Trypanosoma theileri associated with T-lymphocytes isolated from a latently infected cow.

Trypanosoma theileri infection, latent in a mature Hereford cow, could not be demonstrated in routine blood smears or cultures. Throughout the 2-year period an intravenous injection of dexamethasone consistently produced parasitaemia which was detectable in peripheral blood mononuclear cell (PBMC) cultures. Fractionation techniques such as plastic adherence and Sephadex-G10 fractionation, designed to deplete monocytes and enrich T-lymphocytes, increased trypanosome-positive cultures from 25 to 100%. Removal of B-lymphocytes from Sephadex, non-adherent (SE-NA) cells did not reduce the percentage of positive cultures. Light and transmission electron microscopy of SE-NA PBMC cultured for 36 or 45 h revealed numerous trypanosomes and widespread T-lymphocyte destruction. No trypanosomes were observed in 12-h cultures. A close association, either extra- or intracellular, of a parasitic stage of T. theileri with T-lymphocytes is inferred.