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Veterinary Forum June 2007 (Vol 24, No 6)

Clinical Report: "Update on heartworm infection"

by Sophia Yin, DVM, MS (Animal Science)

    Scenario: It is 1:00 am, and through the window you spy wild yellow eyes staring back at you while a black-furred mass huddles nearly motionless. Its mouth hangs open, and the chest rhythmically heaves as the cat tries desperately to suck in oxygen.

    Imagine what this cat's life would be like if you could prevent the underlying disease from occurring and avoid lifelong management with bronchodilators, cortico-steroids or inhalants. In some cases you can, according to the American Heartworm Society (AHS). If the cat in this scenario has heartworm-associated respiratory disease (HARD), a condition that frequently mimics feline asthma, then monthly heartworm preventatives could have altered the course of its life.

    A problem for cats, too

    Many veterinarians think that feline heartworm infection is rare, but Charles Thomas (Tom) Nelson, DVM, president of the AHS, says: "If it's a problem for dogs in your area, then it's also a problem for cats."

    In 1997, when companies initially introduced preventive heartworm treatment for cats, Nelson was a skeptic. To satisfy his skepticism, he contacted several local animal shelters and performed necropsies on euthanized cats. To his surprise, after examining 259 cats, he found that 10% of the cats were infected with adult heartworms, which is a larger percentage than those with FeLV. Since then, the Universities of Florida and North Carolina have found that feline heartworm infection occurs at about 10% to 20% of the rate in dogs.

    This incidence of heartworm infection does not, however, include just outdoor cats, according to Nelson. "The University of North Carolina study found that 27% of cats with heartworm infection were exclusively indoors," he says.

    In the past, it was believed that many cats show minimal signs of heartworm infection; however, more recent research reveals that even infection with maturing larvae can cause severe consequences. In a 2005 study published in the American Journal of Veterinary Research,1 Brown and coworkers looked at pulmonary pathology in three different groups of cats:

    • Group 1 cats tested negative for heartworm antibodies and had no adult heartworms on necropsy.
    • Group 2 cats tested positive for heartworm antibodies but had no heartworms on necropsy, indicating that the cats had a previous infection but none of the larvae had matured to adults.
    • Group 3 cats tested positive for heartworm antibodies, and adult heartworms were found on necropsy.

    Evaluation of lung tissue found that 80% of cats with adult heartworms had occlusive hypertrophy of the small pulmonary arterioles. Of the cats with no heartworms and no heartworm antibodies, less than 13% had some lesions. When cats did have lesions, less than 20% of the vessels were affected. The most interesting finding was that cats with no heartworms but confirmed previous infection had the same hypertrophy of the pulmonary arterioles as those infected with adult heartworms. Fifty percent of these cats had occlusive hypertrophy lesions in 20% to 40% of their vessels.

    "Anywhere from 60 to 80 days after infection, you can find immature worms that measure 1 to 2 inches long in the pulmonary artery of cats," explains Nelson. "Although these heartworms are dying, they are causing the lesions. The cats may have respiratory disease that mimics asthma or allergic bronchitis. Then they seem to self-cure, but the pathology persists."

    In a recent study2 conducted by Byron L. Blagburn, MS, PhD, distinguished university professor, department of pathobiology, and Ray Dillon, DVM, MS, DACVIM, Jack O. Rash professor of medicine and professor of the department of small animal surgery and medicine, both at Auburn University College of Veterinary Medicine, three groups of cats experimentally infected with heartworm were compared. One group of cats received preventive medication (selamectin), one group served as untreated controls, and the third group received ivermectin 84 days after infection to create an abbreviated infection that mimicked the death of late-stage larvae. Each month, the cats underwent pulmonary radiography, bronchoalveolar lavage and blood serology. At 8 months, necropsy was performed on all cats. As expected, cats receiving preventive medication (selamectin) had normal lung pathology. Cats with adult worms had significant changes in the pulmonary artery, arterioles and capillaries, while cats with abbreviated infections had similar lesions almost as severe as those infected with heartworm. Similar studies evaluating cats over an 18-month period are in progress.

    Nelson puts the findings in perspective. "If you look at a room of people, it's hard to tell who smokes cigarettes, but we know that someone who smokes several cigarettes a day is experiencing negative effects on the lungs." Similarly, in cats, signs may not be readily apparent to the owner, but Nelson says, "Sometime later in life they may have another respiratory issue, and you have lost reserve capacity to deal with it. Every season, more damage occurs."

    Luckily, there is an easy solution, Nelson says. "If you are in an area with heartworm disease in dogs, cats should be on heartworm medication, too." Most heartworm preventatives also address intestinal parasites, so you will be addressing two infectious issues at the same time.

    For more information:

    Atkins CE, DeFrancesco TL, Coats JR, et al: Heartworm infections in cats: 50 cases (1985-1997). JAVMA 2000;217:355-358.

    Nelson C, Seward L, McCall J, et al: 2007 Guidelines for the diagnosis, prevention and management of heartworm (Dirofilaria immitis) infection in cats. Accessed from www.heartwormsociety.org (www.vin.com); Jan 2007.

    1. Browne LE, Carter TD, Levy JK, et al: Pulmonary arterial disease in cats seropositive for Dirofilaria immitis but lacking adult heartworms in the heart and lungs. Am J Vet Res 2005;66(9):1544-1549.

    2. Blagburn B, Dillon AR: Feline heartworm disease: solving the puzzle. Vet Med 2007;March(Suppl)2-4.

    References »

    NEXT: Dental Dilemma: "Radical Maxillary Resection of Oral Squamous Cell Carcinoma in a Dog"

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