Factors driving co-occurrence of Schistosoma mansoni and S. haematobium at the micro-geographical level

Brief introduction

Several studies have shown that the two major human Schistosoma species, Schistosoma mansoni and Schistosoma haematobium, co-exist in the same individuals at the micro-geographical level. However, the drivers of this co-occurrence are still largely unknown. Here, we studied whether coexposure and/or host immune responses might explain this phenomenon.

Methods and materials

A multidisciplinary study was conducted in two neighboring rural communities in northern Senegal, where S. mansoni and S. haematobium are co-endemic (n=857). Kato-Katz and urine filtration were used for microscopic detection of the respective Schistosoma species. Households were located using handheld differential global positioning system in the largest community (n=599), and the Kulldorff's scan statistic was used to detect spatial clusters of infection. Schistosoma-specific cytokine responses (IL-10, IL-5, IFN-γ, TNF-a, and IL-2) were assessed in 72h whole blood culture supernatants (n=200), and analyzed by the multivariate technique nonmetric multidimensional scaling (nMDS).

Results

Classical epidemiological analyses showed that the two parasites were more likely to co-occur in the same individuals (p<0.001). Moreover, co-infected subjects had significantly higher infection levels than their mono-infected counterparts (p<0.001; adjusted for age and gender). In contrast, micro-geographical analyses revealed a very focal spatial distribution with S. mansoni clustering in one (p=0.002) and S. haematobium infections in another section (p=0.023) of the community. nMDS analysis of the cytokine data indicated that the characteristic modified Th2 response was most pronounced in co-infected subjects.

Conclusions

The divergent geographical distribution of S. mansoni and S. haematobium in this community could not explain why the two infections cluster in the same individuals. This implies that co-infection is not driven by coexposure, but by within-host interactions. Cytokine profiles suggested that co-occurrence of the two species may be due to host immunological factors and/or parasite-induced immunomodulation. However, other factors may also play a role.