A light and electron microscopic study of changes in blood and bone marrow in acute hemorrhagic Trypanosoma vivax infection in calves.
Eleven 6-month-old calves were tsetse fly challenged with a stock of T. vivax (IL 2337) that causes haemorrhagic infection. The calves were killed at intervals of 4 to 6 days; 2 other calves served as controls. Peripheral blood changes included anaemia, thrombocytopenia, and an initial leukopenia. Later in the course of infection, leukocytosis associated with lymphocytosis and neutropenia developed. Moderate reticulocytosis (highest mean count 3.6±3.7%, maximum count 9.4%) accompanied the first wave of parasitaemia, but poor response (highest mean 0.4±0.0%) occurred during the second wave, despite persistence of severe anaemia. Light microscopic examination of bone marrow samples showed a drop in the myeloid:erythroid ratio with a decrease in granulocytes, particularly metamyelocytes, bands, and segmenters. Increase in lymphocyte counts corresponded with the appearance of lymphoid nodules within the marrow. Megakaryocytic volume increased in infected animals, and some megakaryocytes showed emperipolesis of red cells, neutrophils, and lymphocytes. Transmission electron microscopic examination of the bone marrow showed that trypanosomes had crossed the sinusoidal endothelium into the haematopoietic compartment as early as the second day of parasitaemia. Macrophages proliferated in the bone marrow; and from the second day of parasitaemia until the end of experimental infection, on day 46, the macrophages had phagocytosed normoblasts, eosinophil and neutrophil myelocytes, metamyelocytes, bands, and segmenters, as well as reticulocytes, erythrocytes and thrombocytes. It was concluded that, dyserythropoiesis and dysgranulocytopoiesis were responsible, in part, for the observed anaemia and granulocytopenia, respectively.