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Revalence and Role of HARD<\/h2>\n
FHWD is more widespread and common than previously thought. For many decades it was assumed that cats were resistant to infection, but we now know that this is an inaccurate view of heartworm infection (HWI) in this species.<\/p>\n
Cats are more resistant to heartworm than dogs, but they are considered a susceptible host. While the rate of adult HWI in cats is only 5% to 20% of that in dogs,1<\/sup> these numbers fail to consider the important physical effects caused by immature heartworms that die before they fully mature, and the number of cats whose infections go undiagnosed.<\/p>\n In cats, immature heartworms reaching the pulmonary vessels often die, and in doing so, trigger an intense inflammatory reaction that causes vascular, interstitial, and alveolar disease\u2014a syndrome now termed heartworm-associated respiratory disease<\/strong> (HARD).2<\/sup><\/p>\n When adult HWIs are combined with the far more prevalent HARD, the true incidence of heartworm disease in cats is vastly greater than previously believed.3<\/sup> Whether damage is caused by 9-inch adult heartworms or immature adults, it is still considered “heartworm disease” as far as the cat’s lungs are concerned. Both adult infection and HARD are clinically significant, causing significant disease and even death (in patients infected with adult heartworms) in the cat.<\/p>\n Read Current Feline Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis<\/em>) Infection in Cats<\/strong> at heartwormsociety.org\/images\/pdf\/2014-AHS-Feline-Guidelines.pdf<\/a>.<\/p>\n<\/blockquote>\n Heartworm infection and disease in cats differ significantly from that seen in dogs (Table 1<\/strong>). While primarily considered a pulmonary vascular and secondary heart disease in dogs, in cats, the condition presents itself primarily as a lung disease.<\/p>\n Adult heartworms can live for 5 to 7 years in dogs and 2 to 4 years in cats. Should a cat be unlucky enough to have heartworms that reach the adult stage, even 1 or 2 mature adults represent a substantial worm burden due to the large size of the worm compared with the small size of the cat.<\/p>\n Ectopic infections (in the brain, body cavities, eyes, and systemic arteries), although uncommon, are more frequent in cats than dogs. Same-sex infections also occur more frequently in cats and account for about 1\/3 of feline infections. Unlike dogs, cats rarely have circulating microfilaria; if present, the microfilaria persist for only a short period. Therefore, the cat is considered a dead-end host.<\/p>\n The most common presenting signs in cats are:4<\/sup><\/p>\n The presenting respiratory signs can easily be mistaken for asthma, bronchitis, or an infection.<\/p>\n The largest study to date on client-owned cats demonstrated that 28% of infected cats were asymptomatic.4<\/sup> This may be due to those cats’ sedentary lifestyle\u2014exercise is directly correlated to severity of heartworm disease in dogs.5<\/sup><\/p>\n When the adult worm dies, the dying, fragmenting parasite worsens pulmonary inflammation and thromboembolism, often leading to fatal acute lung injury. Sudden death is sometimes the only sign of HWI in a cat, and such reactions in cats can occur from single-worm infections.<\/p>\n Diagnosing heartworm disease in cats is much more complicated than it is in dogs (Table 2<\/strong>). The American Heartworm Society (AHS) feline guidelines state: “The true prevalence of HWI in cats is probably understated due to diagnostic limitations, and the greater tendency of cats to exhibit only transient clinical signs or die without confirmation of infection.”2<\/sup><\/p>\n Concentrate testing (filter or modified Knott test) for microfilaria is inadequate due to limited and transient microfilaremia, higher incidence of same-sex infections, and low worm burdens. Nevertheless, approximately 10% of cats with HWI are microfilaremic, and this provides a definitive diagnosis. Therefore, when suspicion is high, a test for microfilariae is still indicated.<\/p>\n Serologic testing of cats for heartworm presents several challenges. No single test can accurately detect heartworms at all stages in the cat, so multiple test methods need to be used\u2014and repeated, if infection is suspected.<\/p>\n Antigen Testing.<\/strong> While the antigen test in dogs is considered the gold standard, it can detect only adult female worms. Negative antigen test results in cats can only be considered inconclusive, and false-negative antigen test results can be due to:2<\/sup><\/p>\n Antibody Testing.<\/strong> The antibody test can detect larval infections with both sexes of worms\u2014because the larvae of either sex can stimulate an early, detectable immune response\u2014as early as 2 months after infection. However, positive antibody tests do not indicate the continued existence of an infection, only that an infection (mature or immature, transient or permanent) has occurred.2<\/sup> False-negative results are a concern, because of varying sensitivities of different antibody tests.<\/p>\n Use of Both Tests.<\/strong> Both juvenile and mature adult worms cause clinical disease in the cat; therefore, both tests are useful and help facilitate an accurate diagnosis when combined.<\/p>\n Radiographs:<\/p>\n Approximately 50% of cats with adult HWI demonstrate radiographic changes (Table 3<\/strong> and Figures 1 and 2<\/strong>).6<\/sup> However:<\/p>\n FIGURE 1. Dorsoventral thoracic radiograph from a cat with heartworm disease; pulmonary changes are not dramatic but the right caudal lobar pulmonary artery is enlarged, while the opposite pulmonary artery is somewhat tortuous. Courtesy Dr. Clarke Atkins, North Carolina State University<\/em><\/p><\/div>\n\n
Learn More<\/h3>\n
Feline Versus Canine Heartworm Infection<\/h2>\n
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<\/a><\/p>\nDisease Differences<\/strong><\/h3>\n
Clinical Signs in Cats<\/strong><\/h3>\n
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Diagnostic Testing<\/h2>\n
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<\/a><\/p>\nMicrofilaria Testing<\/strong><\/h3>\n
Serology<\/strong><\/h3>\n
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Radiography<\/strong><\/h3>\n
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