During the ensuing 20 years, the number of new cases of HAT (gambiense and rhodesiense) reported to WHO decreased from more than 25 000 cases at the beginning of the 20th century to fewer than 10 000 cases (9878 cases) in 2009 and to below 1000 cases in 2018. This reduction of more than 96% in 20 years (2000–2019) reflects the control efforts during this period, based on active testing (2–3 million people) in endemic areas and passive case detection in health facilities located in the same areas (increased in the past years). A key element of control was to ensure adequate treatment for all detected cases.

Obviously, the number of cases is under‐detected as the disease is difficult to diagnose and occurs in rural, remote and sometimes unsafe areas where health systems are weak. Nevertheless, epidemiological data and increased surveillance coverage indicate a decreasing percentage of non‐reported cases.

The current epidemiological scenario of HAT is not homogeneous. Four different contexts can be seen.

Gambiense area

• West Africa: very low prevalence. Some historically endemic countries have not reported cases in more than 10 years, or only very sporadic cases (Benin, Burkina Faso, Ghana, Mali, Togo) despite ongoing control activities. Some other countries (Guinea Bissau, Gambia, Liberia, Senegal, Niger, Sierra Leone) do not declare cases, and although control activities are irregular, rough assessments suggest that cases of HAT are not present. In Côte d'Ivoire and Nigeria, transmission is present but prevalence is very low and limited to well defined foci. The situation is different in Guinea, where transmission is still moderately active but also limited to well‐defined foci.
• Central Africa: active transmission with low prevalence. Foci are well known and well limited, with a very low (Cameroon, Equatorial Guinea, Gabon) or low prevalence (Angola, Central African Republic, Chad, Congo, South Sudan, North West Uganda) and a generally decreasing trend. Control and surveillance activities are ongoing. A persistent threat is the unstable security situation in the foci of the Central African Republic and South Sudan. 
• Democratic Republic of the Congo. Although the country bears the most important burden of disease (>70% of total HAT cases), the situation is not homogeneous, with regions of high or moderate prevalence (Bandundu, Kasai, Sankuru) and areas of low prevalence (Bas Congo, Equateur, Kinshasa, Maniema, Oriental Province). 

Rhodesiense area

• East Africa: low number of cases. While the number of cases is low (<200 cases notified annually; 3% of total HAT cases reported), the risk of epidemic outbreaks is not negligible. In the past years, Malawi has presented moderate prevalence in some areas. The epidemiology of rhodesiense HAT is different and the disease should be considered a zoonosis in which humans are occasional hosts. There are two different areas: one with disease transmission mainly related to cattle (Kenya, Uganda) and another with transmission related to wild animals (Malawi, United Republic of Tanzania, Zambia, Zimbabwe). Cases have not been reported in Botswana, Burundi, Ethiopia, Mozambique, Namibia, Rwanda and Swaziland for more than 15 years.

Disease distribution: mapping the foci of human African trypanosomiasis

As the most evident application of the Atlas of HAT, maps of the endemic foci are presented by country and by focus and in different time periods.

Links to maps (http://who-dev.essi.upc.edu/who/hat-distribution.html)

Mapping the risk of human African trypanosomiasis

Based on information on the number and location of reported HAT cases and the geographical distribution of the human population, spatially explicit estimates of the population at risk have been calculated and classified into five categories of risk, ranging from “very high” to “very low”. Approximately 57 million people are estimated to be at different levels of risk of contracting HAT in Africa.

Links to maps of risk of human African trypanosomiasis (http://who-dev.essi.upc.edu/who/hat-risk.html)

In addition to its initial purpose as a tool for understanding and tracking disease distribution and transmission, the HAT Atlas has also become a powerful support to the field-level planning of control activities village by village.As the number of cases continues to decrease and reach unprecedented low levels, the need to rationalize and target interventions increases in order to optimize their cost–effectiveness. Recently, the Atlas has started to be used to facilitate the geographical targeting of HAT active screening by village according to the local risk of transmission, guided by precise data. National HAT control teams can generate lists and maps from the Atlas and use them to identify the villages to be screened, optimize the timing of the visits, and evaluate the capacities and needs of the mobile teams to meet the objectives throughout the year.

Following WHO's strategic recommendations, the list of villages reporting cases in the past 3 years (first priority) and in the 4–5 preceding years (second priority) can be extracted from the Atlas, mapped and distributed by programmatic area. This information provides a concrete and comprehensive vision of the ideal objectives to cover, which can then be used for rational planning of activities considering the available resources and the additional information and experience from the teams.

It is also possible to evaluate how well villages were targeted in the past year, by working on retrospective data also from the Atlas, through a similar process.

The updated and historical data from the Atlas are used also to select the optimal location of sentinel sites for passive surveillance, for treatment facilities and for clinical trials.

As a tool for monitoring HAT elimination, the Atlas tracks and visually displays the information according to the adopted indicators. 

During the past 20 years, strengthened control and surveillance activities have succeeded in progressively reducing transmission of the disease.  The WHO road map on neglected tropical diseases, published in 2012, targeted the elimination of sleeping sickness as a public health problem by 2020. Beyond that, WHO and disease‐endemic countries targeted the elimination of transmission of gambiense HAT, resulting in zero reported cases by 2030. 

Global indicators and milestones for monitoring progress towards the 2020 goal of elimination of HAT as a public health problem were defined as follows: (i) the number of cases reported annually (target: fewer than 2000) and (ii) the area at risk reporting at least 1 case/10 000 people per year, calculated over 5-year periods (target: a reduction of 90% by 2016–2020 compared with the baseline in 2000–2004). 

Three secondary indicators, without specific targets, are also monitored: (a) the geographical distribution of the disease, (b) the at-risk population and (c) the coverage of the at-risk population by control and surveillance activities.

The key data used for monitoring are: the HAT cases reported, captured at the village level and georeferenced; the active screening sessions in all villages, even those that found no cases; and the georeferenced information on health facilities providing HAT diagnosis and treatment.

The timeframe for monitoring via the Atlas is possible from 2000. The time unit is the year, as data are aggregated by village and by year.

It has been also defined an indicator of HAT elimination as a public health problem at country level as the situation where fewer than 1 case/10 000 inhabitants per year (averaged over a 5-year period), are reported in each health district of the country, in conjunction with adequate, functional control and surveillance. In particular, information on the intensity and effectiveness of HAT control and surveillance activities is needed, including active and passive case‐finding, vector control and African animal trypanosomiasis (AAT) control.

References

Simarro PP, Cecchi G, Franco JR, Paone M, Diarra A, Priotto G, et al. Monitoring the progress towards the elimination of gambiense human African trypanosomiasis. PLoS Negl Trop Dis. 2015;9:e0003785.

Franco JR, Cecchi G, Priotto G, Paone M, Diarra A, Grout L, et al. Monitoring the elimination of human African trypanosomiasis: update to 2014. PLoS Negl Trop Dis. 2017;11:e0005585.

Franco JR, Cecchi G, Priotto G, Paone M, Diarra A, Grout L, et al. Monitoring the elimination of human African trypanosomiasis: update to 2016. PLoS Negl Trop Dis. 2018;12:e0006890.

Franco JR, Cecchi G, Priotto G, Paone M, Diarra A, Grout L, et al. Monitoring the elimination of human African trypanosomiasis at continental and country level: update to 2018. PLoS Negl Trop Dis. 2020;14(5): e0008261.