Fish-borne zoonotic trematodes, such as Clonorchis sinensis, heterophyids and others,
constitute a public health concern in parts of northern Vietnam and infections with
these trematodes are often thought to be linked to fish culture. One common fish culture
system is the integrated fish-livestock (VAC) ponds where individual households have
1 or more ponds. Fish fry, mainly of various carp species, produced in hatcheries,
not necessarily local, are introduced into nursery ponds and after approximately 6
weeks, juvenile fishes are transferred to household ponds, referred to as grow-out
ponds. Grow-out ponds are usually fertilized with organic debris, including animal
excreta, to stimulate algal growth and subsequently fish growth. This paper describes
the distribution of freshwater snails and occurrence of trematode infections in these
in VAC ponds and associated habitats as part of a major study on risk factors of FZT
infections in cultured fish in two communes, Nghia Lac and Nghia Phu, Nghia Hung District,
Nam Dinh Province. The area is under intense rice cultivation with an extensive canal
network supplying fields and also household VAC ponds. A total of 16 snail species
was found and four were widely distributed i.e. Angulyagra polyzonata, Melanoides
tuberculata, Bithynia fuchsiana and Pomacea insularum. Snail diversity and counts
were higher in nursery ponds than in grow-out ponds. Species of the families Thiaridae
and Viviparidae were more abundant than other species in VAC ponds while species of
the Bithyniidae, Stenothyridae and Planorbidae dominated in rice fields and small
canals. Trematode infections were found in eight snail species and among these M.
tuberculata had the highest overall prevalence of infection (13.28%). No trematode
infections were found in species of the Viviparidae and Ampullaridae except for metacercariae.
Parapleurolophocercous and pleurolophocercous cercariae constituted the most common
type of cercariae recovered, contributing 40.6% of all infections followed by echinostome
cercariae (35.0%) and xiphidiocercariae (17.3%). Bithynia fuschiana and M. tuberculata
had the most diverse trematode fauna. C. sinensis was not recorded in this study.
The VAC pond system in this area, is very important for transmission of minute intestinal
trematodes while they play little role in transmission of C. sinensis as its intermediate
hosts, bithynid snails, rarely occur in these ponds. From a public health perspective
this is positive as the effects of infections with intestinal trematodes are considered
mild. On the other hand it is possible that even such subtle effects could have importance
in public health as transmission is very intense in the area. And this in combination
with the aquaculture importance, reduced marketability of fishes with high metacercariae
loads, warrants that control efforts against these trematodes are initiated to reduce
transmission in this production system.
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