Do Cryptic Reservoirs Threaten Gambiense-Sleeping Sickness Elimination?

Abstract

Trypanosoma brucei gambiense causes human African trypanosomiasis (HAT). Between 1990 and 2015, almost 440000 cases were reported. Large-scale screening of populations at risk, drug donations, and efforts by national and international stakeholders have brought the epidemic under control with <2200 cases in 2016. The World Health Organization (WHO) has set the goals of gambiense-HAT elimination as a public health problem for 2020, and of interruption of transmission to humans for 2030. Latent human infections and possible animal reservoirs may challenge these goals. It remains largely unknown whether, and to what extend, they have an impact on gambiense-HAT transmission. We argue that a better understanding of the contribution of human and putative animal reservoirs to gambiense-HAT epidemiology is mandatory to inform elimination strategies.

Keywords: Trypanosoma brucei gambiense; elimination; human African trypanosomiasis; reservoir; sleeping sickness; transmission.

Figure I

Outcomes of Human Infection with Trypanosoma brucei gambiense. get infected with T. When naïve persons (uninfected), without specific antibodies (TL−) and without parasites (P−) become infected with T. b. gambiense, they undergo an early phase of the disease with detectable parasitaemia (P+) but without detectable specific antibodies. Thereafter, most of them develop the disease (HAT patient) and are characterised by specific antibodies (TL+) and detectable parasitaemia (P+). Some remain asymptomatic (latent infection) with detectable specific antibodies but without detectable parasites (TL+, P−). Evidence for self-cure comes from asymptomatic people who also eventually become negative for specific antibodies (TL−, P−).

Figure 1

Trypanosoma brucei gambiense in Nonhuman Mammals. The map shows gambiense-human African trypanosomiasis in endemic countries and sites where T. b. gambiense infection in nonhuman mammals has been investigated with direct and indirect methods. Circles represent direct or indirect evidence of presence (red) and of absence (green) of T. b. gambiense in the period 1990–2016. For this period, data are mapped at the village/site level. (Blue) stars represent presence of detection in the years prior to 1990. For this period, data are mapped at the country level. All source references are provided in Tables S1 and S2 in the supplemental information online.

Figure 2

Key Figure: Unknown Elements in Human African Trypanosomiasis Progression and Transmission Solid lines represent progression between disease states, and dashed lines represent transmission of the parasites to and from the tsetse vector. Red boxes denote people or animals that may be infective to tsetse, with the darker shades denoting possible greater infectiveness. The figure highlights key unknown elements in disease progression and transmission including: (1) the probability of an infection leading to latent or stage 1 disease in humans – if, and how frequently; (2) self-cure of infected humans or (3) animals arises; (4) the duration of latent infection in humans, or (5) any infections in animals; and (6) the relative probability of transmission to tsetse from different types of infections (accounting for host feeding preferences).