The finding in this study that fish become infected with FZT as early as in the first week of growth in the nurseries and have a 48.6% infection prevalence when 4 weeks of age is of great consequence for public health and the aquaculture industry. It indicates that infection in nursery juveniles may account for most of the FZT prevalence reported for fish from grow-out ponds (12,17). Furthermore, because juveniles produced at nurseries are sold and distributed to numerous grow-out farms, the risk for FZT infection becomes widely distributed throughout the fish farming areas, complicating efforts to control this zoonosis. For example, the acquisition of FZT infection by overwintered juveniles in Nam Dinh and Ninh Binh provinces has implications for grow-out farms in nearby provinces such as Thanh Hoa, Thai Binh, and Ha Nam, as well as in provinces further north, which are major customers for these fish.

The increase of FZT prevalence during the first month in the nursing ponds is much higher than that during the ≈12-month grow-out period in Nam Dinh Province (19). Among possible explanations for this finding are more risky management practices by nursery operators or biological factors such as age-related fish susceptibility, resistance factors, and immunity. Further studies are urgently needed to understand the causes for this higher prevalence during the nursing phase and to develop efficient prevention management.

Grass carp, one of the favored sources of raw fish for human consumption, had a significantly higher FZT prevalence than other fish species, which is consistent with results of a previous study (24). This may be due to the carp’s food preference for grass and aquatic plants. In these nursing ponds, the grass and aquatic plants used for feeding were often harvested outside the farm, which may have introduced FZT-infected snails into the ponds. According to Phan et al. (19), feeding with green vegetation collected outside the farm is a risk factor for FZT infection in cultured fish. Another factor may be that grass carp spend more time in the littoral zone where vegetation and therefore snails may be concentrated. Furthermore, grass carp might be more susceptible to FZT infection than other fish species.

The higher prevalence of FZT metacercariae in Ninh Binh Province may be due to the common practice of using night soil to fertilize ponds to increase growth of plankton, a major source of carp feed. Nam Dinh Province also has a high human and domestic animal FZT prevalence (3,4,13), making fecal waste from these hosts a highly risky fertilizer. To develop effective control interventions, more research on methods to inactivate FZT eggs in manure must be conducted.

The lack of infection in fish fry is consistent with the report of Thien et al. (24), who investigated fry from hatcheries in the Mekong Delta. Fry were raised in well-controlled hatcheries by using tanks or ponds containing filtered water free of zooplankton, snails, and other microbiota.

One limitation of our study is that bias could occur when sampling fish by using 2 types of net, i.e., cast net and scoop net. However, cast nets cannot be used in fry incubators in hatcheries or to catch 1-week-old juveniles. Different methods for examining and isolating FZT metacercariae in fish fry and juveniles are needed because compression of whole fish bodies is efficient for processing of small fry and detecting any metacercariae present. For larger fish such as juveniles, however, the thickness of the body and muscle tissue precludes using compression, and instead, pepsin digestion (a more laborious procedure) is the method of choice. We also have compared pepsin digestion for several cups of fry with compressing methods and found no significant difference in detection efficiency.

Liver fluke (C. sinensis) metacercariae had a low prevalence (1.5%); most FZT species found were intestinal flukes, particularly Haplorchis spp. and Procerovum varium. Although generally not considered to have a clinical role compared with liver flukes, several intestinal FZTs can cause serious pathologic effects, even death, when infecting the heart, brain, and spinal cord of humans (1,9,11). The dominance of H. pumilio trematodes in juveniles is similar to the findings of other studies of nurseries (24) and grow-out systems (12,17,18). Stellantchasmus falcatus, however, is a major FZT species found in fish from southern and central Vietnam. Although Trung Dung et al. (4) recently recovered S. falcatus trematodes from humans in Nam Dinh, this species was not found in fish from this province, possibly because humans acquired their infections from fish originating from provinces other than Nam Dinh.

The life cycle and epidemiology of FZTs in aquaculture systems is complicated, and fish are vulnerable to multiple risks for infection. However, by developing a hazard analysis of critical control points approach to identifying points for intervention and enlisting the collaboration between different sectors such as human health, animal health, snail vector control, and aquaculture management and extension, we believe a major reduction in FZTs in farmed fish can be achieved, with the benefit of being able to provide aquaculture products free of FZTs for human consumption.

In conclusion, the results from this study strongly suggest that a program for the prevention for FZTs in farmed fish must include nurseries and grow-out farms. An integrated program will, however, require more research on the infection events occurring in the nursery pond, particularly the regulation of snail populations, fish risk behavior, and on methods to inactivate FZT eggs in manure intended for fertilizer use.