minimal dosing schedules. Testing a dog annually for heartworms and other internal parasites is a fundamental part of a complete heartworm prevention program, and is also recommended for dogs who are already on a monthly prevention program. Heartworm infection in cats While dogs are a natural host for D. immitis, cats are atypical hosts. Because of this, differences between canine and feline heartworm diseases are significant. The majority of heartworm larvae do not survive in cats, so unlike in dogs, a typical infection in cats is two to five worms. The lifespan of heartworms is considerably shorter in cats, only two to three years, and most infections in cats do not have circulating microfilariae. Cats are also more likely to have aberrant migration of heartworm larvae, resulting in infections in the brain or body cavities. The infection rate in cats is 1–5% of that in dogs in endemic areas. Both indoor and outdoor cats are infected. The mosquito vector is known to enter homes. Pathology The vascular disease in cats that occurs when the L5 larvae invade the pulmonary arteries is more severe than in dogs. A syndrome related to this inflammatory reaction has been identified in cats: heartworm-associated respiratory disease, which can occur three to four months after the initial infection, and is caused by the presence of the L5 larvae in the vessels. The subsequent inflammation of the pulmonary vasculature and lungs can be easily misdiagnosed as feline asthma or allergic bronchitis. Obstruction of pulmonary arteries due to emboli from dying worms is more likely to be fatal in cats than dogs because of less collateral circulation and fewer vessels. Heartworms can live for 2 to 3 years in cats. Signs and symptoms Acute heartworm disease in cats can result in shock, vomiting, diarrhea, fainting, and sudden death. Chronic infection can cause loss of appetite, weight loss, lethargy, exercise intolerance, coughing, and difficulty breathing. Some cats' immune systems are able to clear a heartworm infection, though the immune system response can cause many of the same symptoms. Also, even if the infection resolves, respiratory damage can cause some symptoms to persist beyond it. Diagnosis Diagnosis of heartworm infection in cats is problematic. Like in dogs, a positive ELISA test for heartworm antigen is a very strong indication of infection. However, the likelihood of a positive antigen test depends on the number of adult female worms present. If only