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Where data are scarce

Method identifies areas at risk for schistosomiasis, even in regions with no information on the disease

086-087_Epidemiologia_220As many as seven million people may be living with schistosomiasis in Brazil, most of them in isolated areas without diagnosis or treatment, according to biologist Ronaldo Scholte, a researcher with the Brazilian Ministry of Health who has authored two surveys of areas with a high prevalence of diseases caused by worms in Brazil. The official estimate provided by the Ministry of Health indicates that one million people were infected between 2003 and 2012. During post-doctoral research he conducted at the Swiss Tropical and Public Health Institute in 2010, Scholte calculated the number of Brazilians with verminosis, including in areas where data on disease transmission are scarce. His group’s contribution, described in articles published in 2013 and 2014 in the journals Geospatial Health and Acta Tropica, involved developing a more comprehensive approach than the one currently used to estimate the risk of contamination from these diseases.

Scholte and other Swiss researchers combined climatic, geographic and socioeconomic variables, along with data on spatial distribution of mollusks that transmit Schistosoma mansoni, the worm that causes schistosomiasis, and data on the prevalence of the disease in Brazil. To interpret these variables, the group used the Bayesian method, a model that is more suitable for geostatistical studies because it better defines and characterizes uncertainty in experiments that involve the production of thematic maps. In geostatistical studies, the logic of the Bayesian method is to arrive at the conclusion by associating uncertainty with the variables involved in the model. Using this model, Scholte identified risk factors related to the transmission of S. mansoni and confirmed that climatic variables and poverty indicators are associated with dissemination of the disease.

The map that Scholte developed also identified areas at risk in regions where there had previously been no specific data on contamination risk. The strip of coastline between the states of Maranhão and Rio Grande do Norte is an example of such an area (see map). According to Scholte, the risk of contamination is greater in the Northeast and the Southeast, owing in part to conditions favorable to proliferation of the host mollusks. In the Southeast, the situation of greatest concern is that of the state of Minas Gerais. In the northern part of the state, the contamination risk is higher than 10%, according to Scholte’s method, which considers a greater range of data and variables than do other statistical methods. In Brazil, these data are currently obtained by searching for cases in endemic areas, as was done in 2009 when the Ministry of Health conducted a survey in 1,004 municipalities and confirmed an average prevalence of 5.3% for schistosomiasis. “Our approach enabled us to identify risk factors associated with transmission of the parasite, which could help lead to the development of measures to prevent and treat the disease in the regions with the highest prevalence,” he says.

Using the same methodology, Scholte and his Swiss colleagues also estimated the number of people with other worm infections in Brazil. More than 50 million people, mainly in the North and the Northeast, are thought to be contaminated, including 29.7 million with Ascaris lumbricoides, or roundworm; 19.2 million with Trichuris trichiura; and 4.7 million with contaminated-soil-transmitted Ancylostoma duodenale and Necator americanos. “In some parts of Pará and Amazônia States, the contamination risk for A. lumbricoides could be as high as 30%,” he notes.

As Scholte explains, surveys are still needed to determine the true prevalence of schistosomiasis and other types of verminosis in Brazil. The most recent national survey of schistosomiasis and geohelminth prevalence conducted by the Ministry of Health—which is expected to conclude in 2014—studied more than 200,000 children between the ages of 7 and 14. “These types of studies are arduous, and they require national-level coordination and considerable funding, so such surveys are done infrequently,” he says. “It is important to develop and improve tools that can help us estimate the prevalence of these diseases,” he points out.

In São Paulo
Schistosomiasis is associated with poverty and lack of basic sanitation, and can be easily treated with single-dose drugs, some of which have low toxicity and few side effects. But remote areas are not the only places where the disease persists. In São Paulo, researchers at the Endemic Diseases Control Authority (Sucen) of the São Paulo State Department of Health found the mollusk Biomphalaria tenagophila, a species that can transmit the S. mansoni worm, in 27 of the 39 municipalities of Greater São Paulo. The data are from a survey of the geographic distribution of the worm’s host mollusk in the São Paulo Metropolitan Region, conducted by Sucen’s Special Programs Division (DPE). According to Fernanda Pires Ohlweiler, a researcher at DPE-Sucen, potential contamination foci were also found in 248 of the state’s 645 municipalities, mainly in outlying urban or rural areas that lack basic sanitation.

In 2009, researchers from the Butantan Institute and the University of São Paulo (USP) had already found the B. tenagophila mollusk in streams in Guarulhos, a city near São Paulo, along the Presidente Dutra highway, Brazil’s most heavily-traveled thoroughfare. When they come into contact with water, S. mansoni eggs release larvae that invade the mollusk. After reproducing, they return to the water, where they remain until they penetrate the skin and then move through the bloodstream to the liver. They cause abdominal pain, vomiting and, in serious cases, enlargement of the liver.

The first foci of schistosomiasis in São Paulo appeared in the early decades of the 20th century and spread in the 1970s with the heavy stream of migrants from other states who settled in areas on the outskirts of the state’s large cities. According to the Ministry of Health, in 2012 there were 1,106 cases of schistosomiasis reported in São Paulo. Over 100,000 cases are diagnosed in Brazil every year. Ohlweiler explains that the vermifuges used to treat the disease are distributed to the population free of charge by the Unified Health System (SUS), but many people who take the treatment are reinfected because they come back into contact with the water that is contaminated with the S. mansoni larvae. “The infrastructure of the areas where those people live needs to be improved,” she comments.

Schistosomiasis accounts for 1.1% of deaths from infectious diseases in Brazil. Between 1994 and 2008 it was responsible for an average of 500 deaths per year, according to the Ministry of Health. In 2010 there were 527 deaths caused by the disease. According to the World Health Organization, 200 million people are thought to be infected.

Diversity of epidemiologically important malacofauna in Greater São Paulo (No. 08/57792-8); Grant mechanism Regular Line of Research Project Award; Principal investigator Fernanda Pires Ohlweiler (Sucen/SES); Investment R$114,295.56 (FAPESP).

Scientific articles
SCHOLTE, R. et al. Predictive risk mapping of schistosomiasis in Brazil using Bayesian geostatistical models. Acta Tropica. V. 132, p. 57-63. 2014.
SCHOLTE, R. et al. Spatial analysis and risk mapping of soil-transmitted helminth infections in Brazil, using Bayesian geostatistical models. Geospatial Health. V. 8, No. 1, p. 97-110. 2013.