THE HEMATOLOGY OF TRYPANOSOMA-CONGOLENSE INFECTION IN CATTLE .1. SEQUENTIAL CYTOMORPHOLOGICAL CHANGES IN THE BLOOD AND BONE-MARROW OF BORANCATTLE

Five adult Boran cattle (Bos indicus), infected with a clone of Trypan osoma congolense IL13-E3 three years earlier and treated, were re-chal lenged with the same clone. Changes in the peripheral blood were monit ored twice weekly, and events in the bone marrow (BM) were assessed by weekly biopsies of the sternal BM, until day 98 postinfection (dpi) w hen the three surviving animals were treated with diminazene aceturate . One animal died on 57 dpi whereas another was treated on 63 dpi when the packed cell volume was 15%. The infected animals developed anaemi a, leucopenia and thrombocytopenia during the first peak of parasitaem ia which persisted until the experiment was terminated. Three phases o f BM response were demonstrated on light microscopic examination of BM smears. The first, the preparasitaemic phase represented by samples t aken on 15 dpi, was an immunological response with slight but signific ant increases in lymphoblasts, lymphocytes, plasma cells and macrophag es (M phi) whereas erythroid and granulocytic cells were unchanged. Th e second, the early parasitaemic or acute phase (21-57 dpi) associated with the development of anaemia, leucopenia and thrombocytopenia, was characterised by intensification of the immunological response, and a n early but transient granulocytic hyperplasia. The third, the late pa rasitaemic or chronic phase (63-98 dpi) associated with persisting pan cytopenia, was characterised by erythroid, megakaryocytic and M phi hy perplasia, dyserythropoiesis, granulocyte hypoplasia and return of lym phoid cell counts to preinfection numbers. Transmission electron micro scopy confirmed these findings and showed that intact trypanosomes wer e not observed in the sinusoids and haemopoietic compartment of the BM . This study demonstrates that T. congolense infection affects haemopo iesis, downregulating or upregulating the various blood cell lineages depending on the stage of infection. This suggests a fine control mech anism, presumably cytokine-mediated. Erythropoiesis, thrombopoiesis an d monocytopoiesis were generally upregulated, whereas granulopoiesis w as downregulated. However, haemopoiesis was generally ineffective as n umbers of circulating blood cells remained below preinfection levels t hroughout the period of the study.