Liver and intestinal trematodes are important public health concerns in many Asian countries [1–3]. People become infected when they eat raw or insufficiently cooked fish containing the infective stage, metacercariae [4–6]. The pathology caused by liver trematode infections in people can be quite severe , while infections with intestinal trematodes generally are considered to be less severe .
Vietnam is endemic for small liver trematodes, Clonorchis sinensis (Cobbold, 1875) and Opisthorchis viverrini (Poirier, 1886), with an estimated 2 million people infected [8, 9]. While there is no estimation of the number of people infected with intestinal trematodes, such as Haplorchis pumilio (Looss, 1886), Centrocestus formosanus Nishigori, 1924 and others, the prevalence is believed to be high, e.g. in the Red River Delta and some provinces in central Vietnam [6, 10, 11].
Light infections by C. sinensis, O. viverrini or O. felineus (Rivolta, 1884) are asymptomatic, but high levels of infection and chronic infection cause damage to the bile duct epithelium, eliciting gastrointestinal problems, damage to the liver and possibly cholangiocarcinoma [2, 12–14]. Opisthorchis viverrini and C. sinensis have been rated as class 1 carcinogens by the International Agency for Research on Cancer . Lun et al.  estimated that 35 million people globally could be infected by C. sinensis, while more than 11 million people are infected with Opisthorchis spp. . Epidemiology and pathogenesis of intestinal trematodes are not well understood. Compared to liver trematodes, infection with intestinal trematodes does not generally present significant clinical symptoms [7, 12], however, some Heterophyidae species can cause significant pathology, often fatal, in the heart, brain and spinal cord of humans . It is thought that approximately 18 million people could be infected globally by these species .
The trematodes not only cause pathology in the final hosts, they also adversely impact on the intermediate hosts. Trematode infection in snails reduces survival and reproduction [19, 20]. Large numbers of Haplorchis pumilio cercariae can be lethal to fry of Oreochromis niloticus (Linnaeus, 1758) within a few hours  and high loads of Centrocestus formosanus metacercariae cause morbidity in their fish hosts , reducing their growth and increasing mortality. Presence of metacercariae in fish raised in aquaculture not only pose a health risk for people eating insufficiently cooked or raw fish, but it also reduce marketability of fish .
Aquaculture in northern Vietnam is to a large extent dependent on family-based integrated pond systems, the so-called VAC system where a garden (Vuon), a fish pond (Ao) and an animal shed (Chuong) constitute a functional unit. The VAC system is considered as an ecological way of using farm products in a natural cycle , which has been encouraged by the Vietnamese government and has had remarkable results from economic, health and nutrition perspective . In this system, manure from the husbandry is used to fertilize ponds, so as to stimulate algal growth and subsequently fish growth, and remnants from garden products and fish remains can be fed to pigs/cattle. Mud from the ponds is used to fertilize gardens or fields . In fish ponds, the main cultured fish are various carp species. Fish for seeding are produced in hatcheries and after about one week, fry is transferred to nursing ponds, where they will be kept for about 9 weeks before being sold for grow-out farmers. Previous studies have shown that fry leaving the hatcheries are free from infection with FZT , while transmission is intense in nursing ponds [6, 28] and grow-out ponds [6, 10, 28, 29].
Controlling the intermediate host snails should be part of an integrated strategy for control of these infections in fishes. Since nursery ponds are very important for the transmission it would be important to control infections in these ponds. Biological control of snails using molluscivorous fishes, such as the black carp, Mylopharyngodon piceus (Richardson, 1846), might be a viable option.
Black carp is a large cyprinid fish, with a maximum reported size of about 1.5 m and a weight of 70 kg . The native range of black carp includes most major Pacific Ocean drainages of eastern Asia from the Amur River Basin to the West-Pearl River Basis and the Red River of northern Vietnam. Larvae and small juveniles feed almost entirely on zooplankton and aquatic insects while larger juveniles and adults feed mainly on molluscs . The shift in diet occurs with the development of pharyngeal teeth at lengths from 3–33 cm, depending on the development rate of the fish and abundance of prey available . The black carp is already used for biological control of snails in various parts of the World [30–35]. The major concern about using black carp outside its natural distribution is its potential spread to large rivers, where it may establish populations, with potential severe effect on native mollusc populations .
We attempted to use black carp in nursery ponds yet this could present a major challenge because the black carp used, should have an appreciable size compared to the fry, so that the fry are protected as the larger carp tend to consume the intermediate host snails (species of Thiaridae and Bithynidae). One concern about using black carp in nursery ponds is that it might switch diet from snails to the fry that they are supposed to protect from infection. Thus it has been observed that fish, which in their natural habitat are specialized molluscivores, will switch to a softer diet when introduced into ponds for biological control . Hung  showed that juvenile black carp could be used in nursery ponds without affecting survival of fry or juvenile fishes. Disturbances to the pond environment caused by the black carp, however, might affect fry survival; for example suspension of bottom sediment in the water due to its feeding at the bottom. This paper describes two field trials using black carp to control intermediate host snails in (1) a pond without other fishes and (2) in real nursery ponds.