Effects of fermentation time and low temperature during the production process of Thai pickled fish (pla-som) on the viability and infectivity of Opisthorchis viverrini metacercariae.

Contamination of a popular fermented fish dish, pla-som, by Opisthorchis viverrini metacercariae (OVMC) is a possible cause of carcinogenic liver fluke infection in Thailand. Affected individuals are at risk of bile duct cancer, which is a major health problem for people in the Greater Mekong Subregion. In order to investigate concerns about food safety, we studied the effects of fermentation time and low temperature on the viability and infectivity of OVMC during the pla-som production process. Pla-som was prepared at room temperature for up to 1 week in duplicate experiments using cyprinid freshwater fish obtained from an O. viverrini-endemic area. OVMC were then isolated and identified under a stereomicroscope. Complete and viable OVMC were found on days 1-4 of fermentation, while their morphology was degenerated thereafter. After OVMC were fed to hamsters, the percentage of the worm recovery after 1 to 2 months of infection was 52%, 44.7%, 11.3% and 1% for days 1, 2, 3 and 4, respectively. In order to measure the effect of low temperature on OVMC, fish were kept in a refrigerator (4 °C) for up to five days and then subsequently fermented for three days. In fish stored in a refrigerator for 1 and 2 days, viable OVMC were clearly observed and were able to infect hamsters, a worm-recovery percentage of 3.3% and 12.7%, respectively. By contrast, in pla-som prepared from fish stored for 3 to 5 days, OVMC were degenerated and could not infect the host. In conclusion, pla-som fermentation for more than four days and refrigerating fish for three days before pla-som processing can prevent O. viverrini infection. This study may increase awareness of fermented-fish dish preparation to prevent liver fluke infection.