Diversity of trematode cercariae among naturally infected lymnaeid snails from Phayao, Thailand.

Lymnaeids are aquatic snails playing an important role in the transmission of many parasitic trematode species of veterinary and medical importance. In this study, we assessed the presence of cercarial flukes in naturally infected lymnaeid snails from Phayao province, Thailand, and determined the species diversity of both the intermediate snail hosts and parasite larvae. A total of 3,185 lymnaeid snails were collected from paddy fields at 31 sites in eight districts of Phayao province between October 2021 and December 2022. Larval fluke infection was assessed using the cercarial shedding method. The collected snails as well as emerging cercariae were identified at the species level via morphological and molecular methods. The sequences of snail internal transcribed spacer region 2 (ITS2) and cercarial 28S ribosomal RNA gene (28S rDNA) and cytochrome C oxidase1 (Cox1) were determined by PCR amplification and sequencing. Three species of lymnaeid snails were detected in this study, including Radix (Lymnaea) rubiginosa (Michelin, 1831), Radix (Lymnaea) swinhoei (Adams, 1866) and Austropeplea viridis (Quoy & Gaimard, 1832), of which R. rubiginosa was the most abundant, followed by A. viridis and R. swinhoei. The overall rate of trematode cercarial infection in the lymnaeid snails was 2.8% (90/3,185); the cercarial infection rate in R. rubiginosa and A. viridis was 3.5% (60/1,735) and 3.1% (30/981), respectively. No larval fluke infection was observed in the studied R. swinhoei (0/469). Nine morphotypes of cercariae were detected at 15 sites from four districts. The emerging cercariae were molecularly identified as Clinostomum sp., Aporocotylidae sp., Apharyngostrigea sp., Trichobilharzia sp., Apatemon sp., Pegosomum sp., Petasiger sp., Echinostoma revolutum and Plagiorchis sp. These findings emphasize the occurrence and diversity of trematode cercariae among naturally infected lymnaeid snails in Phayao province and could contribute to broadening our understanding of the host-parasite relationships between trematodes and their first intermediate hosts as well as developing effective interventions to control trematode parasitic diseases.

Parasitological and molecular characterization of the avian schistosomatid cercariae infecting lymnaeidae snails in Phayao, Northern Thailand.

BACKGROUND AND AIM: Cercarial dermatitis or swimmer's itch is an allergic skin reaction caused by penetrating cercaria of animal blood flukes. It is considered as a zoonotic water-borne skin condition that is found globally. Among the schistosomatid trematodes, avian schistosomes are the most responsible for cercarial dermatitis. Very little is known regarding the occurrence of dermatitis-causing cercariae in Thailand. Therefore, the objective of this study was to preliminarily investigate the presence of larval blood fluke infection among local lymnaeidae snails in Phayao by the incorporation of morphological and molecular methods. MATERIALS AND METHODS: Overall 500 Radix (Lymnaea) rubiginosa (Michelin, 1831) were collected from freshwater reservoirs near Phayao Lake in San Kwan village in Phayao, Thailand, from October to December 2020. The snails were examined for avian blood fluke infection by the cercarial shedding technique followed by morphological and molecular characterization. RESULTS: Only one type of furcocercous cercaria was observed to emerge from six infected snails (1.2%). Our molecular analyses demonstrated that the emerging cercariae showed most similarity to either the 28S ribosomal RNA gene (28S rDNA) or cytochrome oxidase C subunit 1 gene (cox1 or COI) sequences to those of Trichobilharzia species. In addition, phylogenetic tree analyses of both loci revealed similar results; the emerging cercariae were consistently clustered together with Trichobilharzia regenti. CONCLUSION: Our results clearly confirmed that the detected furcocercous cercariae belonged to the genus Trichobilharzia and displayed the highest homology to T. regenti. This study provides important data on the occurrence of dermatitis causing cercariae infection among local lymnaeidae snails, encouraging effective management, and control measures for this zoonotic infectious disease.