Schistosoma mansoni infection in residents of a riverside community in Eastern Amazon

Schistosomiasis is a chronic waterborne parasitic disease that affects approximately 251 million people worldwide. Classified by the World Health Organization (WHO) as a neglected tropical disease, it has significant epidemiological importance in Brazil, where infection by Schistosoma mansoni (S. mansoni) primarily affects individuals living in environments lacking access to clean water and inadequate sanitation infrastructure (Brasil 2014; WHO 2023).

In Pará state, Belém ranks 95th out of the 100 worst sanitation-ranking places in 2023, considering indicators related to water supply, sewage collection and treatment, as well as investment losses (Instituto Trata Brasil 2023). The population uses the extensive number of rivers in the state for different purposes since it serves work activities, recreational and consumption purposes. However, the water analysis shows high levels of total coliforms, which makes it unsuitable for consumption (Neu et al. 2016; Costa et al. 2022).

This inadequate sanitation infrastructure and the numerous open-air canal networks that frequently overflow due to the region's rainfalls facilitates schistosomiasis transmission in Belém. Furthermore, the presence of Biomphalaria intermediate hosts enables the occurrence of new disease foci and the maintenance of transmission areas, increasing risk not only for the urban population but also primarily for those who depend extensively on water use, such as the riverside communities residing on the numerous islands around the city (Neu et al. 2016; Goveia et al. 2019). This study aims to report the occurrence of S. mansoni infection in residents of the Furo do Rio Grande community, at Ilha das Onças, Pará, Brazil.

Stool samples were collected in 70 ml tubes, labeled with the participant information and received at the Basic Health Unit (UBS) of Ilha das Onças, to be later transported to the Laboratório de Parasitoses Intestinais e Esquistossomose (LPIE) at Instituto Evandro Chagas (IEC/SVSA/MS) for examinations by the Kato-Katz Helmtest^® kit (Biomanguinhos, FIOCRUZ, Rio de Janeiro, RJ, Brazil) (Katz et al. 1972), which is the recommended procedure by the World Health Organization (WHO) and the Brazilian Ministry of Health (Brasil 2014; WHO 2023). This technique allows the quantification of individual worm burden by defining the number of eggs per gram (EPG) of feces, establishing the disease burden (Sousa et al. 2017; WHO 2023). A single exam was performed and, from each fecal sample, two slides were made for analysis.

Each participant responded to a structured questionnaire containing sociodemographic and clinically relevant information for the study. Children under 2 years of age, individuals who refused to participate in the research and with incomplete informed consent forms were excluded from the study.

The residences were georeferenced using a Global Navigation Satellite System (GNSS) device, specifically the Garmin GPSMap 62s model (GARMIN INTERNATIONAL INC, Olathe, Kansas, EUA), to obtain geographic coordinates. Cartographic databases (municipal, state, federal boundaries and hydrography) were obtained from the Instituto Brasileiro de Geografia e Estatística (IBGE – http://www.ibge.gov.br/). Sentinel-2 satellite images were obtained from the European Space Agency (ESA – https://sentinels.copernicus.eu/web/sentinel/home).

The spatial distribution map was created in ArcGIS 10.4 (http://www.arcgis.com/) using IBGE datasets, ESA images and S. mansoni cases (houses of individuals infected with S. mansoni).

This study was submitted to the Instituto Evandro Chagas (IEC) Research Ethics Committee (CEP) and approved under certification No. 5.244.276 and Certificate of Presentation for Ethical Appreciation (CAAE): 52200721.5.0000.0019.

The fact that only male individuals tested positive is consistent with studies that report a higher occurrence of schistosomiasis in this gender. This is because men are often more exposed to infection due to their occupational activities, such as farming, fishing, hunting and plant extraction, as well as recreational pursuits frequently taking place in environments with direct water contact. This exposure makes them more susceptible to not only schistosomiasis but also to other waterborne diseases (Silva da Paz et al. 2021).

Malacological surveys were also carried out in the peridomestic area of the infected individuals through active searching in the morning, however, no specimens of Biomphalaria were found. Risk factors for schistosomiasis establishment, such as lack of basic sanitation, direct contact of sewage with water and environmental conditions, are present in this area and enable it to become a potential disease locus. Furthermore, new malacological investigations on the island should be encouraged to enable its monitoring (Brasil 2014; Rodrigues et al. 2019).

The follow-up investigations were performed for cure assessment 4 months after the patients' treatment and no S. mansoni eggs were found. The schistosomiasis cases were reported to the Barcarena Health Department for patient monitoring. All cases were considered allochthonous.

The use of geoprocessing in this study helps not only the assessment of risk areas for schistosomiasis but also combines information about the disease, and environmental and spatial data, favoring the local health agents and health agency's work as a whole. This mapping enables a wide view of the environment, supporting local epidemiological surveillance through the identification of possible risk areas for diseases occurring in a given space (Guimarães et al. 2010; Hino et al. 2011; Nardi et al. 2013).

The difficulty for public health surveillance to access all riverside residential areas is commonly the main factor that makes studies about those populations scarce. The diseases' evaluation occurring among people living in the endemic areas can generate data that will contribute to better management and planning of prevention and control actions such as health educational campaigns, safe access to drinking water, snail monitoring in the area and, in some cases, preventive chemotherapy (Inobaya et al. 2014), ensuring healthcare and health education for these communities (Silva et al. 2014; Rouquayrol et al. 2018).

At Ilha das Onças and the Amazon region as a whole, riverside populations lack basic sanitation and a proper sewage system, which allows the waste to be spread directly into the water flow, contaminating and contributing to vectors and disease proliferation. These conditions can facilitate the migration of S. mansoni and various other parasites, enabling their dispersion and the establishment of new potential contamination risk areas (Correa & Pinheiro 2017; Rodrigues et al. 2019). The absence of sanitation was attempted to be addressed on Ilha das Onças through the implementation of projects that aimed to reduce the impact of waste contamination in effluents. However, these projects are not yet capable of fulfilling the local needs (Neu et al. 2016).

The study revealed the presence of schistosomiasis caused by S. mansoni in residents of Ilha das Onças, an area where there have never been reports of this parasitic disease. Despite being considered a neglected and low-priority disease for public health, it remains a persistent health issue in the state. Therefore, it is necessary to emphasize the importance of epidemiological studies in riverside populations to contribute to health and environmental surveillance actions.

Although the presence of Biomphalaria snails was not identified, Ilha das Onças might be a potential area for schistosomiasis establishment, as it has conditions for the maintenance of its cycle. These findings also support the importance of malacological investigation as a preventive measure since without its intermediate host schistosomiasis cycle cannot be completed, preventing its continuity and, consequently, its establishment in new locations.

The authors would like to thank the technicians from the Parasitology and Pathology Departments at Instituto Evandro Chagas for all the support.

This research was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação Amazônia de Amparo a Estudos e Pesquisa (Fapespa), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade do Estado do Pará (UEPA) and Instituto Evandro Chagas (IEC/SVSA/MS).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.