Introduction
Since the 2000s, Brazil has seen a decrease in the rate of rural-urban
migration, however, the urban population continues to grow and the rural
population is decreasing (Alves and Marra, 2009; United Nations, 2018).
The city of Salvador went through an urbanization process even more
intense than the country as a whole (Souza et al., 2012). This rapid
process produce areas with precarious housing and limited public
utilities that put some communities at risk for transmission of
parasites such as Schistosoma mansoni the cause of
schistosomiasis (Santana and Batista, 2012).
S. mansoni is the second most important parasitic infection after
malaria for its prevalence and morbidity. It infects hundreds of
millions in the Americas, Africa, Middle East, and East Asia. Infection
is commonly considered a disease of rural populations related to
agriculture, fishing, recreation, and other activities associated with
contact with freshwater (McManus et al., 2018). While urban disease is
considered unusual; it has been well documented historically in Africa,
China, and Brazil (Blanton et al., 2015; Klohe et al., 2021). In Brazil,
cities like Salvador, Bahia, in the Northeast have historically seen
transmission of S. mansoni . Pirajá da Silva first distinguishedS. mansoni from the species S. haematobium based on human
cases in the city in 1911 (Katz, 2008). The main conditions for active
transmission of S. mansoni are prevalent in some of the poorer
sections of cities i.e., infected humans, contact with surface waters
(agricultural work, recreation), presence of susceptible snails and poor
sanitation (Zanardi et al., 2019; Klohe et al., 2021). Measures to
eliminate schistosomiasis as a public health threat are aided by
understanding the presence and persistence of the infection in each
location. Rural to urban immigration is likely to contribute, but it is
important for public health planning to understand how much of a factor
this is. The pattern of distribution is also key to the management of
the infection. Transmission in urban centers is thought to be highly
focal (McManus et al., 2018; Montgomery, 2019), but without knowledge of
demography and parasite populations themselves this is difficult to
verify.
Here using population genetics, we addressed for one urban community
whether the presence of a population of S . mansoniresulted primarily from migration or local acquisition. We assessed
place of birth and percentage of time living in a local neighborhood
designated Pirajá, in the city Salvador, Bahia in Northeastern Brazil.
We also evaluated genetic differences by gender, age, and water contact
points. Using epidemiologic and population genetic evidence we show
infection is present primarily due to local transmission in this section
of a major Brazilian metropolis.