Estimates of the duration of the early and late stage of gambiense sleeping sickness

We first model the duration of stage 1 through survival analysis of untreated serological suspects detected during Médecins Sans Frontières interventions in Uganda and Sudan. We then deduce the duration of stage 2 based on the stage 1 to stage 2 ratio observed during active case detection in villages within the same sites.Survival in stage 1 appears to decay exponentially (daily rate = 0.0019; mean stage 1 duration = 526 days [95%CI 357 to 833]), possibly explaining past reports of abnormally long duration. Assuming epidemiological equilibrium, we estimate a similar duration of stage 2 (500 days [95%CI 345 to 769]), for a total of nearly three years in the absence of treatment.Robust estimates of these basic epidemiological parameters are essential to formulating a quantitative understanding of sleeping sickness dynamics, and will facilitate the evaluation of different possible control strategies.Human African trypanosomiasis (sleeping sickness, HAT) has historically been a predominant parasitic infection in Africa, causing many millions of deaths in the late 1800s through early 1900s, and has re-emerged in historical foci after breakdown of control programmes [1]. Two forms of HAT are recognised, due respectively to Trypanosoma brucei gambiense (a mainly human disease found mainly in western and central Africa) and Trypanosoma brucei rhodesiense (a zoonosis observed in eastern and southern Africa) [2]. Gambiense HAT has an insidious onset and progresses over years, while rhodesiense HAT is fulminant, causing death within a few months of infection [3]. In recent years, 10 000–15 000 HAT cases (mostly gambiense) are reported annually [4], but this is likely an underestimate of the true burden.Fundamental aspects of the pathogenesis, clinical profile and epidemiology of HAT remain poorly understood. Infecting trypanosomes first colonise the haemo-lymphatic system, evading specific immunity through antigenic variation, but causing only mild and intermittent symptoms –