Opisthorchis viverrini : intensity and rates of infection in cyprinoid fish from an endemic focus in Northeast Thailand.

The infection rates and monthly variations in intensities of Opisthorchis viverrini infection in cyprinoid fish from an endemic focus in Northeast Thailand were investigated between April 1980 and March 1981. Out of six species of cyprinoid fish examined, four were found to harbour O. viverrini metacercariae. All four species of infected fish showed high rates of infection. Among the four species of O. viverrini infected fish, Cycloccheilicthys apagon and Puntius leiacanthus were found in abundance. The results indicate very little variation in the rates of infection throughout the year. On the other hand, intensities of infection varied from month to month, ranging from 8 to 88 metacercariae per fish for C. apagon and from 8 to 32 metacercariae per fish for P. leiacanthus. O. viverrini metacercariae were found in the fish muscle throughout the body; however, the body muscle harboured the highest density of metacercariae, followed by the dorsal fin, pectoral fin, pelvic fin and anal fin muscles respectively. In lieu of all these findings, it is therefore invalid to assume as indicated by the former workers that any potential intermediate fish host that does not have the O. viverrini metacercariae in its pectoral fin muscle in negative with opisthorchiasis.