Concurrent infections with multiple parasites are common in human populations inhabiting tropical regions of the world. Although morbidities associated with single parasite infections are well-established, much less is known about the risk factors for co-infection, the epidemiological and biological associations among parasite species, or the related health effects of polyparasitism. This cross-sectional study of Plasmodium species and Schistosoma haematobium co-infections among people in a rural village of coastal Kenya was conducted to address some of these questions.

Predictors of polyparasitic infections were identified within the social, environmental and spatial context of households. The relative importance of individual- versus household-level factors in predicting parasite infection also was examined using multi-level modeling techniques, thereby providing insights into mechanisms by which socio-economic position (SEP) and other factors might influence disease risk. In addition, evidence for biologically meaningful associations between parasites was evaluated after adjustment for household clustering of individuals. Finally, potential synergistic relationships between these infections and their effects on anemia and stunting in children were assessed.

Results revealed a heavy burden of parasitic infection in this population, especially in children. Intense Plasmodium species and S. haematobium infections were found to cluster in a subset of children with suggestions of synergistic effects on anemia and stunting. Determinants of heavy infections were age-specific and included household SEP. Individual-level characteristics explained much more of the household-level variation in infection than did household-level variables. Finally, analyses of species-specific Plasmodium infections demonstrated fewer co-infections that expected by chance, suggesting the presence of cross-species interaction.

This research highlights the unacceptable burden of parasitic disease in tropical regions of the world, and suggests that integrated control efforts which consider multiple infections, and which are targeted at school-aged children, should maximize disease reduction under resource-limited conditions.