Is Crohn's Disease Caused By a Relative of TB?

One of the reasons to study the human microbiome--the microbes that live on and in us--is that many diseases might have a microbiological component. One of the best examples of this are gastric ulcers, most of which are caused by an infection by the bacterium Helicobacter pylori. A recent report by the American Academy of Microbiology argues that Mycobacterium avium subspecies paratuberculosis, also known as MAP, may, in part, be responsible for Crohn's disease:

Because the bacterium Mycobacterium avium subspecies paratuberculosis (MAP) causes a Crohn's-like gastrointestinal disease in cattle, it is now considered a prime suspect for causing Crohn's disease in humans, according to a recent report, MAP: Incidental Pathogen or Public Health Threat?, from the American Academy of Microbiology (AAM). From a public health standpoint, research evaluating whether bacteria that cause this disease in cattle can be transmitted through the food chain to humans should be given high priority, according to that report, which derives from a colloquium convened by AAM in June 2007....

According to the report, researchers and physicians have long suspected that MAP could play a role in Crohn's disease because of its unambiguous role in causing Johne's disease in ruminants, a condition that shares many symptoms with Crohn's, including diarrhea, weight loss, and periods of remission. Physicians also note that treating Crohn's patients with antibiotics to target mycobacteria often provides relief from Crohn's symptoms. MAP conceivably is passed from livestock with Johne's disease to humans through contaminated meat and dairy products, providing a scenario for human exposure to the pathogen, according to the report.

Recent studies further strengthen the idea that MAP is tied to Crohn's disease, according to Carol Nacy of Sequella, Inc., in Rockville, Md., who chaired the colloquium. Nacy points out that MAP can be detected in the tissues of seven of eight patients suffering from Crohn's. "I think the association is no longer really in question," she says, but she cautions that finding the bacteria in such patients does not mean that MAP causes Crohn's, since it may be only incidentally present, taking advantage of the environment created by other pathogens or other factors.

Nacy also points out that several factors are probably responsible for causing Crohn's disease, and they may include underlying host genetic or immune factors in addition to one or more infectious agents. This complicated network of factors defies a simple explanation and has confounded efforts to understand this disease, according to Nacy. "It's not just being infected with a bacterium," she says. "It's many things combined together that create the environment that creates disease. And all of those factors need to be systematically sorted out by both basic and clinical research."....

The report separates research priorities into two general areas of emphasis-- one being transmission of MAP from food and the environment to humans and the other being susceptibility of humans to MAP infection and disease. Among other key questions: Is MAP widely present in food, and does it colonize or infect humans who consume it? Also, what is a "normal" immunological response to MAP, and does that response differ in individuals who develop Crohn's?

Oddly enough, I'm part of a group looking at the role of microbes in Crohn's disease. One thing to note is that Crohn's disease may not be one disease, but a set of diseases with similar symptoms. That is one reason why the human genetics are so important: in some people, MAP might be critical, whereas in others, MAP doesn't matter.

If you want to get a copy of the report, email colloquia [at] asmusa [dot] org.