Using Molecular Data for Epidemiological Inference: Assessing the Prevalence of Trypanosoma brucei rhodesiense in Tsetse in Serengeti, Tanzania

Abstract

BackgroundMeasuring the prevalence of transmissible Trypanosoma brucei rhodesiense in tsetse populations is essential for understanding transmission dynamics, assessing human disease risk and monitoring spatio-temporal trends and the impact of control interventions. Although an important epidemiological variable, identifying flies which carry transmissible infections is difficult, with challenges including low prevalence, presence of other trypanosome species in the same fly, and concurrent detection of immature non-transmissible infections. Diagnostic tests to measure the prevalence of T. b. rhodesiense in tsetse are applied and interpreted inconsistently, and discrepancies between studies suggest this value is not consistently estimated even to within an order of magnitude.Methodology/Principal FindingsThree approaches were used to estimate the prevalence of transmissible Trypanosoma brucei s.l. and T. b. rhodesiense in Glossina swynnertoni and G. pallidipes in Serengeti National Park, Tanzania: (i) dissection/microscopy; (ii) PCR on infected tsetse midguts; and (iii) inference from a mathematical model. Using dissection/microscopy the prevalence of transmissible T. brucei s.l. was 0% (95% CI 0–0.085) for G. swynnertoni and 0% (0–0.18) G. pallidipes; using PCR the prevalence of transmissible T. b. rhodesiense was 0.010% (0–0.054) and 0.0089% (0–0.059) respectively, and by model inference 0.0064% and 0.00085% respectively.Conclusions/SignificanceThe zero prevalence result by dissection/microscopy (likely really greater than zero given the results of other approaches) is not unusual by this technique, often ascribed to poor sensitivity. The application of additional techniques confirmed the very low prevalence of T. brucei suggesting the zero prevalence result was attributable to insufficient sample size (despite examination of 6000 tsetse). Given the prohibitively high sample sizes required to obtain meaningful results by dissection/microscopy, PCR-based approaches offer the current best option for assessing trypanosome prevalence in tsetse but inconsistencies in relating PCR results to transmissibility highlight the need for a consensus approach to generate meaningful and comparable data.

Epidemiology of Sleeping Sickness in Boffa (Guinea): Where Are the Trypanosomes?

“…T. b. gambiense has not been found in tsetse, confirming the usual (but poorly understood) very low (<0.1%) mature infection rates of T. b. gambiense in tsetse, even in active sleeping sickness foci ([39]. The same has been reported for T. b. rhodesiense (see [40]). …”

Towards an Early Warning System for Rhodesian Sleeping Sickness in Savannah Areas: Man-Like Traps for Tsetse Flies

“…However, the bloodmeal identifications and infection studies are complex, costly and long-winded. This is especially so with the savannah tsetse for which the proportion of humans in diet [12] and the T. brucei infection rate of the flies [13], [14] are typically very low, so that the confident and timely assessment of any changes would require the examination of thousands of tsetse per month. If the use of traps for routine monitoring for HAT risk in savannah situations is to be practicable, it should involve something quicker, simpler and cheaper, even if less comprehensive.…”

Barcoding in trypanosomes

“…However, this method is time consuming, dependent on operator expertise, unreliable for mixed infections, fails to detect immature infections and, in the case of African trypanosomes, is only useful for distinguishing between parasites to the level of subgenus (Ouma et al 2000;Gibson, 2009;Enyaru et al 2010;Auty et al 2012b;Mugasa et al 2014).…”