Freshwater pulmonate snails and their potential role as trematode intermediate host in a cercarial dermatitis outbreak in Southern Thailand

This study aimed to investigate the pulmonate snail species in the vicinity of the cercarial dermatitis outbreak area in southern Thailand. In 2020, an outbreak of cercarial dermatitis was reported in Chana district, Songkhla Province, caused by the ruminant schistosome Schistosoma indicum and its snail intermediate host Indoplanorbis exustus. In the present study, 1,175 pulmonate snails were collected between October 2021 and October 2022 from five provinces covering 34 locations in southern Thailand. Seven pulmonate snail species were identified based on shell morphology, including Amerianna carinata, Gyraulus bakeri, G. convexiusculus, G. hubendicki, Physella acuta, Indoplanorbis exustus, and Radix rubiginosa. Among these snails, eight species, and five types of cercariae were identified, viz. type (i) Echinostome cercariae consisted of Echinoparyphium recurvatum, Echinostoma spiniferum, and E. revolutum, type (ii) Brevifurcate-apharyngeate cercariae consisted of Schistosoma indicum and S. spindale, type (iii) Brevifurcate-pharyngeate-clinostomatoid-cercariae was represented by Clinostomum giganticum, type (iv) Longifurcate–pharyngeate cercariae (strigea cercaria) was Diplostomum baeri eucaliae, and type (v) Ophthalmoxiphidiocercaria. Among the seven pulmonated snail species, three were found to be infected, viz. G. convexiusculus, I. exustus, and R. rubiginosa, with infection rates of 1.14% (2/176), 0.25% (2/802), and 4.02% (7/174), respectively

Cercarial dermatitis outbreak caused by ruminant parasite with intermediate snail host: schistosome in Chana, South Thailand

A cercarial dermatitis outbreak occurred in Chana district, Songkhla Province, South Thailand, between August and October 2020. A total of 359 cases with cercarial dermatitis were confirmed with three cases of skin biopsy. The species of potential trematodes from infected snails were investigated, and the prevalence of infestation with schistosomes was described. As part of our ongoing studies of trematode diversity in freshwater systems, using morphological traits and sequence data to differentiate species, this study aimed to provide insights into the parasite species that cause cercarial dermatitis in the outbreak area and improve our understanding of parasite species distribution. Snail samples were collected in December 2020 and September and October 2021. Five main areas of outbreaks were investigated, and snails were collected by scooping and examined for infection with cercariae. The cercariae were characterized on the basis of morphological features. We found two species of snails to be infected, viz. Indoplanorbis exustus and Bithynia siamensis siamensis, with infection rates of 2.05% (12/586) and 7.93% (23/290), respectively. Three species of trematodes were found in B. s. siamensis, viz. Gastrothylax crumenifer, Astiotrema monticellii, and Loxogenes liberum. Moreover, three species of trematodes were found in Indoplanorbis exustus, viz. Clinostomum giganticum, Echinostoma spiniferum, and Schistosoma indicum. The latter is a ruminant schistosome, which causes the outbreak of cercarial dermatitis in the area. They were genetically analyzed using the internal transcribed spacer subunit II region to confirm the species identity at generic and infrageneric levels