The changing profile of schistosomiasis in a changing urban landscape

Abstract

Since 2007, most of humanity resides in urban areas, a trend which continues worldwide. Diseases usually associated with rural contexts are now emerging or newly recognised in cities. In the neighbourhood of São Bartolomeu in Salvador, Brazil, the prevalence of Schistosoma mansoni infection in 2011 was >20%. Following enrollment and treatment of a portion of the community, ~25% of the area underwent urban renewal. In 2015, we returned to enrol individuals who had previously participated and a cohort that had not taken part in 2011. Thus, infected individuals in one group experienced specific drug treatment plus improved living conditions and the second group only improved living conditions. Between 2011 and 2015 there were no organised treatment programs, but adequate sanitation increased from 69% to 92% coverage, household flooding decreased, and the presence of indoor toilets increased to 99% of households. Ownership of household appliances also increased significantly. The overall prevalence of schistosome infections was 6.2%. In 2015, the cohort first seen in 2011 had a higher prevalence (8.7%) than those first seen in 2015 (4.8%) and showed a few demographic differences. The 2011 cohort was older, more likely born in Salvador, less likely to have lived outside of Salvador, spent a greater percentage of their lifetime in Salvador, but more likely to have travelled. The population structure of the parasites from both cohorts underwent a marked change with similar increased component and infrapopulation differentiation and >10 fold decrease in effective population size. There was a 4-5 year shift in age-specific prevalence in 2015 for all compared with 2011. While praziquantel may have helped reduce prevalence, our evidence suggests that the structural changes and improvements in living conditions had the biggest impact on schistosomiasis in this community.

Keywords: Community-wide treatment; Differentiation; Effective population size; Elimination; Population genetics; Sanitation; Urbanisation.