Two-year intervention trial to control of fish-borne zoonotic trematodes in giant gourami (Osphronemus goramy) and striped catfish (Pangasianodon hypophthalmus) in nursery ponds in the Mekong Delta, Vietnam.

Fish-borne zoonotic trematode parasites (FZT) pose a food safety and public health problem in Vietnam. The transmission cycle is complex as domestic animals, especially dogs, cats, fish-eating birds and pigs together with humans serve as reservoir hosts and contribute to FZT egg contamination of aquaculture ponds and the environment. This intervention trial was conducted to determine the effectiveness of various on-farm interventions, including reduction in FZT egg contamination through treatment of infected people and domestic animals, reduction in snail density through mud removal from aquaculture ponds prior to fish stocking, and various other measures in reducing FZT infection in juvenile striped catfish (Pangasianodon hypophthalmus) and giant gourami (Osphronemus goramy). Interventions were implemented on 5 farms for each fish species during production cycles in 2009 and 2010 while 5 similar farms for each species served as control. For both fish species, both prevalence and intensity of infection did not differ significantly between intervention and non-intervention farms prior to the interventions. The interventions significantly reduced both prevalence and intensity of FZT infection in the juvenile fish compared to control ponds. For giant gourami, odds of infection in intervention ponds was 0.13 (95% CL: 0.09-0.20; p<0.001) of that in non-intervention ponds after the 2009 trial and 0.07 (0.03-0.14; p<0.001) after the 2010 trial. For striped catfish, these figures were 0.17 (0.08-0.35; p<0.001) after the 2009 trial while after the 2010 trial all ponds with interventions were free from infection. Metacercariae intensity (no. of metacercariae/fish) in giant gourami from intervention ponds was 0.16 (0.11-0.23; p<0.001) of that in fish from non-intervention ponds after the 2009 trial and 0.07 (0.04-0.15; p<0.001) after the 2010 trial; for striped catfish these figures were 0.18 (0.09-0.36; p<0.001) and 0.00 (confidence limits not estimated), respectively. The aquaculture farm pond intervention approaches taken in this trial have the potential to reduce or eliminate FZT infections in fish and may be implemented across the entire region if adjusted to local conditions and fish species.

Effect of pond water depth on snail populations and fish-borne zoonotic trematode transmission in juvenile giant gourami (Osphronemus goramy) aquaculture nurseries.

Infection with fish-borne zoonotic trematodes (FZT) is an important public health problem in many parts of Southeast Asia. People become infected with FZT when eating raw or undercooked fish that contain the infective stage (metacercariae) of FZT. The parasites require specific freshwater snails as first intermediate host and a variety of fish species, both wild caught and cultured, as second intermediate host. Aquaculture production has grown almost exponentially in SE Asia and in order to produce fish free from FZT metacercariae, it is important to mitigate factors promoting transmission to fish. Here we report results from a cross-sectional study to look at the association between pond depth and infection with FZT in giant gourami nursery ponds. Density of intermediate host snails was positively associated with pond depth (count ratio associated with a 1m increase in pond depth was 10.4 (95% C.L.: 1.61-67.1, p<0.5)) and this may partly explain the higher prevalence and intensity of FZT infection in juvenile fish. High fry stocking density (>200 fry m(-3)) was associated with lower host snail density (count ratio=0.15) than low stocking density (<100 fry m(3)). Ponds stocked with 100-200 fry m(-3) had snail counts 0.76 (95% C.L.: 0.33-1.75, p n.s.) of those in ponds stocked with fry density of <100 fry m(-3). Since density of intermediate snail hosts was associated with FZT transmission to fish, effort should be taken to reduce snail density prior to stocking the fry, but focus should also be on habitats surrounding ponds as transmission may occur through cercariae produced outside ponds and carried into ponds with water pumped into ponds.