Urban Schistosomiasis Persists in Salvador, Brazil, but Falls Sharply Over 13 Years

Schistosomiasis, long considered a rural disease, has quietly maintained a foothold in the Brazilian city of Salvador, Bahia. A 13-year series of studies, conducted between 2011 and 2024, tracked the parasite Schistosoma mansoni across neighborhoods of one of Brazil’s largest metropolitan areas, revealing both the persistence of local transmission and a sustained decline in infection prevalence.

The research team carried out repeated cross-sectional surveys to identify the neighborhoods with the highest burden of infection in the Salvador metropolitan region. In total, 6,863 residents provided demographic information and submitted at least one stool sample for Kato-Katz examination. The overall prevalence at the initial examination was 7 percent, a level that, if applied across Salvador’s population, would make the city one of the most important endemic areas in the state of Bahia.

Beyond the initial surveys, the investigators followed 3,964 individuals who were re-examined on up to three occasions at intervals ranging from one to five years. This longitudinal design allowed the team to assess infection dynamics at the individual level, something rarely attempted in urban Brazilian settings, where most evidence has come from single snapshots of prevalence.

The results showed a clear downward trajectory. Regardless of the intervals between treatment rounds, community-wide prevalence declined by more than half and did not rebound. Apparent incidence, the rate of new infections, fell to below 0.5 percent per year following a second period of treatment. These figures suggest that transmission pressure in the studied communities weakened substantially over the study period rather than merely fluctuating.

Perhaps the most revealing finding concerned the origins of infection. In nearly all of the communities studied, most infections occurred among individuals who were born in Salvador and had no travel history outside the city. This pattern indicates that transmission is largely local rather than imported, contradicting the common assumption that urban cases simply reflect migration from rural endemic areas.

The decline in prevalence coincided with observed improvements in sanitation at the community level and reduced contact with surface water. The study’s authors note this association alongside the treatment schedule, without claiming that sanitation alone drove the decline.

The study’s interpretation emphasizes that rapid urbanization in Brazil has shifted the epidemiological landscape of parasitic diseases such as schistosomiasis and malaria. Although typically considered a rural infection, S. mansoni transmission persists in urban settings where sanitation and water access remain uneven.

As one of the first individual-level longitudinal assessments of schistosomiasis persistence in an urban Brazilian setting, the study combines repeated parasitological surveys with demographic and environmental data across multiple neighborhoods. The findings underscore that urban schistosomiasis is a genuine public health concern in Salvador, while documenting a durable reduction in community-wide prevalence over more than a decade of surveillance.

Subject of Research: Long-term epidemiological surveillance of urban schistosomiasis in Salvador, Bahia, Brazil

Article Title: Schistosomiasis as an urban disease: 13 years of studies in the neighborhoods of Salvador, BA, Brazil

Article References: Santos-Muccillo, P., Cedraz, F. M.-A., Silva, L. K., Barbosa, L. M., Moretto, V. T., Zanardi, V. S., Chaves, C. F., Cruz-Filho, J. R. P., Andrade, L. S. A., Santos, C. R. A., Reis, E. A. G., Cardoso, C. W., Teixeira, R. S., Simões, F. M., Nascimento, R. J. M., Costa, J. M. L., Reis, M. G., & Blanton, R. E. (2026). Schistosomiasis as an urban disease: 13 years of studies in the neighborhoods of Salvador, BA, Brazil. PLOS Neglected Tropical Diseases, 20(10), e0013418. https://doi.org/10.1371/journal.pntd.0013418

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0013418

Keywords: schistosomiasis, Schistosoma mansoni, urban health, Brazil, Salvador, Bahia, Kato-Katz, parasitology, sanitation, longitudinal study, public health, neglected tropical diseases