FRI Briefings Mycobacterium paratuberculosis — Another Emerging Pathogen of the Human Gastrointestinal Tract ?

Crohn's Disease (CD) in humans and Johne's disease in cattle both involve a chronic granulomatous enteritis primarily in the small intestine. Symptoms of these diseases are similar to those of intestinal tuberculosis (caused by Mycobacterium tuberculosis), and the agent responsible for Johne's disease is known to be Mycobacterium paratuberculosis. The etiology of CD has so far defied explanation but a great deal of recent research has been devoted to a search for M. paratuberculosis in patients with CD, sarcoidosis, and ulcerative colitis. If this bacterium is associated with any human diseases, then there is the question as to whether people can acquire M. paratuberculosis from animals with clinical or subclinical Johne's disease. It is believed that CD is multifactorial in origin, possibly involving some genetic predisposition and other environmental factors as well as an infectious agent (1). This update will present current information on the possible role of M. paratuberculosis in CD and on potential vehicles for its transmission. Mycobacterium paratuberculosis was first associated with an intestinal disease of cattle about a hundred years ago. At first it was difficult to distinguish this bacterium from tubercle bacilli but eventually they were isolated and characterized and Johne's disease was distinguished from intestinal tuberculosis. Although a number of diagnostic tests are available, it is not always possible to detect these bacteria in sick cattle. The bacteria penetrate the intestinal mucosa, are phagocytized by macrophages, and continue to multiply inside the macrophages. Granulomas, compact foci of inflammatory cells, form at these sites and the animals develop a chronic nonresponsive diarrhea which causes progressive weight loss, debilitation, and death (2). Ruminants, including cattle, sheep, goats, deer, and South American camelids, are infected with M. paratuberculosis more commonly than other animals. Calves may acquire these bacteria early in life—either by ingestion of grass or hay contaminated with feces from infected animals or through their mother's milk. After an extended incubation period of several months to years, a persistent, non-responsive diarrhea develops. Other kinds of animals are occasionally infected: wild rabbits in Scotland (3), a strain of laboratory mice (4), and a colony of stumptail macaques (Macaca arctoides) in the USA (2). Clinical symptoms and pathology of paratuberculosis in the macaques were similar to those observed in ruminants and also resembled Crohn's disease in humans. Mycobacteria were detected in intestinal tissue specimens and in feces, and these were identified as M. paratuberculosis after growth on standard …