Results and Discussion
Of 2,011 residents interviewed, 1,134 (56%) provided at least one stool sample of which 62 were positive for S. mansoni (5.5%, 95% CI 4.2% - 7.0%, Table S2) and 75 for geohelminths (6.6% ). Thus, schistosomiasis in the evaluated group living in Pirajá is five times higher than the national average of 1% and more than twice as high as the Bahia state average of 2.1% (Ministério da Saúde do Brasil, 2014; Katz, 2018). This is typical of many rural areas of the state. There was an association between S. mansoni infection and male sex (OR 3.0, 95% CI 1.7 - 5.1) and age >20 years old (OR 2.4, 95% CI 1.2 - 4.7, Table 1, FigS2). This is consistent with what we have observed in other urban areas of Salvador (Blanton et al., 2015; Barbosa et al., 2016; Silva et al., 2020) and in urban infection in Pernambuco State (Gomes et al., 2022). However, it contrasts with the younger age-specific risk of infection in rural areas (Blanton, 2019; Klohe et al., 2021) and with an urban area of Sergipe (Calasans et al., 2018). The mean intensity of S. mansoni infection was 89 ± SD 172 eggs per gram of feces (epg) was low by WHO criteria (Committee, 2002; World Health Organization, 2019) (Table S2). This was similar to two recent studies of urban schistosomiasis in Brazil (Calasans et al., 2018; Gomes et al., 2022).
Specific local risk was assessed by reported contact with surface water and association with S. mansoni infection. We observed an overall high risk to water contact in any of the evaluated points (OR 3.4, 95% CI 1.9 - 6.0) and specific risk points P1 (OR 2.8, 95% CI 1.7 - 4.8), P2 (OR 3.5, 95% CI 2.0 - 6.0), P3 (OR 2.2, 95% CI 1.2 - 4.2) and P5 (OR 8.6, 95% CI 3.6 - 20.8) (Table 1).
Migration has been indicated as a risk factor for urban schistosomiasis (Blanton, 2019; Klohe et al., 2021), but being born in Salvador was neither protective or a risk for infection. Immigrants (those not born in Salvador) were 17% of the sample (Table S2) and were no more likely to be infected than the native born (OR 1.98, 95% CI 0.84 - 4.67) (Table 1). Distribution of schistosomiasis by age revealed higher risk among male young adults and higher parasitic load among those above 60 years old (FigS2). Mean percentage of lifetime in Salvador was associated with S. mansoni or infection intensity (Table 1). While intensity of infection was numerically higher in immigrants (141 vs 83 eggs per gram, respectively), this was not statistically significant (p = 0.65). The higher intensity of infection in the 60+ age group is likely due to the small numbers of samples. A history of traveling was protective (OR 0.4, 95% CI 0.1 - 0.9) (Table 1). These data suggest that immigration was not associated with risk for schistosomiasis, and local acquisition was more important. Indeed, the more time spent in the city, the greater the risk of infection.
Analysis of genetic differentiation of parasite populations further supports primarily a local transmission pattern. Of the 62 stool samples collected, 51 were successfully genotyped (80%). The average differentiation between infra-populations (Di) was 0.22 (Table S1) and the differentiation between infra-populations from immigrants compared to the native-born (Dc) was 0.06. While the average Di indicates individuals were acquiring genetically different groups of parasites, this is typical of areas with low infection intensities. The Dc between parasites of immigrants and parasites of native-born was low relative to the Di, indicating that the two groups were drawn from the same source.
The genetic distance of individuals from the group is another approach to evaluating whether parasites of immigrants largely originated outside of the Pirajá community. We have shown that communities separated by 6 km on the same river can be distinguished by Dc (Blanton et al., 2011) and principal coordinate with k-means analysis of the Dic or Nei’s genetic distance (Long et al., 2022). Using parasites of those born in Salvador as the component population, no clustering was observed by principal component (PCA) or network analyses (FigureS3). We also stratified by percentage of lifetime in Salvador and no clustering was observed as well (Figure 2). No clustering was observed by PCA analyses when comparing differences by age, sex, history of contact with freshwater bodies when traveling and at the distinct points in Pirajá neighborhood (FigureS4).
It is often unclear if the problem of schistosomiasis is rising or falling with increasing urbanization or if rural-urban migration is fueling the presence and persistence of the infection in cities (Klohe et al., 2021). In Pirajá, immigrants may be more sinned against than sinning (King Lear, 3.2 49-60, Shakespeare) as far as schistosomiasis is concerned. Although some importation of S. mansoni is possible due to immigration, even necessary to stablish local transmission, the evidence in this urban neighborhood suggests that immigrants primarily become infected in the city. The conditions of crowding and sanitation are what perpetuate transmission and focusing on these conditions will powerfully resolve the issue of urban schistosomiasis.