Objectives: To assess the risk of fishborne zoonotic trematode (FZT) infection in fish reared in wastewater-fed ponds in peri-urban areas of northern Vietnam.
Methods: We collected 1200 fish from aquaculture systems receiving wastewater from the cities of Hanoi and Nam Dinh in northern Vietnam and examined them for zoonotic metacercariae. Collection periods were fall (October-November 2005) and spring (April-June 2006). The fish were digested in pepsin and metacecariae, recovered and identified by light microscopy. Data were expressed as both prevalence and intensity of infection.
Results: FZT metacercariae were recovered from fish samples at both sites and during both sampling periods. The overall prevalence of FZT was 5% (2.0% in fall and 6.5% in spring) in fish from Hanoi and 4.6% (2.4% in fall and 5.7% in spring) in fish from Nam Dinh. The higher prevalence in the warmer months was likely due to ecological factors such as temperatures and population dynamics of the snail vectors. All metacercariae recovered were zoonotic intestinal trematodes of the family Heterophyidae. The infected fish included tilapia (Oreochromis niloticus) and three species of carps, including silver carp (Hypophthalmichthys molitrix), which is commonly used for the preparation of raw fish dishes. Intensities of infection were relatively low.
Conclusion: Fish raised in peri-urban wastewater-fed aquaculture systems are at risk of infection with trematode parasites, which can be transmitted to humans through consumption of raw or improperly prepared fish. However, the prevalence was low as compared to previous findings of FZT in non-wastewater fish elsewhere in Vietnam. Studies are needed to assess the relative importance of different risk factors for transmission and control of FZT in wastewater-fed aquaculture.