Introduction: The Baixada Maranhense Microregion currently has the highest prevalence of schistosomiasis in the State of Maranhão, likely because this parasitosis is characterized as an occupational disease, and increased contact with water increases the risk of infection by Schistosoma mansoni. This paper reports the results of the first comprehensive freshwater malacological survey performed in the Baixada Maranhense Microregion. Methods: Freshwater mollusks were collected from the twenty-one municipalities of the Baixada Maranhense Microregion and from Bacurituba and Cajapió and were evaluated for infection by trematodes. Results: A total of 9,129 mollusks were collected (sixteen species), which included the first records of six species in the State of Maranhão: Gundlachia radiata, G. ticaga, Hebetancylus moricandi, Plesiophysa guadeloupensis, Pomacea bridgesii diffusa and Omalonyx sp. Biomphalaria glabrata was found in five municipalities, whereas B. straminea was found in nine. Biomphalaria glabrata and B. straminea were observed in syntopy in Pinheiro and São Bento. Of the 990 specimens of B. glabrata and the 2,109 specimens of B. straminea that were exposed to and/or analyzed for the presence of larval trematodes, only a single specimen of B. glabrata (0.1%) from São Bento shed S. mansoni. Other larval trematodes were first observed in mollusks from the State of Maranhão. Conclusions: These results indicate that the study area is epidemiologically important due to the presence of two natural vectors of schistosomiasis and the active transmission of schistosomiasis, which was confirmed in the infected specimen that was collected in this study.

Foi realizado um levantamento de trematódeos em gastrópodes límnicos provenientes da área da Usina Hidrelétrica de Manso e das proximidades com potencial turístico, como a região do Pantanal. Moluscos parasitados foram encontrados em doze dos dezoito municípios investigados e quarenta e uma interações cercária-gastrópode límnico foram observadas, entre elas Biomphalaria amazônica Paraense, 1966, Biomphalaria occidentalis Paraense, 1981, Marisa planogyra Pilsbry, 1933, Pomacea maculata Perry, 1830, Pomacea scalaris (d'Orbigny, 1835) and Gundlachia radiata (Guilding, 1828) atuando pela primeira vez no Brasil como hospedeiros intermediários de trematódeos. Echinostomatidae e Strigeidae foram as famílias de maior ocorrência (ca. 47%) e a maior variedade de tipos cercarianos foi encontrada em Drepanotrema lucidum (Pfeiffer, 1839). Paramphistomatidae, Schistosomatidae or Spirorchiidae and Notocotylidae or Pronocephalidae foram reportadas pela primeira vez em D. lucidum ampliando o número de famílias que utilizam esse gastrópode como hospedeiro intermediário. Embora nenhum espécime tenha sido encontrado com formas larvais de Schistosoma mansoni Sambon, 1907, outros trematódeos foram observados, como Brevifurcate apharingeate cercaria da família Schistosomatidae que pode causar dermatite no homem. Estudos sobre a taxonomia e a biologia de trematódeos são essenciais para melhor compreender a biodiversidade desses parasitos, assim como os aspectos epidemiológicos para o controle de parasitoses associadas.

A survey for freshwater gastropods carrying trematodes parasites was conducted in Manso Dam and the surrounding areas frequented by tourist, focusing particularly on the Pantanal region. Infected snails were recovered from twelve of the eighteen investigated municipalities and forty-one cercaria-snail pairings were recorded. Among these pairings were several first records of snails serving as intermediate hosts for trematodes in Brazil including Biomphalaria amazônica Paraense, 1966, Biomphalaria occidentalis Paraense, 1981, Marisa planogyra Pilsbry, 1933, Pomacea maculata Perry, 1830, Pomacea scalaris (d'Orbigny, 1835) and Gundlachia radiata (Guilding, 1828). Echinostomatidae and Strigeidae were the most common trematode families (ca. 47%) and the greatest diversity of larvae were obtained from Drepanotrema lucidum (Pfeiffer, 1839). Paramphistomatidae, Schistosomatidae or Spirorchiidae and Notocotylidae or Pronocephalidae were recovered in D. lucidum for the first time extending the number of families which use this gastropod as intermediate host. Although no specimens were found harboring larval stages of Schistosoma mansoni Sambon, 1907 other trematode larvae were discovered, including the Schistosomatidae Brevifurcate apharingeate cercaria that can cause dermatitis in humans. Continued studies on the taxonomy and biology of trematodes are essential to better understand the biodiversity of these parasites as well as the epidemiological aspects for control of associated zoonosis.