trypanosomiasis (trəpăn´əsōmī´əsis), infectious disease caused by a protozoan organism, the trypanosome, which exists as a parasite in the blood of a number of vertebrate hosts. The three variations of the disease that predominate in humans are transmitted by an insect vector. Two types of African sleeping sickness are caused, respectively, by Trypanosoma brucei rhodesiense and T. brucei gambiense, both transmitted by the bite of the tsetse fly. South American trypanosomiasis, or Chagas' disease, is caused by T. cruzi, which is the most common cause of heart disease in South America. It is transmitted by certain species of bugs; the parasite enters the skin when infected bug feces are rubbed into the site of the bite.
The characteristic symptoms of Chagas' disease are edema; hard, red nodular outbreaks of the skin; and damage to the heart muscle. There is no effective treatment. Symptoms of African sleeping sickness may appear at once, after several weeks, or even after years in the Gambian type, which is the most common form. Early disturbances include inflammation at the site of the bite, intermittent fever, enlargement of the spleen; in the Gambian variety the lymph nodes are enlarged. Subsequent signs of heart damage, personality changes, and headache develop. The final stages are marked by tremor, disturbed speech and gait, emaciation, and a prolonged comatose state. African trypanosomiasis is treated with pentamidine or suramin, which are effective when injected in early stages of the disease; in the second stage, when the nervous system is affected, treatment involves melarsoprol or nifurtimox and eflornithine. Even with treatment, organ damage appears irreversible and the disease is often fatal; the prognosis becomes grave after the nervous system is invaded. Prevention involves the use of insecticides and the clearing of vegetation that harbors the tsetse fly. A form of trypanosomiasis known as nagana affects cattle, leading to enormous annual economic losses.