STUDIES ON THE MORPHOLOGY OF CERCARIAE OBTAINED FROM FRESHWATER SNAILS AT ERAWAN WATERFALL, ERAWAN NATIONAL PARK, THAILAND Suluck Ukong1, Duangduen Krailas1, Tunyarut Dangprasert2 and Pasapong Channgarm3

The morphology of cercariae of freshwater snails from Erawan Waterfall, Erawan National Park, Kanchanaburi Province was studied between December 2002 and August 2003. The snail samples were collected by handpicking using a counts per unit of time sampling method. The cercariae, larva stage of a trematode, were investigated using the shedding method where they were categorized into three groups and six species. The f irst group, Pleurolophocercous cercariae, consists of Haplorchis pumilio (C1) and Stictodora tridactyla (C3). The second group, Furcocercous cercariae, consisted of Mesostephanus appendicalatus (C2), Transversotrema laruei (C6) and Cardicola alseae(C4). The third group, Xiphidio cercariae, has only one species which is Loxogenoides bicolor (C5). Out of 1163 snails, only 62 were found to be infected by cercariae, equivalent to a 5.33% infection rate. The infections grouped by species of the cercariae are as follows: C1 22 (1.9%), C3 29 (2.5%), C2 1 (0.1%), C6 1 (0.1%), C4 6 (0.5%) and C5 3 (0.3%). The freshwater snail samples consist of four species. From a total of 1,163 samples, there are 687 Melanoides jugicostis, 91 Tarebia granifera, 296 Thiara scabra and 89 Melanoides tuberculata. Infections were found in 45 (6.5%), 6 (6.6%), 1 (0.3%) and 10 (11.2%), respectively. similar to adult trematodes (Malek, 1922a,b), it is therefore possible to categorize trematodes using their morphological characteristics (Ito, 1980), excretory system (Komiya, 1961), and other internal organs. According to morphological characteristics and excretory systems, trematode cercariae can be classified into 7 groups: Furcocercous cercariae, Pleurolophocercous cercariae, Monostome cercariae, Gymnocephalous cercariae, Echinostome cercariae, Xiphidio cercariae and Amphistome cercariae (Komiya, 1961; Schell, 1962; Malek and Cheng, 1974; Ito, 1980). This classification will be used in our study to distinguish cercariae. The main objective of the study was to evaluate the parasitic infections of freshwater snails in the study areas, which give a rough idea of the infection rates in humans and aniMORPHOLOGY OF CERCARIAE OF FRESHWATER SNAILS Vol 38 No. 2 March 2007 303 mals surrounding the areas. MATERIALS AND METHODS The study started from site surveys and GPS measurements of survey spots (Garmin PLUS III, Taiwan), followed by sample collections by handpicking using a counts per unit of t ime sampl ing method (Ol iv ier and Schneiderman, 1956). Three collectors handpicked snail samples every 20 minutes. The samples were collected three times, once each season, during the study (December 2002, April 2003, August 2003). The snail samples were classified by conchology using Brandt’s taxonomic key and were examined for parasitic infections with trematode cercariae using shedding and crushing methods. The morphological characteristics of cercariae were studied by employing conventional scanning electron microscopes. The cercariae were measured, sketched, and photographed.