Reported is the seasonal pattern of Opisthorchis viverrini metacercariae in cyprinoid fish in north-east Thailand. Samples of fish were collected in 1991-92 at monthly intervals from two areas-Khon Kaen Province, where the opisthorchiasis transmission rate was high, and Mahasarakham Province, where the rate was low. Metacercarial loads in both study areas had similar seasonal patterns. High burdens occurred in the late rainy season and winter (July to January) with low burdens during the summer (March to June). The average burden for Puntius leiacanthus in Khon Kaen was 1.68 metacercariae per fish (127.43 per kg), higher than for all species of cyprinoid fish from the low transmission area. The intensities of infection among P. leiacanthus and Cyclocheilichthys armatus collected in Mahasarakham were comparable, but lower than the intensity of Hampala dispar (0.75 metacercariae per fish) concurrently sampled from the same area (P < 0.05). There was no significant difference in metacercarial load per kg between fish species from Mahasarakham. The results indicate that seasonal variation in metacercariae was a common phenomenon in areas with both high and low endemicity of infection. Also, the metacerarial load in fish was positively associated with infection levels among humans.
PIP: Food-borne trematode infections such as Opisthorchis viverrini are a major cause of morbidity in Asia. Samples of cyprinoid fish collected at monthly intervals during 1991-92 in two provinces (Khon Kaen and Mahasarakham) in North East Thailand revealed substantial seasonal variation of O viverrini metacercariae. The former province is a low transmission area (29.7% average infection rate), while the latter is a high transmission area. Metacercarial intensity was highest from January-March 1991 and October 1991-April 1992, with maximum intensities during February and December 1991. Puntius leiacanthus fish from Khon Kaen bore the highest density of metacercariae per individual fish (1.68). Differences in the intensity of infection for the same species of fish captured in two different geographic areas presumably reflect variations in environmental conditions such as local levels of human and snail infections. Fecal contamination of the study reservoirs is greatest during the early parts of the rainy season as a result of run-off drainage and coincides with the rapid increase in snail populations. The optimal time for large-scale community-based parasite control programs is when the probability of rapid reinfection is lowest and the prevalence and intensity of infection in people are highest (e.g., April-June or the summer months). Most effective is a program that integrates parasite control, environmental management, health education, and food safety.