The Geography and Scale of Soil-Transmitted Helminth Infections

Abstract

The purpose of this review is to describe the distribution of soil-transmitted helminth (STH) infections at different geographic scales, briefly review scale-specific determinants that might explain these geographical differences, and discuss their impact on future opportunities at prevention and treatment. A large volume of research has explored the social, environmental and behavioural determinants of STH infection prevalence and intensity in at-risk regions of the world, but few studies have sought to systematically differentiate how these determinants vary between different geographical contexts or scales. For example, infections are endemic in low- and middle-income countries with warm, moist tropical and subtropical climates because the physical and social environments are conducive to the survival and transmission of soil-transmitted helminths. At smaller scales, parasites appear to cluster within individuals and families/households predisposed to intense infections; however, this is likely due to behavioural and social environmental determinants. Additionally, STH infections are common in many regions of Africa, the Americas and South-East Asia; however, the environmental and behaviour factors that influence prevalence differ between these regions, and in turn, may require different prevention strategies. Transmission of soil-transmitted helminths is highly dependent on a number of environmental and behavioural factors that influence (i) parasite survival, (ii) the degree to which the environment is contaminated with infectious eggs and larvae and (iii) the amount of contact between human hosts and polluted soil. These factors likely explain, at least in part, the geographic distribution of soil-transmitted helminth (STH) infections at the individual, family, municipal, regional and international scale. A more complete understanding of how determinants of infection vary by geography and scale is important for informing the development of location-specific control strategies. In addition, this perspective could help researchers predict the potential effects of both large- and small-scale environmental change (of either a natural or social nature, including anthropogenic climate change, increased deforestation, altered infrastructure and increased or modified agricultural practices) on infection prevalence and intensity. Finally, focusing on geography and scale-specific determinants could help illuminate previously unidentified risk factors of infection.