Identification of human-infective trypanosomes in animal reservoir of sleeping sickness in Uganda by means of serum-resistance-associated (SAR) gene

Background The expansion of sleeping sickness caused by Trypanosoma brucei rhodesiense beyond its traditional focus in southeast Uganda has been linke d with large-scale livestock restocking. To assess the risk presented to th e human population by domestic livestock, human-infective T b rhodesiense m ust be distinguished from non-human-infective T brucei brucei, since both p arasites can be present in cattle. We investigated the use of a simple gene tic marker to characterise parasites collected from cattle in villages with in the new sleeping sickness focus in Soroti District, Uganda. Methods 70 T brucei sl samples of known human infectivity status collected from human beings and cattle in Tororo District, Uganda, from 1989 to 1991 were screened for the presence of the human-serum-resistance-associated (SR A) gene by conventional PCR. In 2000-01, blood samples from 200 randomly se lected cattle in six villages and two markets in Soroti District were scree ned for T brucei sl parasites (ie, both T b brucei and T b rhodesiense) by PCR; positive samples were screened for the presence of the SRA gene. Findings The SRA gene was present in all 29 samples from patients with slee ping sickness in Tororo District. Of the 41 samples collected from cattle a t the same time, the SRA gene was present in the eight samples that tested resistant to human serum in vitro, whereas it was absent from all 33 isolat es that were sensitive to human serum in vitro. Of the 200 cattle sampled i n Soroti District, we estimated that up to 18% (95% CI 12-23) were infected with T b rhodesiense. Interpretation Detection of the SRA gene could provide the basis for a simp le diagnostic test to enable targeted control of T b rhodesiense in the dom estic livestock reservoir, thereby reducing the public-health burden of sle eping sickness in east Africa.