Prevalence of cercarial infections in freshwater snails and morphological and molecular identification and phylogenetic trends of trematodes

Abstract

Objective: To investigate the prevalence of cercarial infections in freshwater snails from several water sources in Nakhon Nayok, Nonthaburi, and Pathum Thani provinces of Central Thailand, and to reconstruct a phylogenetic tree for improved understanding of the relationships in the cercarial stage. Methods: The snail specimens were collected from 34 total sampling sites and investigated for cercarial infections using the crushing method. The cercarial specimens were classified and used for the phylogenetic tree analysis using the Internal Transcribed Spacer 2 (ITS2). Results: A total of 1 921 snail specimens were classified into five families and seven species. The results showed that four snail species were identified as intermediate hosts of the larval stages of trematodes, with an overall prevalence of infection of 2.45% (47/1 921). The infected snail specimens included five groups of the cercarial type: cercariaeum cercariae, echinostome cercaria, megalurous cercaria, parapleurolophocercous cercaria, and xiphidiocercariae. This is particularly true of xiphidiocercariae, which was found to be the dominant type among cercarial infections in bithyniid snails by approximately 38.00%. With regard to molecular identification, the phylogenetic tree was reconstructed using the neighbor-joining method with 10 000 bootstraps and separated the trematodes into three clades: Echinostomatoidea, Microphalloidea and Opisthorchioidea. Conclusions: The study reveals a high prevalence of cercarial infection for each cercarial type and maturation into a definite trematode genus and delineates morphological characteristics and evolutionary trends among each larval trematode in Nakhon Nayok, Nonthaburi and Pathum Thani provinces. In addition, the ITS2 sequence data of cercariae could be used to examine classification of these species at the family level.

Keywords

• cercaria; freshwater snails; molecular identification; phylogenetic tree; prevalence; thailand