Although no intermediate host snails were detected in 26 of the 60
ponds, only 4 of these 26 ponds were also negative for FZT infection in
fish. This could indicate that our sampling effortwas not intense enough
to detect host snails or that ponds are contaminated with FZT cercariae
from surrounding habitats where host snails also are abundant [11,
pers. obs.]. Infections among snails in these surrounding water bodies
in some areas are likely because of the presence of domestic animal reservoir
hosts,mainly dogs, cats and pigs, and fish eating birds, play an important
role in the transmission cycle of FZT in Vietnam[23,24]. Clausen
et al. [10] also observed that in ponds with high FZT prevalence in fish,
snails could be absent, again suggesting an external source of cercariae,
probably from surrounding water bodies that were connected to the
ponds, e.g. sources of water for pond filling. In grow-out ponds daily
water replacement varies from a few percent in low exchange systems
up to 20% or more in high water-exchange systems [25]. The possibility
exists that cercariae enter these nursery ponds with replacementwater
from canals but the extent to which this happens remains to be
investigated.
 
Although no intermediate host snails were detected in 26 of the 60ponds, only 4 of these 26 ponds were also negative for FZT infection infish. This could indicate that our sampling effortwas not intense enoughto detect host snails or that ponds are contaminated with FZT cercariaefrom surrounding habitats where host snails also are abundant [11,pers. obs.]. Infections among snails in these surrounding water bodiesin some areas are likely because of the presence of domestic animal reservoirhosts,mainly dogs, cats and pigs, and fish eating birds, play an importantrole in the transmission cycle of FZT in Vietnam[23,24]. Clausenet al. [10] also observed that in ponds with high FZT prevalence in fish,snails could be absent, again suggesting an external source of cercariae,probably from surrounding water bodies that were connected to theponds, e.g. sources of water for pond filling. In grow-out ponds dailywater replacement varies from a few percent in low exchange systemsup to 20% or more in high water-exchange systems [25]. The possibilityexists that cercariae enter these nursery ponds with replacementwaterfrom canals but the extent to which this happens remains to beinvestigated.
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