Methods
Study population
The study area is located in the Kaengkhoi district, Saraburi Province, Thailand, which is approximately 100 km north of Bangkok.The area includes a small city with some industry; it is surrounded by rural villages and residents depend on agricul- ture for their income. Data from the 2001 census maintained by government health-care officers show a total population of 80 141 in the catchment area, including 5686 (7%) children