Transmissibility of Trypanosoma brucei during its development in cattle

Recent outbreaks of Trypanosoma brucei rhodesiense sleeping sickness in Soroti District of eastern Uganda have demonstrated the important role cattle can play as reservoirs of this parasite. To clarify the epidemiological importance of the cattle reservoir, experiments were conducted to determine the ease with which T. brucei is transmitted during the course of its development in Friesian cattle. The development of T. brucei in cattle is characterized by an acute phase with high levels of parasitaemia and a decline in PCV. The acute phase is followed by a chronic phase during which the PCV remains low but stable and the parasitaemia is low. Parasites are often difficult to detect using parasitological diagnostic tools during this chronic phase. Challenge of chronically infected cattle with T. congolense results in a sudden increase in the T. brucei parasitaemia. Despite significant differences in parasitaemia, the proportion of tsetse flies that developed metacyclic infections after a first bloodmeal on the infected cattle did not differ significantly between the acute and chronic phases or the phase of mixed T. b. brucei/T. congolense infection. This suggests that, throughout the observation period, the parasitaemia was above the threshold above which infection rates of tsetse are independent of the parasitaemia. The repercussions of the research findings for the understanding of the epidemiology, spread and the control of T. b. rhodesiense sleeping sickness are discussed.