Prevention and Control of Fish-borne Zoonotic Trematodes in Fish Nurseries, Vietnam

Jesper Hedegaard Clausen

, Henry Madsen, K. Darwin Murrell, Phan Thi Van, Ha Nguyen Thi Thu, Dung Trung Do, Lan Anh Nguyen Thi, Hung Nguyen Manh, and Anders Dalsgaard

Author affiliations: University of Copenhagen, Copenhagen, Denmark (J.H. Clausen, H. Madsen, K.D. Murrell, A. Dalsgaard); Research Institute for Aquaculture No.1, Bac Ninh, Vietnam (P.T. Van, H.N.T. Thu); National Institute of Malariology, Parasitology, and Entomology, Hanoi, Vietnam (D.T. Do); National Institute of Veterinary Research, Hanoi (L.A.N. Thi); and Vietnam Academy of Science and Technology, Hanoi (H.N. Manh)

Abstract

Worldwide, >18 million persons were infected with fish-borne zoonotic trematodes in 2002. To evaluate the effectiveness of interventions for reducing prevalence and intensity of fish-borne zoonotic trematode infections in juvenile fish, we compared transmission rates at nurseries in the Red River Delta, northern Vietnam. Rates were significantly lower for nurseries that reduced snail populations and trematode egg contamination in ponds than for nurseries that did not. These interventions can be used in the development of programs for sustained control of zoonotic trematodes in farmed fish.

Liver and intestinal trematodes are major fish-borne zoonotic parasites of humans. The World Health Organization and the Food and Agriculture Organization of the United Nations estimate that >18 million persons were infected with fish-borne zoonotic trematodes (FZTs) in 2002 (1,2), and the World Health Organization recently added FZT infections to its list of emerging infectious diseases. FZTs are especially problematic in Asian countries, where fish are a main source of protein (3). They are highly prevalent in Vietnam, in cultured and wild-caught fish (4–7), and cause major concern for food safety (8,9).
In aquaculture systems, the main risk factors for FZT infection and transmission include contamination of pond environments with FZT eggs from infected hosts, i.e., humans, cats, dogs, pigs, and fish-eating birds. Factors that promote the diversity and population growth of snail intermediate hosts (families Thiaridae and Bithynidae) also increase risk (10–14). In Vietnam, a large proportion of aquaculture production, mainly of cyprinid fishes, originates from small family farms that integrate other agricultural activities. These integrated fish–livestock systems are called VAC systems (in Vietnamese, the word vuon means garden, ao means pond, and chuong means pigpen). Prevalence studies have demonstrated that in Vietnam, these integrated systems pose the highest risk for FZT transmission (12,15). The ponds become contaminated through use of animal, and occasionally human, fecal waste as pond fertilizer and through runoff water from pond banks and adjoining fields. Fish fry (newly hatched fish) are introduced into these ponds at about 2–3 days of age, and when they become juveniles (after 6 weeks), they are transferred to grow-out ponds. Efforts to control snail invasion and their population growth in fish ponds are usually minimal. FZT egg contamination of pond environments and FZT transmission are increased by the practice of feeding fish waste to domestic animals and by the cultural preference for eating raw or inadequately prepared fish dishes (16).
Recent findings from an investigation in northern Vietnam (4,15,17) revealed high rates of FZT transmission, especially in fish nurseries, where FZT prevalence among stocked FZT-free fish fry increased to 14.1%, 48.6%, and 57.8% after 1 week, 4 weeks, and when overwintered in ponds, respectively. The juvenile fish raised in nurseries are eventually transferred to grow-out farms, thereby potentially seeding a large number of grow-out farms with FZTs.
Our study objective was to evaluate the effectiveness of interventions to prevent FZT transmission in nurseries in northern Vietnam. Intervention goals were to reduce FZT egg contamination and to control snail populations in nursery ponds. The study is the first stage in developing a practical and sustainable program for prevention of FZT infections in aquaculture.
The project objective, planned interventions, and sampling procedures were approved by all participating institutions and their research committees in Vietnam. Risks, rights, and benefits were explained to all participating intervention farmers at the beginning of the project. Farmers were informed that they were free to withdraw from the study at any time. Persons from households associated with nonintervention nurseries were examined at the end of the study, and treatment was provided for free to those with egg counts indicating infection with trematodes. Thus, all household members received the same health benefits.