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Studies on Schistosomiasis Japonica in Formosa

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.