Although the genus Trypanosoma contains many species of protozoans, only T. cruzi, T. brucei gambiense, and T. brucei rhodesiense cause disease in humans. T. cruzi is the etiologic agent of Chagas' disease in the Americas; T. b. gambiense and T. b. rhodesiense cause African trypanosomiasis.
Chagas' disease, or American trypanosomiasis, is a zoonosis caused by the protozoan parasite T. cruzi. Acute Chagas' disease is usually a mild febrile illness that results from initial infection with the organism. After spontaneous resolution of the acute illness, most infected persons remain for life in the indeterminate phase of chronic Chagas' disease, which is characterized by subpatent parasitemia, easily detectable antibodies to T. cruzi, and an absence of associated signs and symptoms. In 10–30% of chronically infected patients, cardiac and/or gastrointestinal lesions develop that can result in serious morbidity and even death.
LIFE CYCLE AND TRANSMISSION
T. cruzi is transmitted among its mammalian hosts by hematophagous triatomine insects, often called reduviid bugs. The insects become infected by sucking blood from animals or humans who have circulating parasites. Ingested organisms multiply in the gut of the triatomines, and infective forms are discharged with the feces at the time of subsequent blood meals. Transmission to a second vertebrate host occurs when breaks in the skin, mucous membranes, or conjunctivae become contaminated with bug feces that contain infective parasites. T. cruzi can also be transmitted by the transfusion of blood donated by infected persons, by organ transplantation, from mother to unborn child, by ingestion of contaminated food or drink, and in laboratory accidents.
Initial infection at the site of parasite entry is characterized by local histologic changes that include the presence of parasites within leukocytes and cells of subcutaneous tissues and the development of interstitial edema, lymphocytic infiltration, and reactive hyperplasia of adjacent lymph nodes. After dissemination of the organisms through the lymphatics and the bloodstream, primarily muscles (including the myocardium) (Fig. 27-1) and ganglion cells may become heavily parasitized. The characteristic pseudocysts present in sections of infected tissues are intracellular aggregates of multiplying parasites.
Trypanosoma cruzi in the heart muscle of a child who died of acute Chagas' myocarditis. An infected myocyte containing several dozen T. cruzi amastigotes is in the center of the field (hematoxylin and eosin, 900×).
In individuals with chronic T. cruzi infections who develop related clinical manifestations, the heart is the organ most commonly affected. Changes include thinning of the ventricular walls, biventricular enlargement, apical aneurysms, and mural thrombi. Widespread lymphocytic infiltration, diffuse interstitial fibrosis, and atrophy of myocardial cells are often apparent, but parasites are difficult to find in myocardial tissue by conventional histologic methods. Conduction-system abnormalities often ...