Society, 2014). Right heart failure and cor pulmonale, if they do occur, can lead to pleural effusion (hydrothorax or chylothorax) as well as ascites in affected cats (Atkins, 2015). Late in the course of heartworm disease, the adult worms begin to die off. This represents the third stage of feline heartworm infection. This heartworm die-off can lead to another increase in pulmonary inflammation, as well as the formation of pulmonary thromboemboli that can lead to fatal pulmonary damage (American Heartworm Society, 2014). While adult heartworms typically live five to seven years in dogs, evidence suggests that the lifespan of heartworms in cats is much shorter, often only two to four years (Stannard, 2015). Therefore, any cat that is diagnosed with HARD is at risk of developing further lung injury two to four years later, as the adult worms begin to die off. Cats that survive the death of their adult heartworm infections enter the fourth stage of feline heartworm disease. These cats often experience permanent pulmonary damage, caused by the replacement of normal Type I alveolar cells with hyperplastic Type II alveolar cells (ESDA, 2017). In many cases, this leads According to one study, approximately 28% of heartworm- infected cats are completely asymptomatic (Stannard, 2015). Additionally, 82% of heartworm-infected cats will self-cure their heartworm infections (ESDA, 2017). These two factors can make diagnosis very challenging, especially given the lack of sensitive tests for feline heartworm disease. Even cats that do show clinical signs of disease may be difficult to diagnose, because heartworm disease may or may not appear on the differential diagnosis list. Signs vary significantly and are often not attributed to heartworm infection. Many infected cats lack any clinical signs beyond a vague appearance of malaise, or they may have nonspecific signs, such as intermittent vomiting (American Heartworm Society, 2014). In fact, 16% of cats with heartworm disease present to the veterinarian for vomiting that is unrelated to eating (Stannard, 2015). Vomiting can have a wide range of causes in cats and it is not uncommon for heartworm disease to not even make it onto the differential diagnosis list in these cats. In the early stages of heartworm infection, when cats are experiencing HARD, the patient’s clinical signs may be indistinguishable from feline asthma. Dyspnea is reported in 48% of cats with heartworm disease, while 38% of affected cats are noted to demonstrate coughing (Stannard, 2015). In some cats, a peracute syndrome may occur, resulting in respiratory
to permanent respiratory dysfunction and chronic respiratory disease. In summary, clinical signs are highest during Stage I, lowest during Stage II, then rise again in Stage III before declining slightly in Stage IV. This waxing and waning nature of clinical signs further contributes to the challenges associated with diagnosing feline heartworm disease. Cats are widely regarded as a dead-end host for heartworms. Approximately 1/3 of infections are same-sex infections and circulating microfilariae are rarely observed (Stannard, 2015). Cats are unlikely to serve as a reservoir for infection or be capable of infecting mosquitoes or transmitting heartworm disease to other hosts. Rarely, heartworms can migrate aberrantly and lead to ectopic infections in the brain, eyes, systemic arteries, and body cavities. Although this is not a common occurrence, it is more common in cats than it is in dogs and further points to cats being an aberrant, dead-end host for Dirofilaria immitis (Stannard, 2015; ESDA, 2017).
CLINICAL SIGNS OF HEARTWORM INFECTION IN CATS
distress, ataxia, collapse, and possible sudden death (American Heartworm Society, 2014). Cases of HARD often appear to increase and decrease in frequency based upon a seasonal pattern. Heartworm transmission in many areas of the U.S. peaks in July and August (American Heartworm Society, 2014). The acute inflammatory stage of HARD typically lasts six to eight months, which results in a seasonal peak in cases that typically occurs four to six months after peak heartworm transmission season (“A Roundtable Discussion,” 2008). Therefore, in the United States, cases of HARD typically peak in November through February. Any cat that presents for respiratory signs during this time of year should be carefully evaluated for the presence of HARD. In the later stages of heartworm infection, if and when adult worms develop, pulmonary and cardiac signs may be observed. Some cats may demonstrate a heart murmur, if adult worms lodge in the right atrioventricular valve and interfere with tricuspid valve function (American Heartworm Society, 2014). Signs of heart failure may also be present, such as dyspnea and jugular venous distention (Atkins, 2015). Less commonly, signs of feline heartworm disease can include seizures, ascites, pneumothorax, hydrothorax, and chylothorax (American Heartworm Society, 2014). In some cases, adult heartworm infection can remain asymptomatic until worm death results in pulmonary thromboembolism, which may be fatal.
DIAGNOSIS
Although heartworm testing in dogs is a relatively straightforward matter, involving a single heartworm antigen test, this is rarely the case in cats. Feline heartworm infections tend Antigen testing Although antigen testing is highly reliable for the diagnosis of heartworm disease in dogs, it is far less useful in the diagnosis of feline heartworm disease. Antigen tests detect an antigen that is found primarily in female heartworms. In general, these tests are capable of identifying most occult heartworm infections in which a single female adult heartworm is present. In dogs, large numbers of adult heartworms are often present. The likelihood of a single female worm being present is high, making the test very sensitive. In cats, however, approximately 30-50% of heartworm infections do not involve an adult female heartworm (American Heartworm Society, 2014). This makes antigen testing significantly less valuable in cats. Antigen testing is also capable of detecting infections in which over five dying adult male heartworms are present, but this is also an uncommon finding in cats because they tend to have very low worm burdens (Norsworthy, 2018). Ultimately, heartworm antigen testing is capable of detecting
to involve very low worm burdens and often involve single-sex infection. These two factors significantly increase the challenges associated with accurately diagnosing heartworm disease in cats.
only about 75% of adult heartworm infections (Norsworthy, 2018). And, as stated above, many cases of feline heartworm disease do not involve any adult worms; antigen testing is of no value in detecting these larval or immature adult infections. Therefore, the true sensitivity of heartworm antigen testing is very low in most cats. In recent years, a considerable amount of discussion has occurred surrounding the idea of heat-treating blood samples prior to heartworm antigen testing. In this protocol, samples are heated at 104°C for 10 minutes prior to performing a heartworm antigen test (Little et al., 2014). This heat treatment is intended to disrupt antigen-antibody complexes, making more heartworm antigen available for detection by a standard heartworm ELISA test. While this technique is primarily used in dogs, recent experimental infection studies have shown that it may reduce the number of false-negative antigen tests seen in feline heartworm disease as well (American Heartworm Society,
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