Effect of environmental factors in reducing the prevalence of schistosomiasis in schoolchildren: An analysis of three extensive national prevalence surveys in Brazil (1950-2018)

Author summarySchistosomiasis mansoni is a neglected tropical disease caused by infection from parasitic worms of the species Schistosoma mansoni. Due to the complexity of the mechanism of transmission and maintenance of schistosomiasis, several preventive actions on diverse conditioning factors can promote disease control. Active search, timely treatment of cases, stool tests, and epidemiological investigations are the initial actions under programs for epidemiological surveillance of the disease. Thus, national surveys on prevalence of the disease covering a large time span can provide valuable information about its epidemiological pattern over the years. Our study addressed three national surveys with historical coverage (1947-1953, 1975-1979, and 2010-2015) that mapped the prevalence of the disease in children aged 7-14 for nearly seven decades. We also employed statistical models to investigate which environmental, economic, or demographic factors are associated with the disease at municipal level. The results showed that the decrease in schistosomiasis from the 1950s to the 2010s was statistically significant and suggests that improvements in water supply and sanitation require structured and systemic approaches for controlling the transmission of schistosomiasis. BackgroundOver seven decades, Brazil has made admirable progress in controlling schistosomiasis, and a frequent question about the explanation for this reduction refers to the effect of improving environmental factors in the country. This article seeks to identify factors related to the change in the epidemiological situation of schistosomiasis mansoni infection by analyzing three national prevalence surveys conducted since 1950. Methodology/principal findingsThis is an ecological study analyzing an unbalanced panel of data based on national surveys and considering the municipality as the unit of analysis. The sample consisted of 1,721 Brazilian municipalities, in which a total of 1,182,339 schoolchildren aged 7-14 were examined during the three periods corresponding to each survey (1947-1953, 1975-1979, and 2010-2015). The percentage of municipalities with zero cases of schistosomiasis was: 45.4%, 54.2% and 73.7%, respectively for those periods. A zero-inflated Poisson regression model, with fixed and random effects, was fitted to assess the association between candidate factors and disease prevalence using a significance level of 5%. There was a significant decrease in disease prevalence between the first and last periods analyzed (RR 0.214, CI 0.184-0.249), with a protective association with access to sanitation (RR 0.996, CI 0.994-0.998), urbanization (RR 0.991, CI 0.989-0.993), and living in own households (RR 0.986, CI 0.983-0.989); and an inverse association with piped water supply (RR 1.010, CI 1.008-1.011). ConclusionThe findings of this study indicate a decrease in the prevalence of schistosomiasis over seven decades in schoolchildren from the analyzed Brazilian municipalities, associated with environmental factors and social conditions. The increased access to piped water in the municipalities apparently triggers other ways of contact with unsafe water bodies, generating new transmission routes and suggesting the need for a systemic approach concerning contact with water.