Date Published: December 23, 2008
Publisher: Public Library of Science
Author(s): Rachel L. Pullan, Jeffrey M. Bethony, Stefan M. Geiger, Bonnie Cundill, Rodrigo Correa-Oliveira, Rupert J. Quinnell, Simon Brooker, Giovanna Raso
Abstract: BackgroundIndividuals living in areas endemic for helminths are commonly infected with multiple species. Despite increasing emphasis given to the potential health impacts of polyparasitism, few studies have investigated the relative importance of household and environmental factors on the risk of helminth co-infection. Here, we present an investigation of exposure-related risk factors as sources of heterogeneity in the distribution of co-infection with Necator americanus and Schistosoma mansoni in a region of southeastern Brazil.MethodologyCross-sectional parasitological and socio-economic data from a community-based household survey were combined with remotely sensed environmental data using a geographical information system. Geo-statistical methods were used to explore patterns of mono- and co-infection with N. americanus and S. mansoni in the region. Bayesian hierarchical models were then developed to identify risk factors for mono- and co-infection in relation to community-based survey data to assess their roles in explaining observed heterogeneity in mono and co-infection with these two helminth species.Principal FindingsThe majority of individuals had N. americanus (71.1%) and/or S. mansoni (50.3%) infection; 41.0% of individuals were co-infected with both helminths. Prevalence of co-infection with these two species varied substantially across the study area, and there was strong evidence of household clustering. Hierarchical multinomial models demonstrated that relative socio-economic status, household crowding, living in the eastern watershed and high Normalized Difference Vegetation Index (NDVI) were significantly associated with N. americanus and S. mansoni co-infection. These risk factors could, however, only account for an estimated 32% of variability between households.ConclusionsOur results demonstrate that variability in risk of N. americanus and S. mansoni co-infection between households cannot be entirely explained by exposure-related risk factors, emphasizing the possible role of other household factors in the heterogeneous distribution of helminth co-infection. Untangling the relative contribution of intrinsic host factors from household and environmental determinants therefore remains critical to our understanding of helminth epidemiology.
Partial Text: People living in poor areas of the tropics commonly harbour multiple parasitic infections, including infection with multiple helminth species ,. An increasing number of studies demonstrate that individuals infected with multiple helminth species tend to harbour the most intense infections – and can be at an increased risk of infection-related morbidity –. For example, a study of Brazilian school children showed those harbouring concomitant infection with Ascaris lumbricoides and Trichuris trichiura were at increased risk of stunting , whilst another Brazilian study found the risk of anaemia among school children infected with Schistosoma mansoni and two or three soil-transmitted helminth (STH) infections was significantly higher that those harbouring single STH species . The occurrence of extensive polyparasitism in human communities also has important implications for a multiple infection approach to control .
We employed a combination of spatial statistics and hierarchical multinomial modelling to investigate spatial patterns and household and environmental factors influencing occurrence of mono- and co-infection by the helminths N. americanus and S. mansoni. Our multi-level approach has the advantage of taking into account household clustering of infection, a commonly observed feature of helminth epidemiology ,,. The results suggest that, in addition to age and sex, characteristics associated with lower socioeconomic status (relative socio-economic status, household crowding) and residential environment (living in the eastern watershed or in areas with less vegetation) were significantly associated with the risk of co-infection relative to being uninfected with either species. Risk factors for co-infection reflected those associated with mono-infection, with no identified risk factors specific to co-infection.