High reinfection rate after preventive chemotherapy for fishborne zoonotic trematodes in Vietnam.

BACKGROUND: The World Health Organization aims for complete morbidity control of fishborne zoonotic trematodes (FZT) in endemic areas by 2020. The main intervention tool for achieving this goal is regular use of preventive chemotherapy by offering praziquantel to those at risk in endemic areas. The purpose of this study was to investigate the effectiveness of preventive chemotherapy to control FZT in an endemic area in Northern Vietnam. METHODOLOGY AND PRINCIPLE FINDINGS: We followed a cohort of 396 people who fulfilled the criteria for receiving preventive chemotherapy. Stool samples were examined by Kato-Katz technique for the presence of trematode eggs before, and two, 16, 29 and 60 weeks after preventive chemotherapy. The prevalence of trematode eggs in stool was 40.2% before, 2.3% two weeks after and increased to a cumulative prevalence of 29.8% sixty weeks after preventive chemotherapy. CONCLUSIONS: The effectiveness of preventive chemotherapy as a main component in control of FZT is not well documented in most endemic areas. We found a high reinfection rate within the first year after preventive chemotherapy. Since these trematodes are zoonoses, preventive chemotherapy may not have sufficient impact alone on the transmission to have a lasting effect on the prevalence. Animal reservoirs and farm management practices must be targeted to achieve sustainable control of fishborne zoonotic trematode infections, hence control programs should consider a One Health approach.

Occurrence and species distribution of fishborne zoonotic trematodes in wastewater-fed aquaculture in northern Vietnam.

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.