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.