Wannaporn Ittiprasert1, Manas Chongsa-nguan1, Dutchanee Manatrakul2, Prapasri Jongsuksuntigul2, Vara Meesomboon2, Thitima Vongsaroj2, Viroj Kitikoon3, Yuwaporn Sakolvaree1, Hideo Hayashi4, Wanpen Chaicumpa1 
1Department of Microbiology and Immunology, Faculty of Tropical Medicine, Mahidol University, Bangkok 10400, Thailand; 2Department of Communicable Diseases Control, Ministry of Public Health, Nonthaburi 11000, Thailand; 3Department of Social Medicine and Environment, Faculty of Tropical Medicine, Mahidol University, Bangkok 10400, Thailand; 4Department of Microbiology, Institute of Basic Medical Sciences, University of Tsukuba, Japan

Abstract
Serum samples were collected from 339 inhabitants of Ubon Ratchathani province, Thailand, which is located in the immediate vicinity of Kong island, Lao PDR, where schistosomiasis mekongi is endemic (group 1). The samples were subjected to indirect- and dot-blot ELISA using a Schistosoma heterophile substance, i.e. keyhole limpet hemocyanin (KLH), as the antigen. Thirtyfive serum samples of residents of Bangkok, a low risk area for the blood fluke infection, were tested concurrently (group 2). It was found that 10 of 339 samples of group 1 revealed KLHindirect ELISA optical densities higher than the uppermost limit of samples of group 2. These 10 serum samples were negative when tested by the indirect ELISA using affinity purified, specific antigen of Trichinella spiralis. The finding implied that the high optical densities found when the samples were tested by the KLH-indirect ELISA, were not incited by T. spiralis, the infection which had been known to stimulate antibodies reactive also to the KLH. All sera of both groups 1 and 2 were negative when tested against KLH and specific T. spiralis antigen in dot-blot ELISA. Evaluation of the ages and occupations of the 10 individuals of group 1 revealed that their age range was 18 to 72 years, and 50% or more were farmers with a high chance of exposure to water where the S. mekongi infective cercariae might be present. Moreover, none of the 10 individuals denied swimming in the Mekong river. Thus, the possibility exists that the high optical densities of the 10 serum samples in the KLH-ELISA is due to natural exposure or undetectable (light) infection by the Schistosoma mekongi.