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.