Crohn's Disease and Johne's Disease

Two more pieces of the puzzle that might some day provide the definitive answer to that question have recently come to light.

But first some background information. Mycobacterium avium ssp. paratuberculosis (MAP) is the bacteria that is the cause of Johne's Disease (JD) in cattle. It appears that JD was first recognized as an infectious disease in 1895 by Drs. Johne and Frothingham, and it was first described in a publication in the United States in 1908 (right here in Pennsylvania, nonetheless!). JD is a chronic bacterial disease affecting primarily a portion of the small intestines of cattle. It eventually progresses to diarrhea and weight loss due to marked thickening of the intestinal wall. Surveys suggest that more than 20% of dairy herds in US have at least one JD positive cow, and estimates are that at least 3-4% of animals are infected.

"Crohn's Disease" (CD) is an inflammatory bowel disease which often affects the small intestine. It was first described, at least in a published report, in 1913. This disease, which has many similarities to Johne's disease in cattle, causes a chronic inflammation in the small intestines, and is usually accompanied by diarrhea and weight loss. It most commonly affects young people and may require surgery to remove a section of the affected intestine. There is no agreement within the medical community concerning the cause of CD. Many different theories have been proposed, including that it has an auto-immune, genetic, nutritional or infectious cause. The level of CD diagnosed in industrialized countries has increased substantially over the past 30 to 40 years.

While it is difficult to experimentally prove a causative relationship between MAP and CD there are a number of pieces of evidence which appear to support a link between the two. For example, a number of studies have shown that a higher percentage of CD patients have evidence of MAP in their intestines than patients who underwent intestinal surgery for reasons other than CD disease. The organism has also been found in the lymph nodes and breast milk of people afflicted with CD.

A recently publication in the Lancet by Naser et al. (vol 364, Sept 18, 2004) reports the findings of a study in which blood samples from 52 participants were examined for evidence of the MAP organism. The bacteria was cultured from the blood of 50% of the CD patients (n=28) in the study and from 0% (zero) of the patients who did not have some form of inflammatory bowel disease (n=15). The investigators also found the organism in two of nine (22%) patients with ulcerative colitis (another type of inflammatory bowel disease). While this does not prove a causal relationship, it does add to the evidence that MAP may play a role in at least some CD cases.

But, the question might be, how could the MAP organism get from an infected animal into people?

The MAP organism can be isolated from the milk of infected cows. Research conducted in the United Kingdom has also shown that viable MAP organisms could be isolated from pasteurized milk available on grocery store shelves. A subsequent laboratory study undertaken by researchers with the USDA's Agricultural Research Service suggested that MAP does not survive high-temperature, short-time (HTST) pasteurization which is commonly used in the US for fluid milk.

However, another recently released study by Ellingson from the Marshfield Clinic in Wisconsin has documented the presence of viable MAP organisms in pasteurized milk which was purchased from retail stores in California, Minnesota and Wisconsin (presented at the International Association for Food Protection Annual Meeting on August 9, 2004). Almost 3 percent of the 702 samples tested were positive, suggesting that there is the potential for people to be exposed to this bacteria when they consumed milk. Admittedly, this does not prove that MAP causes CD, or that this is even the main route by which people are exposed to the organism, but the dairy industry would be remiss to ignore the findings of this study and the one published in the Lancet.

Possible exposure from a contaminated environment is also a concern. Although there are other domesticated and wild animal species that can also be infected with MAP, cattle can shed billions of organisms in their manure thereby contaminating their environment. Contamination of surface waters by manure - either directly deposited there by the cow or other animals, or through runoff from contaminated ground, is also possible. Although the organism does not grow and multiply outside of a suitable host, it can survive for long periods of time in the environment. In fact, in one study viable MAP bacteria were still present in a sample of tap water that had been inoculated more than 1 year earlier.

It should be pointed out that just as there are researchers who feel there is good evidence establishing a link between Mycobacterium avium ssp. paratuberculosis and Crohn's disease in people, there are those who believe that the evidence does not support such a conclusion. Although discussions about the interpretation of the data are likely to continue for some time, it seems prudent that the dairy industry collectively and seriously addresses this issue as soon as possible.

Ernest Hovingh, Veterinary Science Extension