Cercariae of Trematodes in Freshwater Snails in Three Climatic Zones in Sri Lanka

Cercariae of trematodes in freshwater snails were studied in 1694 snails collected from rivers and streams in three climatic zones of Sri Lanka during 2005 to 2008. Four snail species; Thiara scabra, T. tuberculata, Paludomous sphearica and Gyraulus saigonensis were collected from nine sites in four districts; Matale, Kandy, Kegalle, and Kurunegala. Snails were allowed to release cercariae under direct sunlight or artificial light in the laboratory. Of the four snail species collected, Thiara spp. was the most preferred intermediate host with high prevalence of infection. Eight different morphological types of cercariae were released namely; oculopleurolophocercous, distome, gymnocephalous, echinostomous, gymnophallus, xiphidiocercariae, macrocercous and two furcocercous type cercariae. These cercariae belonged to six trematode families; Heterophyidae, Schistosomatidae, Psilostomidae, Echinostomidae, Gymnophallidae, Lecithodendriidae and two unidentified groups. Of the snails collected, 16% was infected with one or more types of cercariae. The highest overall infection was 77.4% in snails collected from Kurunegala district, of which more than 90% was Thiara spp. One snail with mixed infections was found. Diversity and prevalence of cercariae varied among the three climatic zones. The highest diversity was observed in the wet zone (Shannon Diversity Index, H = -1.423) showing six morphotyes. The diversity of infection in the dry zone was very low (H = -0.192) with only two morphotypes. Although, the cercariae diversity was very high in the wet zone, the prevalence of infection was very low (10.1%) compared to the dry (77.4%) and intermediate (33.9%) zones.