male worms are present, the test will be negative. Even with female worms, an antigen test usually only becomes positive seven to eight months after infection. Therefore, a cat may have significant clinical signs long before the development of a positive test. Heartworm-associated respiratory disease can be found in cats that never develop adult heartworms and therefore never have a positive antigen test. An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to D. immitis, so a cat that has successfully eliminated an infection may still be positive for up to three months. The antibody test is more sensitive than the antigen test, but it does not provide direct evidence of adult infection. It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for heartworm-associated respiratory disease. X-rays of the chest of a heartworm-infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries. Treatment and prevention Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) topical solution. Ivermectin, milbemycin, and selamectin are approved for use in cats in the US. Arsenic compounds have been used for heartworm adulticide treatment in cats, as well as dogs, but seem more likely to cause pulmonary reactions. A significant number of cats develop pulmonary embolisms a few days after treatment. The effects of melarsomine are poorly studied in cats. Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended, and no drugs are approved in the US for this purpose in cats. Treatment typically consists of putting the cat on a monthly heartworm preventive and a short-term corticosteroid. Surgery has also been used successfully to remove adult worms. The prognosis for feline heartworm disease is guarded. Heartworm infection in humans Dirofilaria are important medical parasites, but diagnosis is unusual and is often only made after an infected person happens to have a chest X-ray following granuloma formation in the lung. The nodule itself may be large enough to resemble lung cancer on the X-ray, and requires a