A study on the infection status of intermediate hosts of Paragonimus, snail and crab, was conducted on Che Ju Island for four years to understand ecology of Paragonimus in the area and to evalute the control program by mass chemotherapy The infection rate of the first intermediate host of Paragonimus, snail, showed higher rate in the area where human association was more frequent and had higher human infection rate compared with the area with lower human infection and less contact by people. Larger snails were infected more frequently than smaller ones. After the mass treatment on human population, the infection rate of snail dropped markedly in four years. The infection rate and the intensity of infection of crabs were influenced by many factors:by frequency of human association, by prevalence of paragonimiasis in the population and volume of water flow, dilution effect. The distribution of metacercariae in infected crabs varied by body sites; the most frequently in gill, next in legs, internal organs and thoracic muscle in order. If crabs were examined for existence of metacercariae only gills, about 6% of infected crabs of may be missed as negative. The infection status of the crabs was not different by distance from the actual site of pollution. Also seasonal fluctuation of crab infection was not remakable. The crab infection by size showed time sequence of exposure; smaller crabs had lower infection rate and metacercaria density reaching to the maximum by 3 cm of size. As shown in the snail infection, the crab infection remarkably decreased after the control measure of human paragonimiasis, mass chemotherapy, in four years.
