Prevalence of zoonotic trematode parasites in fish fry and juveniles in fish farms of the Mekong Delta, Vietnam

Zoonotic parasites are a significant food safety problem, particularly in Asia. In Vietnam fishborne zoonotic trematodes (FZT) are highly prevalent in fish cultured in grow-out farms. However, FZT infection status of juveniles produced and distributed by hatcheries and nurseries is unknown. Here we report an epidemiological investigation on FZT in fry and juveniles of major cultured freshwater fish species in four provinces of the Mekong Delta of Vietnam. No FZT infections were found in 14 species of fry sampled from hatcheries. In contrast, nursery juveniles of river catfish, hybrid catfish, giant gouramy, climbing perch, common carp, kissing gouramy, silver barb, silver carp, grass carp, Indian carp, pacu, tilapia and snakeskin gouramy were frequently infected with FZT metacercariae (range 1.2–29.7%). Seasonal variation in prevalence was observed: prevalence in river catfish and hybrid catfish were maximal in January, at the end of the flooding season, while the prevalence in juveniles of giant gouramy, climbing perch, common carp, kissing gouramy, silver barb, silver carp grass carp, mrigal and pacu were higher in the wet season, June to November. Overall, FZT prevalence was highest in climbing perch and giant gouramy (29.7% and 27.8%, respectively) and the lowest in river catfish (1.2%). The density of FZT metacercariae in fish varied seasonally only in climbing perch which was maximal in the wet season (P < 0.05), compared to the dry season (430 vs 28 metacercariae/100 g of fish). These results demonstrate that acquisition of infected seed stock from nurseries is a serious risk for Vietnamese grow-out fish farms, and stress that interventions to control FZT must focus also on these stages of the cultured fish production cycle.