Mekong schistosomiasis: 2. Evidence of the natural transmission of Schistosoma japonicum, Mekong strain, at Khong Island, Laos

Abstract

Of 30 sentinel mice exposed for 15 hr in floating cages on the Mekong River in March, 1971, in an area thought to be a transmission site for schistosomiasis, 26 mice survived and none acquired schistosomiasis. By contrast, in April, 1972, of 20 mice exposed under somewhat different circumstances for a longer period (40 hr), 18 survived and of these 11 acquired schistosomiasis. In April and May, 1972, an intensive search for infected specimens of the snail, Lithoglyphopsis aperta, produced a total of 29,659 snails from 9 localities around the perimeter of Khong Island. Of these, 15,790 were collected in water adjacent to Khong Town and they included 48 specimens positive for S. japonicum, Mekong strain, as determined by shedding of cercariae. This represented a natural infection rate of 0.30%, or 0.16% if the snails from the other collecting sites were included. The data suggest that the risk of acquiring schistosomiasis on Khong Island may be greatest in Khong Town, may be intermittent in time and place, and may be limited to the months from the end of March to the beginning of the rains in mid June, when the snails are present in largest numbers.