The pathogenicity of cyclically transmitted C16 clone 1 of Trypanosoma brucei metacyclics which developed within normal and hypertrophied DNA virus-infected salivary glands of Glossina morsitans morsitans was studied in the inbred, BALB/c mice. Microscopic examination of salivary glands obtained from G. m. morsitans with combined virus and T. brucei infections (i.e. the flies which transmitted the trypanosome infection to the test group of mice, group A) revealed that the glands comprised hyperplastic epithelial cells, some of which were fragmented, and numerous metacyclic trypanosomes. All the mice which contracted trypanosomiasis from G. m. morsitans with only T. brucei, i. e., the control mice (Group B; n=4) and the mice in the test group (n = 3) developed high parasitaemia and died. Repeated-measures analysis of variance using adjusted mean rate revealed that the mean prepatent periods and the mean times to death in both the control and the test groups of the BALB/c mice were not significantly different (P > 0.05). Analysis of log-transformed data fitted to a logistic growth model revealed that the rate of multiplication of T. brucei parasites in the blood of the test group BALB/c mice (r=2.373) was greater than in the control mice (r=0.808) and that the maximum carrying capacity was also attained earlier in the former group. These observations imply that the development of T. brucei metacyclics within hypertrophied salivary glands and their co-existence with the DNA virus particles might have enhanced their infectivity and virulence in the mice.

The epidemiological importance of differences in transmissibility, infectivity, antigenicity, pathogenicity and drug sensitivity among species and strains of the African pathogenic trypanosomes is well known. These aspects of trypanosomiasis have been studied particularly intensively in the case of Trypanosoma brucei, but less attention has been given to T. congolense and T. vivax, the two most serious trypanosomal pathogens of domestic livestock. Our recent studies have included experiments involving the cyclical transmission by Glossina morsitans of stocks of T. congolense from locations in East and West Africa to rabbits, calves and sheep, and examinations of formation of local skin reactions and the antigenicity of the trypanosomes developing in these hosts. Differences have been observed in the occurrence of local skin reactions in different host species, and antigenic differences have been found between trypanosome stocks from different areas by serological tests and immunisation and challenge experiments. Differences in the serological responses of rabbits to different trypanosome stocks have also been observed during estimations of antibody levels using enzyme-linked immunosorbent assays.