Epidemiology and Pathology

Transmission of Chagas' disease can occur by blood transfusion, and this is becoming increasingly important, since many blood donors in hyperendemic areas are already infected. One of the most useful tests for detecting Chagas' disease is the ELISA method, which is also used in AIDS testing, but a 1990 report showed that only 60% of Brazil's 5.5 million blood transfusions each year use blood that has been tested. The poor, who need blood transfusions after surgery or trauma, cannot afford to pay for tested blood; in the worst areas 10% of blood was found to be infected. Poorer countries, such as Bolivia with more than 500,000 infected people in a population of over 7 million have few resources available to test blood for Chagas' disease carriers. Rarely, transmission may occur through food contaminated by feces or urine from an infected animal. Fetuses may be infected by transplacental transmission during either the acute or chronic phase of Chagas' disease. Sequelae from such an infection may include spontaneous abortion, intrauterine growth retardation, myocarditis, neurological changes, and perinatal death. Transmission may also occur by organ transplantation and, in these cases, infection is more severe due to immunosuppressive therapy.

Trypanosoma cruzi was first described by Carlos Chagas in the state of Minas Gerais, Brazil, in 1909. It is a tiny pleomorphic protozoan which divides its life cycle between man (and animals) and the bug vectors. Trypomastigotes, the flagellated forms of trypanosomes, circulate in the blood of infected individuals. In stained blood smears they appear as C- or U-shaped flagellated organisms (unlike African trypanosomes responsible for sleeping sickness, which are S-shaped) (Fig.4.4A). Amastigotes, also called leishmanial forms, are intracellular. When a trypomastigote invades the host tissues it enters the host cells, especially reticuloendothelial cells, muscle and glia. In so doing, it loses its flagellum and undulating membrane and rounds up into an amastigote, a tiny round body 3 to 5 micra in diameter (Fig. 4.4B). Division occurs by successive binary fission to form intracellular masses of amastigotes (pseudocysts). When the cell ruptures or disintegrates, amastigotes are transformed to trypomastigotes which invade more host cells and enter the blood.

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Fig. 4.4 (A) Photomicrograph of a blood smear containing several trypomastigotes. The parasite can be recognized by its large anteriorly placed kinetoplast and its C or U shape. x1020. AFIP 56-3455. (B) High magnification of myocardium in acute Chagas' myocarditis. A mononuclear cell infiltrate surrounds a degenerating myofiber in which large numbers of amastigotes are enclosed within a pseudocyst. The heart was extensively involved with a pancarditis.