Further studies on acquisition of potential infectivity for man in closely related Trypanosoma Trypanozoon brucei clones.

Twelve clones were isolated from relapsing populations in an infection initiated with a clone-derived human serum resistant Trypanozoon. Isolates were serotyped by the immunotrypanolysis test and four distinct variable antigen types (VATs) designated WITat 4, 5, 6 and 7 were identified. Contemporaneous blood incubation infectivity testing (BIIT) showed that most clones isolated in the course of infection were resistant to normal human serum while some clones isolated after series of relapses were susceptible. Four clones (G1, G2, G7 and G15) designated WITat 5, WITat 6, WITat 6 and WITat 5 respectively behaved differently in the BIIT. One of each pair was readily sensitive while the two others were highly resistant to human serum. This observation suggests that resistance to human serum might be dissociated from variable antigen type in a serodeme of Trypanosoma rhodesiense. The resistance or susceptibility to normal human serum by Trypanozoon is not immutable but is determined by the environment in which the trypanosomes live.