The idea that MAP may be making people sick is not new. In their 2003 report to the National Academy of Sciences, Davis and seven other scientists proposed a possible link to Crohn’s disease.

"This has been a controversial subject attracting strong adherents to both positions, but its resolution is essential for the development of rational approaches to disease management and prevention,” they wrote.

In 2006, the U.S. Department of Agriculture issued a fact sheet acknowledging researchers’ concerns that MAP is being transmitted to humans in undercooked meat, unpasteurized milk and water.

"Because of continued potential public-health concerns related to Johne’s disease, animal-production industries must give it more attention,” the document says.

In 2008, a report by the American Academy of Microbiology said that people with Crohn’s disease are "seven-fold more likely” to have MAP in their gut tissues. While the cause of Crohn’s is unknown, the authors wrote, "the possible role of this bacterium, which could conceivably be passed up the food chain to people, has received too little attention from the research community.”

And just last year, Davis and another WSU researcher published a analysis funded by the National Institutes of Health citing more cases of MAP found in the human population, making further research on cattle essential. More precise testing tools reveal MAP is present in healthy humans as well, said Davis – a finding that "tells us that all humans are susceptible to MAP infection but that its presence doesn’t always lead to disease. While not destroying MAP, the human immune system is keeping it in check.”

Similarly, not all cattle come down with Johne’s, said Davis. Those that do apparently keep MAP under control for two years or so, staying symptom free until something triggers a breakdown in immunity and allows the disease to progress.

"There’s no cure so, from then on, they get sicker and sicker,” he said. "For dairy farmers, it can be a devastating loss.”

Culprit or innocent bystander?

It’s not unusual to find foreign microbes living inside the human gut where thousands of others reside naturally. But how to determine if MAP is harmful or simply hanging out? That’s what further research can establish, said Davis, who wants to investigate the immune response in animal models. It takes extensive study to determine whether a bacterium is primary or secondary, he said.

As for all the people walking around with MAP anchored in their intestines who aren’t ill, the reason is the presence of protective immunity - similar to what’s seen with MAP in cattle, said Davis.