This study was conducted in Lahanam zone of Songkhone district of Savannakhet province, Lao P.D.R. There are six villages in Lahanam zone, and there are five primary schools in the area. Stool examination and treatment were performed for all school children (total number = 670 children) from grade 1 to grade 5, registered at Lahanam in September 2004. Liver fluke, Opisthorchis viverrini (Ov) was the most common intestinal parasite with prevalence of 49 % , followed by hookworm with 32 % . Ov egg-positives were treated by praziquantel at schools (selective mass-chemotherapy) and all the children were treated for hookworm by albendazol (mass-chemotherapy). In September 2005, after one year of treatment, stool examination was repeated for the same children in schools. Due to the move to the new academic year, school children of the previous grade 5 had already graduated and we did not include them. Prevalence of Ov and hookworm after one year of treatment were 29% and 12% , respectively. Children attending Bengkhamlay primary school located about 5 km from Banghiang River had lower prevalence for Ov. And higher prevalence of hookworm than other schools located within 2 km from the river (p<0.001). In December 2005, community stool examination was carried out in Bengkhamlay The subjects were all the villagers in Bengkhamlay age 5 years old and over. The prevalence of Ov in those over 15 years old (284 people) was 66.2% and the age group showing the highest prevalence (77.5% ) was 30‒39 years old. Prevalence of hookworm was 31.3 % in those over 15 years old. Intensity of Ov. Infection was low to moderate with increasing tendency by age up to middle age, and become low after 60 years old
