Risk assessment in Crohn's disease.

MD The hygiene hypothesis says that as regions become more industrialized or westernized, an increase in diagnoses of inflammatory bowel conditions, among other autoimmune diseases, is seen. The idea is that, with increased hygiene comes less opportunity to build immunity against pathogens. Being in a more hygienic or sterile environment impacts gut flora so that certain bacteria may be treated as foreign in some people. Genetics plays a strong role, and so those individuals with a genetic susceptibility to gut disorders are most vulnerable. If we look at India or other Asian countries as an example of westernization and industrialization, we see an increasing incidence of inflammatory bowel disease (IBD) in these advancing countries. Interestingly, the pattern of IBD emergence in many countries as they industrialize is one in which increased incidence of ulcerative colitis is seen that then is overtaken by more Crohn’s disease–like forms of IBD. This phenomenon may be because flora change as a region becomes more westernized. We classify IBD into being either ulcerative colitis or Crohn’s disease, but 110 of the 163 genes that have been identified as playing a role in IBD overlap between the 2 disease entities. The same genes, therefore, can go in either direction to cause a person to be at risk for development of ulcerative colitis or Crohn’s disease. The flora likely influence in which direction the genes go. Perhaps, early on, a change in the environment or hygiene predisposes certain people in a newly industrialized region to a gene-bacterial interaction that gives rise to a more ulcerative colitis–like phenomenon. As industrialization of the region progresses, a gene-bacterial interaction that selects a more Crohn’s disease–like phenomenon emerges.