Isolation of trypanosomes stocks from the field

One of the enduring problems in the epidemiology of sleeping sickness is that there are 3 morphologically indistinguishable subspecies of Trypanosoma brucei involved in a complex transmission cycle between humans, tsetse and reservoir hosts. Two subspecies, ir: b. gambiense and ir: b, rhodesiense, are infective to man and cause gambian and rhodesian sleeping sickness, respectively. The third subspecies ir: b. brucei cannot by definition infect humans, but coexists with the other trypanosomes in reservoir hosts and vectors. The advent of molecular methods for taxonomy brought the hope that unequivocal biochemical markers for the 3 subspecies would quickly be found. In particular, markers for the human infective trypanosomes would have enabled identification of reservoir hosts without recourse to experiments with human volunteers or serum resistance tests. However, the results of molecular characterization revealed a much more complex picture, with several subdivisions within 'I: brucei, rather than the 3 expected. Most isolates of T. b. gambiense fitted into one clearly demarcated group, but the other subdivisions did not correspond to the recognised subspecies. More significantly, analysis of the data using population genetics methods led to the discovery of genetic exchange in ir: brucei. The importance of genetic exchange in generating diversity among ir: brucei stocks in the field is still controversial, but the principle of gene flow destroys any remaining hope that stable markers for T. brzicei subspecies exist. In this chapter, we will first describe the methods for isolation and characterization of trypanosomes from the field and the mathematical methods of data analysis, before considering the implications of this work for epidemiology.