Vol 35 (Suppl 1) 2004 291 TREMATODE INFECTION RATES OF FISH FROM A WASTEWATER TREATMENT FACTORY POLISHING POND AND A CANAL IN PHUKET, THAILAND D Krailas, T Janecharat, S Ukong,W Junhom, S Klamkhlai, N Notesiri and P Ratanathai Department of Biology, Faculty of Science, Silpakorn University, Nakhon Pathom, Thailand Abstract. The infection rates of trematode metacercariae were studied in fish from a polishing pond of a wastewater treatment factory and from Bang Yai Canal, Mueang District, Phuket Province, Thailand. The investigation was carried out from April 2001 to April 2002. Nile tilapia (Tilapia nilotica) were collected from the polishing pond, while mullet (Valamugil cunnesius) were collected from Bang Yai Canal. The metacercariae were examined from the fish’s dorsal fins, pectoral fins, ventral fins, digestive tracts, and gills. One hundred and twenty-eight Nile tilapia were collected and seven were found infected with metacercariae (5.5%). One hundred and thirty- one mullet were collected and 128 were infected with the parasites (97.7%). In the Nile tilapia, the metacercariae were found from the dorsal fins, pectoral fins, and gills at 3 (42.9%), 3 (42.9%), and 1 (14.3%) cysts, respectively. For the mullet, the metacercariae were found from the dorsal fins, pectoral fins, ventral fins, digestive tracts, and gills at 1,887 (42.4%), 339 (7.6%), 329 (7.4%), 869 (19.5%), and 1,031 (23.1%) cysts, respectively. One-way ANOVA tests (F test) were used to compare the infections of trematode metacercariae statistically. It was found that the infection rates of metacercariae at different months exhibited significant differences (p < 0.05), whereas the number of metacercariae found at different parts of the fish samples were not significantly different (p > 0.05). MATERIALS AND METHODS The investigation was carried out from April 2001-April 2002. Nile tilapia (Tilapia nilotica) (Fig 1) were collected from the polishing pond of a wastewater treatment factory, while mullet (Valamugil cunnesius) (Fig 2) were collected from Bang Yai Canal. The metacercariae were examined from the fish’s dorsal fins, pectoral fins, ventral fins, digestive tracts, and gills. They were individually examined under stereomicroscope and photographed. The statistically significant differences in infection rates of metacercariae at different months and parts of the fish samples were studied by one-way ANOVA tests (F test). RESULTS One hundred and twenty-eight Nile tilapia were collected and seven were found infected with metacercariae (5.5%) (Table 1). One hundred and thirty-one mullet were collected and 128 were infected with the parasites (97.7%). The metacercariae were investigated from muscle under the dorsal fins, pectoral fins, ventral fins, digestive tracts, and gills. In the Nile tilapia, the metacercariae were found from the dorsal fins, pectoral fins, and gills at 3 (42.9%), 3 (42.9%), and 1 (14.3%) cysts, respectively. Cysts were spherical, 280 x 300 μm, with thick double walls. The only shape of the Correspondence: D Krailas, Department of Biology, Faculty of Science, Silpakorn University, Nakhon Pathom 73000, Thailand. E-mail: kduang@su.ac.th INTRODUCTION Trematode infections are public health and veterinary problems that have been reported over a wide range of areas in Thailand (Saenphet et al, 2001). It is well known that freshwater and brackish water fish are not only a major protein source for humans but also common intermediate hosts of many kinds of trematode parasites, such as small intestinal flukes and liver flukes (Pearson and Ow Yang, 1982; WHO, 1995; Radomyos et al, 1998; Sukontason et al, 1998). In a previous study, trematode metacercariae were reported to occur in freshwater fish from northern and northeastern Thailand (Manning et al, 1971; Radomyos et al, 1983; Namue et al, 1998). Since there was no report concerning trematode infection in the southern part, studies of helminthic infections in rats, cats, dogs, and even humans should be done in this region, especially surveys for larval stages in fish intermediate hosts. Thus, in this study, trematode metacercariae were investigated in fish from a polishing pond of a wastewater treatment factory and from Bang Yai Canal, Mueang District, Phuket Province, Thailand. The study results can be used as a public health and veterinary knowledge base.