In this study, Salmonella is used as the standard bacterium for evaluation of pollution of water environments. The author continued to improve Salmonella detection by visualization of hydrogen sulfide production, which is characteristic of Salmonella. At the same time, the positivity rate of Salmonella in rural Lao was investigated. According to the survey conducted so far, it has been found that the Salmonella positivity rate in residents of Laos is markedly higher than that in Japan. It is conceivable that the level by contamination of Salmonella in sewage drained from each house in Laos will be higher than that in Japan. There is no sewage treatment facility in Laos even in the capital Vientiane. In the city, a vast wetland named That Luang Marsh functions as sewage reception. In the case of contamination that exceeds the purification capacity of the wetland, detection of Salmonella in the water of the Mekong River will indicate the deterioration of water quality.