Studies on Schistosomiasis Japonica in Formosa

Abstract

Discussion and Summary The historical review of the epidemiology of schistosomiasis japonica in Formosa from Japanese literature as given above shows clearly that the chief endemic area of this disease is Changhua Hsien (Taichu Prefecture) in the mid-western part of this island. Possibly another endemic area exists at Laonung in Kaohsiung Hsien (Takao Prefecture), in the south-western part of Formosa. The current idea abroad that Shinchiku (= Hsinchu Hsien) in the northwestern part of the island is the main endemic area of this disease is without foundation in the Japanese literature. Dr. S. Yokogawa, in a personal communication, dated March 25, 1951, agrees with us and states, “It is true that there was no schistosomiasis japonica in Shinchiku, either human case or animal case.” There is, furthermore, no report in the literature of the finding of living Oncomelania snails in Shinchiku. The above review also shows that the riddle of the human cases of schistosomiasis as given by Takegami should be solved, the incidence of animal infection verified, and the distribution of the intermediate host, O. formosana, in this island thoroughly re-studied. Summary 1. Changhua Hsien is situated in the center of Formosa on the western seaboard and in the vicinity of the Tropic of Cancer. The arable lowland, being an alluvial deposit, is one of the most important rice growing centers in Formosa. 2. Regarding the snails, O. formosana, in this area, they are usually found in the small irrigation ditches of about 0.5 meter in width, especially in such ditches as are shaded by a row of bamboos. They can occasionally be found in ditches of about 1–2 meters in width, but very rarely in wider ditches. The fact that O. formosana lives in ditches irrigated by water of high turbidity deserves mention, as such a habit has never been recorded for the Oncomelania snails in other endemic areas in the Orient. 3. O. formosana was found in 140 out of 628 villages surveyed. They comprise an area of about 273 sq. kms., and are distributed mainly in the central part of this county. 4. Concerning the incidence of cercarial infection, 38,241 O. formosana from 140 ditches in 140 villages were examined (300 or less from each ditch). On the average, 1.3 per cent were positive, and in the infected ditches the range of infection was from 0.3 to 30 per cent, with its mean at 2.4 per cent, median at 1.2 per cent, and mode at 0.3 per cent. The frequency curve of the percentage of infection, if plotted, will be a curve of positive skewness. 5. The peculiar habits of O. formosana in this district, and the significance of the high incidence of infection of the snails in this county are discussed. Importance of the study of the incidence of infection of the Oncomelania snails in relation to the epidemiology of schistosomiasis japonica is emphasized.