villages" of Rokakanda and Deam Te. The measurement of antibodies by the ELISA was carried out in the Department of Medical Zoology, Dokkyo University School of Medicine, according to the method described by Matsuda et al. (l984). The results obtained are summarized in Table 1 and Figure 2. The OD value over 0.2 was regarded to be antibody positive in the present study. More than 90% IgG positive rates were observed among school children in villages upstream of Kratie, while 20% and less IgG positive rates were seen in villages downstream of Kratie. High IgG positive rates, 94.1% and 75.0%, were found among floating villagers in Roka Kandal and Peam Te, respectively, only 2-3 river km downstream of Kratie. Not a few IgM positive cases were found throughout upstream of Kratie, suggesting that endemic transmission of schistosomiasis still exists in the area.
In our surveys conducted in June 1997, 37 (97.3%) out of 38 blood specimens from school children in Pong Ro, ca. 40 river km downstream of Kratie, were found to be IgG negative, despite that they have not so far been treated with praziquantel. No IgM positive was observed in the 38 specimens. Even though the extent of the surveys was limited due to lack of security, difficulty of transmission, etc., the southern limit of endemic transmission of Schistosoma mekongi infection in the Mekong River appeared to occur between Henchey and Pong Ro or its environs..
In the present surveys, a total of 160 blood samples (80 in Talous, 4 river km downstream of Kratie; and 80 in Kanh-chor, 18 river km downstream of Kratie) were collected by venipuncture from school children. After blood donation, each child received tablets of vitamin-complex, paracentamol and mebendazole together with a note-book and bool point pen. One soccer ball was presented to Kanh- Chor Primary School. The sera was separated with the new centrifuge in the laboratory in Kratie, The serum specimens will be subjected to immunological analyses in the Department of Medical Zoology, Dokkyo University School of Medicine. We'll let you know the results by late July.
II. Malacological surveys
When the search for Neotricula operta commenced on 30 April, the Mekong River was low, being ideal condition for the snail surveys. We collected snails by lifting submerged stones and taking them by teeth brushes from the stones to which they attached. Thick rubber gloves (upper-arm length) and rubber boots (waist length) were worn to avoid getting infected with schistosome ceracariae while collecting the snails.
Large numbers of the gamma race snails were observed on stones from the shores of Klokor, 3 river km upstream of Kratie. The gamma snails were extreamely predominant and abundant here, being the snail population density 6-18 / 100 sq. cm. The gamma snails were also collected from approximately 50 meters off the shore of Kampe, 13 river km upstream of Kratie. The great majority were adult snails with 4.0-5.5 whorls. No snails were collected from the sandy shores of Talous, 5 river km downstream of Kratie and Kanh Chor, 18 river km downstream of Kratie.
