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

• Deeper ponds had higher density of FZT intermediate host snails.
• Snail host density is positively related to prevalence and intensity of FZT infection in fish.
• High stocking density with fry of Giant gourami is negatively associated with snail host density.
• Transmission of FZT to fish in ponds may also occur from immigration of cercariae produced outside ponds.

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 1 m 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 m3). 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.

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