Intestinal protozoa are hypothesized to stimulate immunosurveillance against colon cancer

Summary Colon cancer in humans results in considerable morbidity and mortality throughout most of the world. During the twentieth century, there was a rapid rise in colon cancer within modernizing countries that has not been adequately explained, although the role of diet has been widely explored. Previously, we showed that the presence of the endemic Eimeria spp. protozoan in intestinal tissues is associated with regions of low tumorigenesis in the large and small bovine intestine and that an Eimeria surface protein is a potent activator of dendritic cells and a useful immunomodulator, with anti-cancer and anti-viral properties. Therefore, we hypothesize that the persistent presence of such an intestinal protozoan enhances immunosurveillance by elevating the intestinal alert status and that the loss of these organisms could lead to a higher incidence of colon cancer. Preliminary support of this hypothesis derives from the observations that domestic animals, known to maintain this protozoan, have very low colon cancer incidence. We propose that this also may occur in human populations that use human excrement (night soil) as a fertilizer, a practice that serves to complete the life cycle of this type of microbe. We examine some evidence for this hypothesis in Japan’s mortality patterns, where we show that colon cancer increased after the cessation of night soil use, but before the change to a western diet. We conclude that this hypothesis, a variation of the hygiene hypothesis, is worth further consideration and continued elaboration.