The role of the bone marrow in bovine trypanotolerance I. Changes in bloodand bone marrow in Trypanosoma congolense-infected cattle

This study compared the changes in the bone marrow (BM) of five trypanotole rant N'Dama cattle with those of four trypanosusceptible Boran cattle durin g trypanosome infection. In the early parasitaemic phase, from 12 to 21 day s postinfection (DPI), tsetse-transmitted primary Trypanosoma congolense IL 1180 infection induced parasitaemia, slight depression in packed cell volu me (PCV), marked leucopenia due to lymphocytopenia and eosinopenia, and thr ombocytopenia which were of similar intensity in Boran and N'Dama cattle. H owever, from 28 DPI until the end of the experiment on 112 DPI, the parasit aemia was higher in the Boran than in the N'Dama. Severe anaemia and leucop enia characterised by lymphopenia, neutropenia, eosinopenia and monocytopen ia persisted in Boran cattle. In contrast, the PCV values dropped gradually in N'Dama cattle and from 77 DPI recovered slowly to values just below pre infection levels by 112 DPI. The total and differential leucocyte counts of the N'Dama cattle stabilised at approximately two-thirds of preinfection v alues between 28 and 112 DPI, and were double those of the Boran. Marked th rombocytopenia occurred in both breeds. The anaemia was initially macrocyti c hypochromic but terminally became microcytic hypochromic in both breeds. Light and electron microscopic studies of sequential biopsies of the BM of these animals showed that the BM response was the key to these differences between the N'Dama and Boran. The biopsies of the BM of the N'Dama cattle w ere hypercellular (scored 4.5 +/- 1.0 compared to 4.0 for controls) with mi ld hyperplasia of erythroid cells and mild hypoplasia of myeloid cells from 28 to 112 DPI, endowing the animals with higher haemopoietic potential tha t enabled them to replace most lost cells. In contrast, the Boran cattle ha d hypocellular (scored 2.4 +/- 1.1) BM biopsies with relative erythroid hyp erplasia and myeloid hypoplasia. resulting in low capacity of cell replacem ent manifested as severe unremitting anaemia and leucopenia. The BM of both breeds showed moderate hyperplasia of cells of the mononuclear phagocyte s ystem. Therefore,, this study showed, for the first time, that BM response is a key determinant factor or trypanotolerance as it determines the animal 's capability for blood cell regeneration.