Quantitative evaluation of the strategy to eliminate human African trypanosomiasis in the Democratic Republic of Congo

Abstract

Background: The virulent vector-borne disease, Gambian human African trypanosomiasis (HAT), is one of several diseases targeted for elimination by the World Health Organization. This article utilises human case data from a high-endemicity region of the Democratic Republic of Congo in conjunction with a suite of novel mechanistic mathematical models to address the effectiveness of on-going active screening and treatment programmes and compute the likely time to elimination as a public health problem (i.e. <1 case per 10,000 per year).

Methods: The model variants address uncertainties surrounding transmission of HAT infection including heterogeneous risk of exposure to tsetse bites, non-participation of certain groups during active screening campaigns and potential animal reservoirs of infection.

Results: Model fitting indicates that variation in human risk of tsetse bites and participation in active screening play a key role in transmission of this disease, whilst the existence of animal reservoirs remains unclear. Active screening campaigns in this region are calculated to have been effective, reducing the incidence of new human infections by 52-53 % over a 15-year period (1998-2012). However, projections of disease dynamics in this region indicate that the elimination goal may not be met until later this century (2059-2092) under the current intervention strategy.

Conclusions: Improvements to active detection, such as screening those who have not previously participated and raising overall screening levels, as well as beginning widespread vector control in the area have the potential to ensure successful and timely elimination.

Fig. 1

The top figure shows the total level of screening and detection in the Yasa-Bonga and Mosango health zones between 2000 and 2012. The bottom figure shows the annual incidence based on an assumed population size of 289,030 in relation to the WHO 2020 goal of elimination as a public health problem (shown in green)

Fig. 2

Multi-host model of HAT with various host groups able to acquire and transmit HAT infection (humans and reservoir animals), further non-reservoir animal species (others) and tsetse. Human hosts follow the progression which includes an infectious stage 1 disease, I _H1, infectious stage 2 disease , I _H2, and a non-infectious (due to hospitalisation) disease, R _H. Unfed tsetse are susceptible, S _V, and following a blood-meal become either exposed, E _V, or have reduce susceptibility to the trypanosomes , G _V. Tsetse select their blood-meal from one of the host species. Any blood-meals taken upon non-reservoir hosts do not result in infection. The transmission of infection between humans/tsetse and reservoirs/tsetse is shown by grey paths. Additional humans follow the same progression as the first human type but may receive more bites (high-risk) or may not participate in screening. Transmission from additional humans to tsetse is not shown here but occurs in the same way as humans to tsetse

Fig. 3

The reported incidence data and the corresponding Models 1, 4, 6 and 7 under posterior median parameterisation. Green lines show the simulated new infection incidence as well as passive, active and total reported cases under each model. N.B Model 1 has a different y-axis scale to the others figures

Fig. 4

Projected incidence for HAT beginning in 2013 for Models 1, 4, 6 and 7 under posterior median parameterisation. Stars show the simulated years of elimination as a public health problem under either (i) continuing with the mean level of screening or (ii) continuing with the maximum level of screening achieved between 2000 and 2012. The imperfect specificity of the diagnostic algorithm used in active screening results in persistent reporting of actively detected cases even after elimination has been achieved