worm infections. Juvenile infections (approximately three months post infection) are more common than adult infections in cats, but can be just as harmful. Recent studies from the University of Florida and Auburn University have shown that juvenile infections in cats can cause significant pathology, such as heartworm-associated respiratory disease (HARD).1

Which cats to test?

Because the prevalence data in cats have not included juvenile infections, these estimates are just capturing the "tip of the iceberg." Current thinking suggests that for every cat infected with an adult heartworm infection there may be as many as 10 cats afflicted with HARD. The true prevalence and impact of feline heartworm disease and HARD are being overlooked.

2. Why should cats be tested for heartworm infection?

Most experts agree that awareness needs to be increased of how feline heartworm disease affects patient populations. Given the historic lack of understanding about juvenile worms and the pathology associated with HARD, it is reasonable to screen patient populations broadly with an eye toward improving veterinary and pet owner awareness about the risk to cats and the need for prevention. It is also reasonable to evaluate whether a cat is free of adult infection before prescribing heartworm prevention.

3. What are the advantages and disadvantages of a feline heartworm antigen test vs. an antibody test?

Antigen tests are sensitive and highly specific for adult infections, rarely producing false positives. Antigen tests are reported to be 68% to 86% sensitive but cannot detect a single male infection because male worms produce very low antigen levels.2, 3 It is believed that single male infections occur in about 30% of the adult heartworm infections in cats. However, because antigen tests detect heartworm infections beginning five to seven months after infection, they miss juvenile infections that occur in the first two to three months of infection and the associated HARD pathology.

Antibody tests are sensitive to early infection, detecting infection as early as two months after infection, but are not conclusive. The sensitivity of antibody tests wanes as infection matures to the juvenile stage, and an antibody-positive result does not indicate whether or not the cat is presently infected.

4. How can routine heartworm testing and prevention improve the health of your feline patients and your practice?

Based on the awareness of HARD and various prevalence studies, feline heartworm is a significant disease that is under-recognized.4 Routine testing can raise awareness among your staff and clients, helping to drive the adoption of preventives in cats at risk for heartworm infection. Today, only 4% of cats are placed on preventives, compared with about 60% of dogs. Raising the standard of care with respect to feline heartworm will benefit cats, drive preventive sales, and stimulate regular appointments.

References

1. Blagburn BL, Dillon AR. Feline heartworm disease: solving the puzzle. Veterinary Medicine (supplement) 2007; March: 7-14.

2. Snyder PS, Levy JK, Salute ME, et al. Performance of serologic tests used to detect heartworm infection in cats. J Am Vet Med Assoc 2000;216:693-700.

3. Berdoulay P, Levy JK, Snyder PS, et al. Comparison of serological tests for the detection of natural heartworm infection in cats. J Am Anim Hosp Assoc 2004;40:376-384.

4. Lorentzen L, Caola A. Incidence of positive heartworm antibody and antigen tests at IDEXX Laboratories: Trends and potential impact on feline heartworm awareness and prevention. Vet Parasitol 2008: in press.