2016). This is perhaps easy to understand, as cats often have even lower levels of circulating antigen than dogs and they mount a significantly greater inflammatory response. Disrupting antigen-antibody complexes may help free small levels of circulating antigen enough to allow detection. While heat treatment of samples prior to heartworm antigen is not currently recommended for routine heartworm testing, it may be valuable if a cat has a negative heartworm antigen test despite a strong Antibody testing Heartworm antibody tests are an alternative serologic test that can be used to diagnose feline heartworm disease. While a positive antibody test does not indicate ongoing infection with heartworms, it does indicate previous exposure to heartworm larvae. Antibody tests in cats typically turn positive approximately two months after exposure to heartworm infection, meaning that antibody tests can be beneficial in the diagnosis of HARD (which tends to occur early in the course of heartworm infection). These tests are highly specific (up to 98%), but have a relatively low Microfilariae testing Although microfilariae testing is often used as a component of heartworm testing in dogs, this test is rarely performed in cats. Infected cats rarely have detectable microfilariae, because most infections are single-sex infections and adult heartworms typically die without producing microfilariae. The chances of finding microfilariae, however, may be increased with the use of a Knott’s test or filter technique (American Heartworm Society, Thoracic radiographs Thoracic radiographs cannot definitively diagnose heartworm disease, but they can help support a clinical suspicion of Dirofilaria immitis . It is important to keep in mind, however, that the radiographic findings associated with feline heartworm disease may be entirely absent or transient in nature (ESDA, 2017). This significantly reduces the diagnostic utility of thoracic radiographs, except as a preliminary screening technique. Cats with heartworm disease may develop visible enlargement of the main lobar and peripheral pulmonary arteries. These changes can be observed as tortuosity, loss of taper, and truncation in the caudal lobar branches (especially on the right side). Typically, these changes are most visible on the ventrodorsal view. Main pulmonary artery enlargement may also be observed in cats with adult heartworm infection, but this is Echocardiography Echocardiography can be a valuable technique for visualizing adult heartworms within the main and right branches of the pulmonary artery, right ventricle, and other areas of the heart. On echocardiogram, heartworms are visible as short, segmented, parallel linear artifacts, due to the strong echogenicity of the heartworm body. In some cases, it is even possible to determine whether a heartworm is dead or alive based on echocardiography; dead heartworms typically are collapsed on their sides, resulting in the appearance of two lines that are not quite parallel (American Heartworm Society, 2014). It is important to resist the temptation to count worms on echocardiography; the worms are often serpentine, coiling back and forth on themselves, and this makes obtaining an accurate worm count impractical (ESDA, 2017). While not all heartworms will be in a location that allows visualization with echocardiography, echocardiography does Takeaway messages regarding testing When performing serologic testing for heartworm disease, sensitivity can be improved by performing both an antigen and an antibody test concurrently. Additional tests, such as imaging, may be required for the diagnosis of many cases.
clinical suspicion of heartworm infection (for example, a positive antibody test or supportive radiographic findings). In summary, a positive heartworm antigen test in a cat confirms adult heartworm infection. A negative heartworm antigen test in a cat decreases the likelihood of adult heartworm infection, but does not rule it out completely.
sensitivity. Necropsy studies of naturally infected cats found in animal shelters suggest that heartworm antibody tests will accurately detect disease in anywhere from 32-89% of cats infected with adult heartworms (American Heartworm Society, 2014). A positive heartworm antibody test in a cat indicates exposure and a heightened risk of active or ongoing heartworm infection but does not confirm active infection. A negative heartworm antibody test in a cat decreases the likelihood of heartworm infection but does not completely rule it out. 2014). Using these methods, approximately 10% of heartworm- infected cats have detectable microfilariae (Stannard, 2015). Given the low sensitivity of this test, it is rarely used in cats. If Dirofilaria immitis microfilariae are observed, however, it is diagnostic for heartworm disease. not a consistent finding and can only be appreciated on a lateral thoracic radiograph (American Heartworm Society, 2014; ESDA, 2017). The radiographic findings associated with Toxocara and Aelurostrongylus are very similar to those associated with heartworm disease, so it is important to keep these differentials in mind when evaluating feline thoracic radiographs. Additionally, radiographs of cats with HARD may be difficult to distinguish from the radiographs of feline asthma patients. In summary, thoracic radiographs cannot provide a definitive diagnosis or rule out feline heartworm disease, but they may support a clinical suspicion and suggest a need for further testing. allow the diagnosis of many cases of adult heartworm infection. Depending on the operator’s level of skill, the sensitivity of echocardiography for the detection of adult heartworms is 88- 100% (ESDA, 2017). False-positive results on echocardiography are rare but can occur when chordae tendineae are mistaken for adult heartworms in the right ventricle (ESDA, 2017). Unlike dogs, cats with heartworm disease do not typically develop pulmonary hypertension. Therefore, echocardiographic signs of right atrial and right ventricular hypertrophy and high- velocity tricuspid regurgitation are rarely observed with adult heartworm infection (ESDA, 2017). A positive echocardiogram provides definitive evidence of heartworm disease, while a negative echocardiogram does not rule out heartworm infection.
When interpreting heartworm test findings, the significance of results can be summarized as follows: ● Heartworm antigen test: ○ Positive : Confirms infection with adult heartworms.
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