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 b 0.5)) and this may partly explain the higher prevalence and intensity
of FZT infection in juvenile fish. High fry stocking density (N200 fry m−3) was associated with lower host
snail density (count ratio = 0.15) than low stocking density (b100 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 b100 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.