An updated atlas of human helminth infections: the example of East Africa

Abstract

Background: Reliable and updated maps of helminth (worm) infection distributions are essential to target control strategies to those populations in greatest need. Although many surveys have been conducted in endemic countries, the data are rarely available in a form that is accessible to policy makers and the managers of public health programmes. This is especially true in sub-Saharan Africa, where empirical data are seldom in the public domain. In an attempt to address the paucity of geographical information on helminth risk, this article describes the development of an updated global atlas of human helminth infection, showing the example of East Africa.

Methods: Empirical, cross-sectional estimates of infection prevalence conducted since 1980 were identified using electronic and manual search strategies of published and unpublished sources. A number of inclusion criteria were imposed for identified information, which was extracted into a standardized database. Details of survey population, diagnostic methods, sample size and numbers infected with schistosomes and soil-transmitted helminths were recorded. A unique identifier linked each record to an electronic copy of the source document, in portable document format. An attempt was made to identify the geographical location of each record using standardized geolocation procedures and the assembled data were incorporated into a geographical information system.

Results: At the time of writing, over 2,748 prevalence surveys were identified through multiple search strategies. Of these, 2,612 were able to be geolocated and mapped. More than half (58%) of included surveys were from grey literature or unpublished sources, underlining the importance of reviewing in-country sources. 66% of all surveys were conducted since 2000. Comprehensive, countrywide data are available for Burundi, Rwanda and Uganda. In contrast, information for Kenya and Tanzania is typically clustered in specific regions of the country, with few records from areas with very low population density and/or environmental conditions which are unfavourable for helminth transmission. Information is presented on the prevalence and geographical distribution for the major helminth species.

Conclusion: For all five countries, the information assembled in the current atlas provides the most reliable, up-to-date and comprehensive source of data on the distribution of common helminth infections to guide the rational implementation of control efforts.

Figure 1

The spatial distribution of survey sites included in the East African Atlas. The Atlas currently includes 2,612 surveys conducted between 1980 and 2009. First-level administrative boundaries are indicated in grey. Population density is based on a 100 m gridded population surface [41].

Figure 2

Time period of included surveys. The number of prevalence surveys identified by year in Burundi, Kenya, Tanzania and Uganda, East Africa, 1980–2009. The majority (134/138) of surveys in Rwanda were conducted in 2008 and therefore are not presented here. The graphs show a recent increase in the number of surveys conducted since 2000 in East Africa, especially in Tanzania and Uganda.

Figure 3

The known geographical distribution of schistosomiasis in East Africa. The geographical distribution of (A) Schistosoma haematobium and (B) S. mansoni infection, based on available survey collected between 1980 and 2009, and categorized according to WHO prevalence thresholds (n = 2,405). First-level administrative boundaries are indicated in grey. S. mansoni infection is most prevalent around Lake Victoria basin, North-west Uganda and the central highlands of Kenya. In contrast, S. haematobium infection is distributed along the Kenyan and Tanzanian coast, Tana River in Kenya and Lake Victoria in Kenya and Tanzania, but absent from Uganda.

Figure 4

The known geographical distribution of soil-transmitted helminths in East Africa. The geographical distribution of (A) hookworm, (B) Ascaris lumbricoides, and (C) Trichuris trichuira, based on available survey collected between 1980 and 2009, and categorized according to WHO prevalence thresholds (n = 1,948). First-level administrative boundaries are indicated in grey. The relatively wide distribution of hookworm is apparent in most surveyed areas in East Africa, except in northern Kenya and northeast Uganda. The distribution of A. lumbricoides and T. trichuria infection is more restricted, with high prevalence estimates reported in Burundi and Rwanda, central and western Kenya, southeast Uganda, northeast Tanzania and Zanzibar (Pemba and Unguja).