RESUMEN
Nan Province, located in northern Thailand, is hyperendemic for parasite infections; the helminthic infection rate in 1,010 schoolchildren was 60.0% in 2001. Mass anthelmintic chemotherapy has been conducted with schoolchildren, and selective treatment has been given to people in the community, from 2002. The modified cellophane thick smear method was used to examine the prevalence and intensity of helminth infections in schoolchildren and community people once a year during the period 2002-2004. The prevalence of helminth infections decreased slowly from 60.0 to 40.3% in schoolchildren and from 70.8 to 60.0% in the older age population. Three parasite species were common: hookworm, Ascaris and Haplorchis, an intestinal trematode. Hookworm presented throughout the whole district. Ascaris infection occurred at high rates in some villages, while in some villages none was found. The villages where Ascaris infection was nil had high rates of Haplorchis infection, and vice versa. Most hookworm and Trichuris infections were of light intensity. Heavy intensity infection was found in 12.8-18.1% of Ascaris cases examined. Parasite infection rates in Chaloem Phra Kiat District can be classified as low prevalence.