Infectivity and virulence of Trypanosoma (Trypanozoon) brucei for mice. IV. Dissociation of virulence and variable antigen type in relation to pleomorphism

Previous observations have indicated the possibility that the virulence of a trypanosome is closely associated with its variable antigen type. A set of clone populations has now been examined, all of which bear the same variable antigen. The clones isolated close to the point of isolation were found to be of moderate virulence to mice, whereas those isolated from the same stock following repeated passage were of extreme virulence. Virulence and variable antigen type need not, therefore, necessarily be associated and would appear to depend on the degree of pleomorphism. The degree of virulence exhibited by the clones was found not to be determined by their apparent rate of growth from inoculation to first patency.