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Wildcat Creek Cattery Blog
The Facts About Feline Heartworm DiseasePrevalence and Role of HARD 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. 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, 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. 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—a syndrome now termed heartworm-associated respiratory disease (HARD). 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. 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.  Feline Versus Canine Heartworm Infection Heartworm infection and disease in cats differ significantly from that seen in dogs (Table 1). While primarily considered a pulmonary vascular and secondary heart disease in dogs, in cats, the condition presents itself primarily as a lung disease.  Disease Differences 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. 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. Clinical Signs in Cats The most common presenting signs in cats are:4
The largest study to date on client-owned cats demonstrated that 28% of infected cats were asymptomatic. This may be due to those cats’ sedentary lifestyle—exercise is directly correlated to severity of heartworm disease in dogs. 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. Diagnostic Testing Diagnosing heartworm disease in cats is much more complicated than it is in dogs (Table 2). 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.”  Microfilaria Testing 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. Serology 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—and repeated, if infection is suspected. Antigen Testing. 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:
Use of Both Tests. 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.
Radiographs:
Approximately 50% of cats with adult HWI demonstrate radiographic changes (Table 3 and Figures 1 and 2). However:
Echocardiography When identified, adult heartworms appear as parallel linear artifacts, commonly called railroad tracks or equal signs (Figure 3). They are found most often in the proximal main and right lobar branches of the pulmonary artery, making it necessary to methodically probe these areas. Due to the small size of feline pulmonary arteries relative to the larger size of adult heartworms, worms are more likely to extend from the peripheral branches into the proximal segments in cats compared with dogs. In cats, adult heartworms (%) are detected in the:
Treatment No approved treatment is available for adult FHWD; in fact, melarsomine is toxic in cats at doses as low as 3.5 mg/kg. Supportive CareFor acutely ill cats, the best course of action is supportive care, including:
For infected cats with clinical signs and/or radiographic evidence of lung disease, prednisone in diminishing doses is recommended: The starting dose is 2 mg/kg PO Q 24 H, declining gradually to 0.5 mg/kg every other day by 2 weeks; then discontinuation after an additional weeks. Treatment can be repeated if clinical signs recur. Contraindicated Medications Diuretics are inappropriate, even for infected cats with severe interstitial or patchy alveolar lung patterns. Aspirin and other nonsteroidal anti-inflammatory drugs have failed to produce demonstrable benefit and may actually exacerbate parenchymal pulmonary disease. Disease Monitoring Monitoring both antibody and antigen test results periodically, along with follow-up chest radiography, is appropriate for assessing progress.
Prognosis for cats with FHWD and HWI is roughly equivalent to that of cats with hypertrophic cardiomyopathy. In a study of 50 cats with naturally acquired HWI (n = 48, with adequate follow-up), the median survival duration was 1.5 years (range, 0–4015 days). Heartworm Prevention FHWD has been diagnosed in all 50 states and can occur wherever the disease is seen in dogs. Nevertheless, relatively few cats receive heartworm preventives because of long-held assumptions that cats are resistant to HWI and that the disease is rare. Heartworm prevention in cats is especially important given the lack of an approved or safe and effective treatment. The AHS recommends year-round use of heartworm preventives for cats as well as dogs, and owners have the option of oral or topical monthly preventives, with varying parasite spectra. All cats in heartworm-endemic areas should receive a preventive agent because multiple studies have shown that indoor cats are susceptible to HWI. An added benefit to administering heartworm preventives to cats is that several products also protect cats from fleas, ticks, and ear mites, as well as zoonotic parasites, such as Toxocara and Ancylostoma species, which present a year-round threat in many areas of the country. Summary Until recently, conventional wisdom held that heartworm disease in cats was a relatively minor disease threat. In reality, the overall rate of positivity for feline heartworm antigens is substantial. According to a study by IDEXX, the infection rate ranges from 0.9% to 4.6%, depending on the region of the country sampled.11 This compares to a nationwide average heartworm prevalence rate of 1.2% in dogs, a feline leukemia virus prevalence of 1.9%, and a feline immunodeficiency virus prevalence of 1%. The inaccurate perception of low heartworm incidence rates in cats can be traced to 2 factors:
NEW STUDY SHOWS TREATMENT WITH FELINE REVOLUTION® (SELAMECTIN) PRIOR TO HEARTWORM INFECTION PREVENTED LUNG DAMAGE IN CATSFLORHAM PARK, N.J., November 13, 2014 -- New research published in the Journal of Veterinary Parasitology shows that administering the monthly heartworm prevention REVOLUTION® (selamectin) prior to heartworm infection (i.e., mosquito season) prevented heartworm-associated lung damage in cats. The lungs of the cats pre-treated with REVOLUTION showed no difference from the uninfected control group. This study was conducted by a team of investigators led by A. Ray Dillon, DVM, MS, MBA; DACVIM (IM) at the Auburn University College of Veterinary Medicine.
The study, “Effect of pre-cardiac and adult stages of Dirofilaria immitis in pulmonary disease of cats: CBC, bronchial lavage cytology, serology, radiographs, CT images, bronchial reactivity, and histopathology,” demonstrated that pre-cardiac stages of D. immitis-induced chronic lung disease was prevented by the administration of REVOLUTION one month prior to infection. Radiographs and CT-scans of the heartworm infected, selamectin-treated cats were normal, antibody tests were negative and lungs were histologically the same as normal cats throughout the study. The authors concluded that, “The clinical implication of the study is that heartworm prevention should be initiated at least one month before the first risk of infection.” About the Study A controlled, masked research study was conducted to define the initial inflammatory response and lung damage associated with the death of precordia stages of D. immitis in cats as compared to adult heartworm infections in normal cats. In the study, eighteen cats (>6 mo.) were divided into three groups: infected, pretreated with selamectin and infected, and uninfected control. The selamectin pretreated and infected cats were administered 100 L3 by subcutaneous injection. The selamectin pretreated and infected cats were treated according to label directions 32 days and 2 days prior to L3 heartworm infection and continued to receive monthly treatments for the duration of the experiment. After infection, all cats were observed for 240 days. The full study, A. Ray Dillon, et al., “Effect of pre-cardiac and adult stages of Dirofilaria immitis in pulmonary disease of cats: CBC, bronchial lavage cytology, serology, radiographs, CT images, bronchial reactivity, and histopathology.” Vet Parasitol (2014) can be found here. About Heartworm Disease in Cats The incidence of heartworm disease in cats closely correlates with the infection rate in dogs, but in cats the disease is often a result of immature worms that never become adults. Serology demonstrates that in the endemic areas of the U.S. about 25 percent of cats develop positive antibody titers to heartworms. About REVOLUTION for Cats REVOLUTION is the veterinarian’s #1 choice in feline parasite protection. FDA-approved REVOLUTION is a unique medication proven to be safe and effective against adult fleas and flea eggs, heartworms, roundworms, hookworms and ear mites. REVOLUTION is available in three- and six-dose packs for a broad range of cat sizes.
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AuthorI'm Crystal Rhine, owner of Wildcat Creek Cattery LLC. Follow along to watch cute videos, see testimonies, cute pictures and Maine Coon information! 
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