Infection of animals and, rarely, humans with nematodes of the genus Dirofilaria; cats are an atypical host and are susceptible to very samll worm burdens, even a few worms may lead to sudden death. In dogs, the definitive hosts, clinical signs include dyspnea, cough, reduced exercise tolerance, and weight loss. In cats, sudden death, vomition, and less commonly, a syndrome resembling asthma may occur. Microfilariae circulate in the bloodstream where they are picked up by mosquito vectors; death of the adult worms during treatment may release antigens in the bloodstream, which can lead to serious side effects including anaphylaxis, expecially in cats. Human infection is typically subclinical, with development of small pulmonary nodules that are often only picked up when they are confused with small tumors on X-ray.