Aquaculture and Zoonotic Trematodes, Vietnam | CDC EID

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Volume 16, Number 12–December 2010
Research
Freshwater Aquaculture Nurseries and Infection of Fish with Zoonotic Trematodes, Vietnam

Van Thi Phan, Comments to Author Annette Kjær Ersbøll, Thanh Thi Nguyen, Khue Viet Nguyen, Ha Thi Nguyen, Darwin Murrell, and Anders Dalsgaard
Author affiliations: Research Institute for Aquaculture No.1, Bac Ninh, Vietnam (V.T. Phan, K.V. Nguyen, H.T. Nguyen); University of Southern Denmark, Odense, Denmark (A.K. Ersbøll); Vinh University, Vinh, Vietnam (T.T. Nguyen); and University of Copenhagen, Copenhagen, Denmark; (V.T. Phan, D. Murrell, A. Dalsgaard)

Suggested citation for this article

Abstract
Residents of the Red River Delta region of northern Vietnam have a long tradition of eating raw fish. Fish-borne zoonotic trematodes (FZTs) are estimated to infect ≈1 million persons in Vietnam. It remains uncertain at what stages in the aquaculture production cycle fish become infected with FZTs. Newly hatched fish (fry) from 8 hatcheries and juveniles from 27 nurseries were therefore examined for FZT infection. No FZTs were found in fry from hatcheries. In nurseries, FZT prevalence in juveniles was 14.1%, 48.6%, and 57.8% after 1 week, 4 weeks, and when overwintered in ponds, respectively. FZT prevalence was higher in grass carp (p<0.001) than in other carp species. Results show that nurseries are hot spots for FZT infections in fish. Thus, sustainable FZT prevention strategies must address aquaculture management practices, particularly in nurseries, to minimize the risk of distributing infected juveniles to grow-out ponds and, subsequently, to markets for human consumption. Liver and intestinal infections caused by fish-borne zoonotic trematodes (FZTs) are increasingly being recognized as serious public health problems and with FZTs incorporated among causes of neglected tropical diseases (1,2). FZTs are especially widespread in Southeast Asia, including Vietnam, Lao People's Democratic Republic, Thailand, Cambodia, People's Republic of China, and North and South Korea (3–9). Liver flukes are associated with high incidence of bile duct cancer (1,10), and intestinal flukes cause serious pathologic changes in the heart, brain, and spinal cord (1,2,11). The epidemiology of FZTs is complex because humans and reservoir hosts, such as dogs, cats, pigs, and fish-eating birds, harbor egg-shedding adult stages (12,13). These hosts are infected by consumption of raw, inadequately cooked, or pickled fish. For many inhabitants in the Red River Delta provinces of northern Vietnam, the consumption of such fish dishes is a traditional behavior that is difficult to alter (14–16). In the Nam Dinh and Ninh Binh provinces, the widespread habit of eating raw fish is associated with a high FZT prevalence of 30%–40% in humans (3,4). Aquaculture fish species commonly used to prepare raw fish dishes, such as carp, frequently also have high a prevalence of FZT metacercariae (12,17–19). The influence of FZTs on the food safety of aquaculture products can have a noticeable adverse economic and public health effect because fish farming in Asia is expanding rapidly. Farm-raised fish are a main protein source consumed domestically and an essential product for exporting to other countries (2,10,20). Therefore, the production of FZT-free fish for human consumption should be a key objective for the aquaculture industry. Achieving this goal is seriously hampered, however, because the present state of knowledge on FZT infection in the fish production chain is inadequate to devise practical and sustainable prevention strategies, especially for small-scale and integrated freshwater aquaculture. The available knowledge of FZT infection is mainly obtained from studies of fish in grow-out ponds, where fish are harvested for human consumption (12,17,19). Freshwater fish hatcheries in Nam Dinh, Ninh Binh, and Bac Ninh provinces include facilities such as a water reservoir, water storage facilities, breeding tanks, and incubators for hatching eggs. Depending on the hatchery, the water reservoirs are cement tanks or consist of earthen ponds from which the water is either pumped into cement breeding tanks or supplied from a tower to the breeding tanks and egg incubators. Water used for the breeding tanks and incubators is filtered through a net to remove different microbiota, e.g., zooplankton. Brood stock are moved from earthen ponds into cement breeding tanks for induced spawning. Fertilized eggs are then incubated in round cement incubators with running water for ≈5 days, depending on fish species and temperature. Newly hatched fish are termed fry. Fry are kept in tanks for 3–5 days, after which they are sold and subsequently stocked in earthen ponds in so-called nurseries. The fish raised in nursery ponds are called juveniles. The nursery ponds in Nam Dinh and Ninh Binh provinces are mainly backyard earthen ponds located close to households and premises housing livestock and poultry. Juveniles are nursed up to 4 weeks and then sold for further nursing to bigger size or to be stocked to reach market size in grow-out ponds. Juveniles may be kept in ponds during the winter months (overwintered juveniles) for sale in early spring. Management of nurseries in northern Vietnam often involves the application of livestock manure as fertilizer before stocking fish to increase the density of plankton that serves as a food source for juveniles. Additionally, farmers may also apply night soil (human manure) as fertilizers to the nursing ponds. We report an investigation that aimed to determine the FZT infection status in integrated small-scale hatcheries and nurseries in Nam Dinh, Ninh Binh, and Bac Ninh provinces, which are major areas endemic for FZTs in Vietnam. By assessing the FZT metacercariae prevalence in fish from the initial stages of production, namely the hatcheries and nurseries, the study provides knowledge needed for a comprehensive assessment of FZT infection during the entire fish production cycle. full-text: Aquaculture and Zoonotic Trematodes, Vietnam | CDC EID

Suggested Citation for this Article

Phan VT, Ersbøll AK, Nguyen TT, Nguyen KV, Nguyen HT, Murrell D, et al. Freshwater aquaculture nurseries and infection of fish with zoonotic trematodes, Vietnam. Emerg Infect Dis [serial on the Internet]. 2010 Dec [date cited]. http://www.cdc.gov/EID/content/16/12/1905.htm

DOI: 10.3201/eid1612.100422

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Van Thi Phan, Centre for Environment Disease Monitoring in Aquaculture–Research Institute for Aquaculture No. 1, Dinh Bang Tu Son, Bac Ninh 84, Vietnam; email: phanvan@ria1.org