The differentiation of Trypanosoma rhodesiense from T. brucei by means of human serum

A review is made of the Blood Incubation Infectivity Test (BIIT) introduced by Rickman and Robson (1970) to differentiate Trypanosoma rhodesiense from T. brucei in the laboratory. This test consists of incubating the trypanosomes with human blood and then injecting them into a rat; if the rat becomes infected, the trypanosomes are probably T. rhodesiense, and conversely. In the present paper it is shown that most of the effect of the human blood or serum is exerted after injection into the animal, and not during the previous incubation in vitro; moreover, the effect is proportional to the dose of blood per weight of animal. Accordingly, improved methods of conducting the test are recommended, especially the inoculation of the trypanosomes—blood mixture into 5 mice (rather than 1 rat), each animal to receive at least 0·4 ml. human blood per 20 g. weight. Further, the test should be interpreted according to statistical definitions of what is to be accepted as “serum-resistance”. Provisionally a strain which can infect 20% of the animals (treated with human blood as above) might well be regarded as “serum-resistant” (probably T. rhodesiense) and so potentially dangerous to man.