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Surveys carried out in Brazil, between 1947 and 2015, to verify the prevalence of two important diseases caused by worms, schistosomiasis and hookworm, found a decrease in the occurrence of these diseases. Now, a survey by Fiocruz Minas has evaluated the factors related to this reduction, verifying, mainly, the influence of sanitary improvements. The results showed that the drop in the number of cases of both diseases is directly related to a common factor: access to sanitary sewage. The study is the first to characterize a historical evolution of the population’s living conditions over the course of almost seventy years and to outline an overview of the determinants of the decline of these diseases throughout the national territory, by analyzing data from municipalities located in the five regions of the country.

Image/Robert Herriman

“Surveys showed a drop in the prevalence of diseases, but did not investigate what could be leading to this decrease. Our work aimed to establish this relationship and, for that, we characterized the evolution of a series of conditions in the Brazilian population, over the seven decades in which the surveys were carried out. Confirming our hypothesis, access to sanitary sewage was associated with the reduction of both diseases”, explains Mariana Cristina Silva Santos, who developed the research during her doctorate carried out through the Postgraduate Program in Public Health at Fiocruz Minas, under the guidance of researcher Léo Heller and co-supervised by professor Sueli Mingotti, from the Federal University of Minas Gerais (UFMG).

To carry out the analysis, Mariana was based on data from the three national surveys on the prevalence of diseases already carried out in the country, in the periods 1947-1953, 1975-1979 and 2010-2105, as well as on census data from the Brazilian Institute of Geography and Statistics (IBGE) and the Institute of Applied Economic Research (Ipea), where information was obtained on demographic, socioeconomic aspects and on the provision of services. The researcher also carried out a bibliographical review of these diseases in other Latin American countries, through which other conjunctural information was raised in relation to the analyzed period. Associations between disease prevalence and possible factors were made using statistical models.

In the analysis of schistosomiasis, data from the three surveys were used, resulting in a sample composed of 1721 municipalities, in which 1,182,339 students aged between 7 and 14 years were examined. For hookworm, only the first and last were used, since the disease was not included in the second survey, covering 1428 municipalities and a total of 745,983 schoolchildren aged 7 to 14 years. The years 1950, 1977 and 2013 were adopted as the reference period for data analysis.


Among the municipalities that make up the schistosomiasis sample, the prevalence of the disease was 8.3% for the reference period 1950, 4.8% for 1977, and 0.8% for 2013. the prevalence was 47.4% in 1950, falling to an average of 0.3% in 2013.

A factor most strongly associated with the reduction of the two diseases, the coverage of the sewage network increased 12 times among the municipalities that make up the schistosomiasis sample, from 2.6% in 1950 to 30.6% in 2013. municipalities included in the hookworm sample, the result was very similar, going from 2.6% in 1950 to 31% in 2013, representing an increase of 11.9 times.

“Although, over these seven decades, sanitation policies have been unstable and discontinuous, interventions aimed at improving access to the sewerage network have significantly contributed to reducing the prevalence of diseases. This means that, if we have more consistent policies, the results will be even better”, says Mariana.

Researcher Léo Heller agrees that, even with so many limitations in public sanitation policies, there were many benefits in terms of disease prevalence, but highlights that there is still much to be done. “Because what does the sewage system do? It removes waste from people’s homes, but it is of no use if they are taken to water collections. It is necessary to treat the sewage, removing the eggs of the disease-causing parasites”, he explains.

In this sense, according to the researcher, the changes suffered by the sanitation framework in July 2020 are cause for concern, strongly encouraging the provision of services by private companies. “This new framework is very economical, and the implementation of sewage treatment presupposes investments that reduce profits. In addition, the tendency is for private companies to be willing to offer services only in profitable regions, which can generate even more inequalities, as poor and small municipalities may be uncovered. This all associated with fragile regulation is quite worrying”, he assesses.

The association between the availability of a sewage system and the reduction in the prevalence of diseases verified by the study is in line with the results of other national surveys, carried out so far at the local level, which also relate improved sanitation to a lower probability of infections.

The results also dialogue with a document prepared by Fiocruz, through Fio-Schisto, in which the importance of sanitation for disease control is highlighted. The document, delivered to the Ministry of Health recently, analyzes the feasibility of applying the recommendations made by the World Health Organization (WHO) in Brazil, which proposes, among other measures, mass medication to control schistosomiasis. For Fiocruz, this recommendation does not fit the Brazilian reality, since the prevalence is low, and therefore, investing in sanitation is the most effective measure.

Other factors

Another aspect that was associated with the decrease in the prevalence of both diseases is urbanization. According to the study, the percentage of predominantly urban municipalities ranged from 25.6% in 1950 to 68.4% in 2013.

Regarding schistosomiasis, another factor associated with the reduction in the prevalence of the disease was the occupation of the household, with living in one’s own house having a positive influence. The prevalence of hookworm disease was impacted by GDP per capita, which is the sum of the wealth of the municipality divided by the number of inhabitants. A higher GDP was associated with a decrease in the number of cases of the disease.

“These two factors, occupation of the household and GDP per capita, are related to economic status and demonstrate that the socioeconomic structure can produce and condition the distribution of diseases among the population. Our findings therefore show that the reduction in the prevalence of these diseases over seven decades can be explained by the combination of several factors, which include environmental, demographic and socioeconomic issues”, says Mariana.

About the diseases

Schistosomiasis, also known as schistosis, is a disease caused by Schistosoma mansoni , a parasite that has humans as its definitive host, but requires snails as intermediate hosts to develop its evolutionary cycle. The transmission of this parasite occurs by the release of its eggs, through the feces of infected people. In contact with water, the miracidia hatch and infect the snails, which, some time later, release cercariae, which penetrate the skin or mucosa of people, restarting the cycle.

The disease has two stages during its evolutionary process: acute and chronic. In the first phase, the main symptoms can be redness and manifestations of itching and dermatitis on the skin, high fever, weakness, nausea, vomiting, cough, diarrhea and rapid weight loss. In the chronic phase, the most serious, symptoms are enlargement of the abdomen and organs such as the liver and spleen, bleeding when defecating, fatigue, colic, constipation, cirrhosis and the appearance of lumps in various parts of the body.

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Hookworm disease, popularly known as yellow fever or Jeca Tatu disease, is a disease caused by nematode worms of the species Necator americanus and Ancylostoma duodenale . The life cycle is direct and does not have intermediate hosts, with humans being able to serve as definitive hosts. In the feces of the person contaminated with the adult worm, there are eggs that develop into larvae in the environment and, when in contact with the human body, pierce the skin and reach the circulation. Through the blood, they are taken to the heart and lungs, until they reach the small intestine, where they settle, feed, grow and reproduce.

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Among the symptoms of the disease are pallor, discouragement, difficulty in reasoning, fatigue, weakness, hypertension, dizziness and muscle, abdominal and headache. If there is no timely treatment, hookworm is even more dangerous in pregnant women, affecting the development of the fetus, and in children, and can cause mental and physical retardation.

To learn more about the study, access the recently published articles:

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

Sewerage as a protective factor for prevalence of hookworm infection in schoolchildren in Brazil: A multilevel ecological analysis of national prevalence surveys (1950–2018)

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