Fish-borne Trematodes in Vietnam

From 1976 to 2000, over 30,000 human stool samples from 38 communes in 15 provinces were examined. The average infection rate of Clonorchis and Opisthorchis in the provinces examined was 21%. In these provinces, the infection rates were distributed as follows: 0.6% in Ha Giang, 0.2% in Thai Binh, 16.3% in Bac Giang, 13.1% in Hai Phong, 5% in Hoa Binh, 16% in Ha Tay, 3-37.5% in Nam Dinh, 3% in Ha Nam, 2030% in Ninh Binh, 11% in Thanh Hoa, 0.9% in Nghe An, 0.3% in Da Nang, 11.9% in Binh Dinh, 36.9% in Phu Yen, and 7.6% in Dak Lak. The infection rate of the liver flukes was 3 times higher in men than women, and higher in adults than children. The highest prevalence (50.2-51.6%) was recorded in the age group 40-50 years. In the north of Vietnam, C. sinensis infection was microscopically identified and confirmed by molecular approach (PCR technique). In the south of Vietnam, O. viverrini infection was microscopically detected only and has not been molecularly identified yet. The infection rates of Clonorchis/Opisthorchis in dogs and cats, animal reservoir hosts, were 28.6 and 64.2%, respectively, among which 814 flukes were particularly detected in the liver of one cat. Intermediate hosts, fish, have been examined for trematodes in the northern regions of Vietnam. Ten species of freshwater fish were examined, of which 7 species were positive for Clonorchis metacercariae. The main heavily-infected fish species was Hypophtalmichthis molitrix, at rates ranging from 44.5-92.9%, with an average of 75 metacercariae per fish. In the southern provinces, one (Carassius carassius) of 10 species of freshwater fish was infected with Opisthorchis metacercaria (10-29%). Snail examinations carried out in the northern (Red River Delta) region revealed that the infection rate of Clonorchis cercariae was 10.2% (in Melanoides tuberculatus) and 5.1% (in Parafossarulus striatulus). In the southern provinces, the cercarial infection rates of Opisthorchis were 2.6-8% (in Melanoides tuberculatus). The habit of eating raw fish is very common throughout Vietnam, with a long tradition. The rates of eating raw fish were 70-80% among populations in some provinces. In Vietnam, treatment of clonorchiasis/opisthorchiasis has been done by praziquantel 25 mg/kg /day x 3 days, with cure rates of 65-91%. Control of diseases in Vietnam by case management combined with IEC activities, reduced infection rates and intensity of infection. MATERIALS AND METHODS Human prevalence surveys Mass surveys in populations have been undertaken in areas where the local people have a habit of eating raw fish. Stool samples were analyzed by Kato and Kato-Katz techniques. For some human surveys, data on age, sex, and habit of eating raw fish were analyzed. Adult worms were collected from patients, stored in alcohol 70% in -20 ̊C for molecular analysis (PCR). Intermediate host surveys Fish examinations for metacercariae were performed either by direct compression of muscle or fins between glass for microscopic viewing, or by pepsin-HCl digestion of 10 gram muscle samples (WHO, 1995). Snails examined for cercariae were crushed, and a drop of the released liver-pancreatic fluid was added and examined with a microscope. Reservoir host surveys Potential mammal reservoir hosts for zoonotic trematodes were obtained from known endemic areas; these included cats and dogs. Cats and dogs were necropsied and their organs (liver/bile ducts) searched for adult worms. Adult worms were fixed, stained, INTRODUCTION Parasitic zoonoses are widespread in Southeast Asia, particularly Vietnam. Among fish-borne trematode infections caused by many species, only clonorchiasis and opisthorchiasis have been determined in Vietnam. The documented history of these parasites is quite long. As early as 1887, Clonorchis sinensis was reported from North Vietnam (Tonkin) by Grall (Bernard et al, 1924), with a prevalence of 28% in northern Vietnam (Mathis and Leger, 1910). Opisthorchis felineus was reported in Vietnam in 1907 by Verdun et al (1924), and Opisthorchis viverrini by De et al (1996) and Chuong et al (1997). Since 1976, epidemiological research on food-borne trematode infections, paticularly fish-borne trematodes has been carried out in Vietnam by the National Institute of Malariology, Parasitology and Entomology (NIMPE) in Hanoi. These studies have contributed to strategies of food-borne trematode control in Vietnam. Correspondence: Nguyen Van De, National Institute of Malariology, Parasitology and Entomology, Hanoi, Vietnam. SOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 300 Vol 35 (Suppl 1) 2004 and prepared for taxonomic studies according to standard methods (Schell, 1970). Experimental infections Metacercariae recovered from either fish were administered to laboratory animals (cats or dogs), to obtain adult stages for species identification. Taxonomic determinations The species determinations of eggs (feces), cercariae, metacercariae or adult trematodes were made by reference to the taxonomic literature (for example, Schell, 1970). Morphological identification was done and confirmed by PCR technique. Food habit surveys People were interviewed about their habits of eating raw or undercooked fish, using questionnaires. Statistical analysis When relevant, the data collected were analyzed statistically using SAS version 8, and chi-square and SeqEd 1.0.3; Assembly LIGN 1.0.9c for sequencing.