Clonorchiasis is an important food-borne parasitic zoonosis in East Asia including Korea, Japan, China, Taiwan and northern part of Vietnam. People in endemic areas acquire infection by eating raw or undercooked freshwater fish. At present, clonorchiasis is recognized as one of the most important endemic diseases in Korea (Rim, 1990).

Clonorchiasis could be prevented if the metacercariae in the fish are killed or inactivated. Studies on the use of radiation to kill or weaken infectivity of not only protozoa but also helminth parasites have been carried out (Lee et al., 1989; Chai et al., 1995; Dubey et al., 1998), and most of the irradiated helminths are either significantly attenuated or killed depending on the dose of radiation. Recently, irradiation of foods such as fish has been attempted to control various kinds of human pathogens (Dela and Banzon, 1989; Steinhagen et al., 1998).

When parasites are exposed to radiation, they show the growth retardation, vacuolization of the interstitium, elevation of the integument, malformation or underdevelopment of reproductive organs and weakened pathogenicity to the host (Ikeda and Tani, 1984). The effects of ionizing radiation on parasites depend on the radiation dose, developmental and physiological status of worms, and other various physical parameters during or after the irradiation (Hall, 1978). Only a small proportion of Schistosoma mansoni cercariae which are irradiated with 23 Gy grow to adults, and the majority of females are sterile (Bickle et al., 1979). The migration activity of schistosomula is also weakened by irradiation (Bickle et al., 1979). Such effects of radiation have also been reported in other helminths (Barriga and Myser, 1987; Chai et al., 1995).

However, mechanisms of irradiation-induced chromosomal aberration of Clonorchis sinensis have not been studied yet. The purpose of the present study was to investigate the effects of irradiation of the metacercariae on the survival and change of the chromosomes of C. sinensis.

The effects of radiation on trematodes were dependent on the irradiation dosage, preconditioning and physiological status of animals to irradiation (Villella and Weinbern, 1965). The percentage of metacercariae developed in control rats in this study was similar to other reports (Concepcion and Barriga, 1985; Lee et al., 1989). The groups which were fed with metacercariae exposed to 25 Gy and 30 Gy showed a significant (p < 0.05) reduction in the recovery rate compared to that of the control group. No worms were recovered from the 50 Gy irradiation group. Clonorchis sinensis in the present study appeared to be quite sensitive to radiation, compared with other helminths. Paragonimus ohirai metacercariae were also sensitive to irradiation and could not grow in the host after exposure to 20 Gy (Ikeda and Tani, 1984). A remarkable difference of radio-sensitivity was observed depending on species of parasites: for examples, trematodes are more sensitive to irradiation in general than nematodes or cestodes. However, the difference of radiosensitivity may also be seen in the same species at various stages of development, in which the frequency of cell division is different. In Schistosoma mansoni, spermatogenesis and oogenesis are suppressed by 23 Gy (Bickle et al., 1979). Oliver et al. (1972) have noted clumping of chromatin material after irradiation and recorded chromosomal bridges and rings. Radiation is well known to destroy DNA structures directly or by forming hydroxy radicals in the tissue. Therefore there may be three times more cell destruction in the oxygenated state than in the anoxic one (Hall, 1978). Variations in the chromosome number from 1 to 5 among different cells within the same individual were observed by irradiation with 20 Gy, 25 Gy and 30 Gy. Supernumerary chromosomes have generally been defined as those which have additional number to the karyotype and are non-homologous or homologous to chromosomes of the normal complement (White, 1973). Doses of up to 20 Gy of gamma irradiation in this study increased the number of supernumerary chromosomes in metaphase stage of C. sinensis.

The chromosome numbers and karyotypes of C. sinensis have been reported (Park et al., 2000). In this study, we observed the same chromosome number. Sasaki (1961) have been reported that highly contracted chromosomes tend to have their centromeres more median than less contracted ones in different cells, while longer chromosomes tend to contract more strongly on colchicine treatment than shorter ones in the same cell. In this chromosomal study, we made preparations under the same conditions, and in fact, this air-drying method produced more uniform and complete flattening of cells than could be achieved by the usual squash methods. Results of present study did not indicate any change in the number of chromosomes during spermatogenesis among the different radiation groups, but did show slightly reduced in chromosome size in the worms from 25 and 30 Gy irradiated groups. In general, the chromosomes gradually become shorter as the irradiation doses increased. Nonetheless, irradiation doses used in the present study did not alter chromosome numbers of the survived parasites after gamma irradiation.