Johne’s disease is a contagious, chronic, and usually fatal infection that primarily affects the small intestine of ruminants. It is found worldwide. Signs of Johne’s disease in cattle include weight loss and diarrhea with a normal appetite but are rarely evident until two or more years after infection. The causative bacteria are Mycobacterium avium subspecies paratuberculosis (MAP), and transmission is primarily fecal/oral to calves less than six months of age.

Based on the 2007 Dairy NAHMS study, about 68 percent of U.S. dairy herds have at least one cow that tests positive for Johne’s with herd prevalence approaching 100 percent in large dairy herds. In the typical herd, for every obvious case of Johne’s disease among dairy cattle on the farm, 15 to 25 other animals are likely infected. The clinical case represents only the “tip of the iceberg” of Johne’s infection. Infected animals often shed the organism through feces even before they show clinical signs, which allows for continued, unnoticeable infection of herd mates.