Comparative Risk of Liver and Intestinal Fluke Infection from Either Wild-Caught or Cultured Fish in Vietnam.

A survey for fish-borne zoonotic trematode (FZT) metacercariae in fish from the Northern Mountain Region of Vietnam was conducted from May to August 2014. The major objective was to evaluate the impact of water body types on the prevalence of FZT. A total of 846 fish (31 species) were collected from three water body types, including aquaculture ponds, rivers, and reservoirs. A pepsin digestion method was used for the recovery of metacercariae from fish. Overall, 29 of the 31 fish species were infected with FZTs. Prevalence was 56.8% in river fish, 46.7% in pond fish, and 29.3% in reservoir fish. The prevalence of Clonorchis sinensis metacercariae was 10.6% in reservoir fish, and 2.6% in river fish; fish from ponds were uninfected. The infection intensity of C. sinensis was 29.7 metacercariae/reservoir fish and 2.1 metacercariae/river fish. The prevalence of C. sinensis was highest (25.9%) in Toxabramis houdemeri. The intestinal flukes Haplorchis pumilio, Haplorchis taichui, Haplorchis yokogawai, Centrocestus formosanus and Procerovum varium were recovered from 29 fish species, with an overall prevalence of 46.8% and a mean infection intensity of 23.6 metacercariae. The results indicate that a C. sinensis sylvatic cycle involving wild fish species is important in the epidemiology of liver fluke and that consumption of raw or inadequately prepared wild-caught fish is a the major risk factor for human Clonorchiasis in this region. Both wild-caught and cultured fish present a high risk for infection with intestinal flukes. The habitat requirements of the snail host may be the important determining factor in favoring a sylvatic cycle of C. sinensis. This information is relevant to the development of effective prevention and control strategies, and emphasizes the priority that must be given to education of the local communities on the risks of consuming raw or improperly prepared fish dishes.

Relationship between snail population density and infection status of snails and fish with zoonotic trematodes in Vietnamese carp nurseries.

BACKGROUND: Fish-borne zoonotic trematodes (FZT) are a food safety and health concern in Vietnam. Humans and other final hosts acquire these parasites from eating raw or under-cooked fish with FZT metacercariae. Fish raised in ponds are exposed to cercariae shed by snail hosts that are common in fish farm ponds. Previous risk assessment on FZT transmission in the Red River Delta of Vietnam identified carp nursery ponds as major sites of transmission. In this study, we analyzed the association between snail population density and heterophyid trematode infection in snails with the rate of FZT transmission to juvenile fish raised in carp nurseries. METHODOLOGY/PRINCIPAL FINDINGS: Snail population density and prevalence of trematode (Heterophyidae) infections were determined in 48 carp nurseries producing Rohu juveniles, (Labeo rohita) in the Red River Delta area. Fish samples were examined at 3, 6 and 9 weeks after the juvenile fish were introduced into the ponds. There was a significant positive correlation between prevalence of FZT metacercariae in juvenile fish and density of infected snails. Thus, the odds of infection in juvenile fish were 4.36 and 11.32 times higher for ponds with medium and high density of snails, respectively, compared to ponds where no infected snails were found. Further, the intensity of fish FZT infections increased with the density of infected snails. Interestingly, however, some ponds with no or few infected snails were collected also had high prevalence and intensity of FZT in juvenile fish. This may be due to immigration of cercariae into the pond from external water sources. CONCLUSIONS/SIGNIFICANCE: The total number and density of potential host snails and density of host snails infected with heterophyid trematodes in the aquaculture pond is a useful predictor for infections in juvenile fish, although infection levels in juvenile fish can occur despite low density or absence infected snails. This suggests that intervention programs to control FZT infection of fish should include not only intra-pond snail control, but also include water sources of allochthonous cercariae, i.e. canals supplying water to ponds as well as snail habitats outside the pond such as rice fields and surrounding ponds.

Prevalence and risks for fishborne zoonotic trematode infections in domestic animals in a highly endemic area of North Vietnam.

Fishborne zoonotic trematodes (FZT) are endemic in humans and cultured fish in Vietnam but little is known about FZT in domestic animals. A study was designed to determine FZT prevalence and species diversity, and risk factors for infection, in dogs, cats and pigs. Faecal samples from 186 dogs, 94 cats and 168 pigs belonging to 132 households in Nghia Hung district, Nam Dinh province, were examined for small trematode eggs; those were trematode eggs with length less than 50 microm. Prevalence of FZT varied significantly between cats (70.2%), dogs (56.9%) and pigs (7.7%). Forty-nine of the egg-positive animals (25 dogs, 20 cats and 4 pigs) were necropsied to obtain adult trematodes for identification. The liver fluke, Clonorchis sinensis, and 11 species of intestinal flukes including Haplorchis, Stellantchasmus, Stictodora and Centrocestus were recovered from the infected animals. The practice of feeding raw fish to the animals was a significant risk factor for infection; this risk was reduced if the animals were periodically treated with anthelmintics. Based on the high prevalence of FZT and certain risky husbandry practices, domestic animals are likely to be major contributors of FZT eggs to the environment. Therefore, education of farmers to avoid feeding raw fish and to perform regular anthelmintic treatment of dogs, cats and pigs is needed in integrated FZT control programs.