TEMPORAL VARIATIONS OF OPISTHORCHIS VIVERRINI AND OTHER TREMATODE INFECTION RATES IN BITHYNIA SIAMENSIS SIAMENSIS FROM O. VIVERRINI-ENDEMIC AREAS, CHACHOENGSAO PROVINCE,CENTRAL THAILAND

A field study was conducted to investigate monthly variations in cercarial trematode infections in Bithynia siamensis siamensis snails in an Opisthorchis viverrini-endemic area of Chachoengsao Province, Thailand. A total of 45, 511 snails collected each month from September 2010 to August 2011, were examined for trematode infection, resulting in overall 0.10, 4.94,1.03, and 0.01% prevalence of O. viverrini, xiphidiocercaria, amphistome, and furcocercous cercaria, respectively in B. s. siamensis. Highest prevalence of O. viverrini (0.33%) and amphistome (2.13%) were observed during the dry season (mid-February to mid-May) when rice paddies were dry, while highest prevalence of xiphidiocercaria (7.92%) occurred in the rainy season (mid-May to mid-October) when rice paddies were flooded. Monthly snail density ranged from 8.40 snails/m(2) as observed in January 2011 to 32.39 snails/m(2) observed in November 2010. These observations indicated dynamics of B. s. siamensis snail intermediate host of liver flukes in central Thailand depended on population density, cercarial trematode prevalence and ecology.