Effect of strains of mice and challenge dose on the infectivity and virulence of Trypanosoma vivax

Factors influencing the infectivity and virulence of clone-derived Trypanosoma vivax for mice were investigated using a titration technique capable of recognizing differences in magnitude of about 10-fold. Marked differences were observed in the susceptibility and duration of infection in four inbred strains of mice after low dose (10(2.2)ID63) challenge. Only mild differences in survival period were observed when challenge doses of 10(3.2) and 10(4.2)ID63 per mouse were used. In all tests, Balb/C mice were most susceptible while CBA mice were most resistant. Long survival of infected mice was highly correlated with frequent occurrence of remission of parasitaemia. Infectivity of T. vivax isolates varied during the course of an infection, being high at the early stages of the infection and very low at later stages.