The global limits and population at risk of soil-transmitted helminth infections in 2010

Abstract

Background: Understanding the global limits of transmission of soil-transmitted helminth (STH) species is essential for quantifying the population at-risk and the burden of disease. This paper aims to define these limits on the basis of environmental and socioeconomic factors, and additionally seeks to investigate the effects of urbanisation and economic development on STH transmission, and estimate numbers at-risk of infection with Ascaris lumbricoides, Trichuris trichiura and hookworm in 2010.

Methods: A total of 4,840 geo-referenced estimates of infection prevalence were abstracted from the Global Atlas of Helminth Infection and related to a range of environmental factors to delineate the biological limits of transmission. The relationship between STH transmission and urbanisation and economic development was investigated using high resolution population surfaces and country-level socioeconomic indicators, respectively. Based on the identified limits, the global population at risk of STH transmission in 2010 was estimated.

Results: High and low land surface temperature and extremely arid environments were found to limit STH transmission, with differential limits identified for each species. There was evidence that the prevalence of A. lumbricoides and of T. trichiura infection was statistically greater in peri-urban areas compared to urban and rural areas, whilst the prevalence of hookworm was highest in rural areas. At national levels, no clear socioeconomic correlates of transmission were identified, with the exception that little or no infection was observed for countries with a per capita gross domestic product greater than US$ 20,000. Globally in 2010, an estimated 5.3 billion people, including 1.0 billion school-aged children, lived in areas stable for transmission of at least one STH species, with 69% of these individuals living in Asia. A further 143 million (31.1 million school-aged children) lived in areas of unstable transmission for at least one STH species.

Conclusions: These limits provide the most contemporary, plausible representation of the extent of STH risk globally, and provide an essential basis for estimating the global disease burden due to STH infection.

Figure 1

Geographical distribution of available survey data geo-located to the point or district level. Data were available for 4,840 unique survey locations, plus an additional 1,583 second administrative level areas.

Figure 2

Relationship between climatic factors and prevalence of STH infection. Climatic factors estimated from interpolated weather station data, and prevalence of STH infection from 4,840 unique survey locations world-wide, stratified by world region. Estimates were derived for each survey point location and data for locations experiencing the same temperature are averaged for presentation.

Figure 3

Climatic suitability for (A) hookworm, (B)A. lumbricoidesand (C)T. trichiuratransmission defined by land surface temperature and aridity. Areas were defined as stable (dark green), unstable (light green, where infection prevalence <2%), or no risk (light grey). Exclusion masks were developed in a step-wise fashion on the basis of species and region specific climatic thresholds.

Figure 4

Box and whisker plots of the distribution of point and district level data by transmission category. Figures show hookworm (A, B), A. lumbricoides(C, D) and T. trichiura(E, F). The thick black line indicates the median, the box, the 25^th and 75^th percentile, whiskers indicate the range of the data and dots extreme outliers.

Figure 5

Boxplots showing the distribution of helminth survey data by population density classification, for (A) hookworm, (B)A. lumbricoidesand (C)T. trichiura. Urban areas are defined using Urban Extents (UE) from the Global Urban Rural Mapping Project (GRUMP); peri-urban defined as <15 km from the UE edge and having population densities >250/km², rural as >15 km from the UE edge and/or population density <250/km2 and extreme rural population density <1/km² using an updated Gridded Population of the World (GPW3) population database. The thick black line indicates the median, the box, the 25^th and 75^th percentile, whiskers indicate the range of the data and dots extreme outliers.

Figure 6

Relationship between national gross domestic product and mean country infection prevalence, stratified by species. Mean prevalence is calculated from surveys conducted 2005–2011. GDP (per capita in US$) for 2010; Gross Domestic Product per capita (current US$).

Figure 7

Distribution of soil-transmitted helminth (STH) in 2010, applying climatic exclusion limits. Indicating areas at stable risk of transmission for at least one STH species (dark green), unstable transmission of at least one species (light green) or at no risk of transmission for any STH species (dark grey). Pink hatching indicates countries excluded on the basis of socioeconomic status; also shown are irrigated areas (blue), urban extents (dark pink) and surrounding peri-urban extents (light pink).