How does the Johnes agent cause disease?
The organism, Mycobacterium paratuberculosis, usually gains access to the host via oral ingestion, although other routes are possible. The M. paratuberculosis bacterium initially infects the last segment of the small intestine. The growth of the organism and spread of infection is slow and usually takes several years before any disease is detectable in the individual animal. As the amount of small bowel inhabited by the bacterium increases, the numbers of organisms shed in the feces increases. These M. paratuberculosis organisms in the feces of cattle are the primary way Johne’s is spread from animal to animal. With time, the infection in the intestinal tract becomes more widespread and the lymph nodes near the bowel become infected with M. paratuberculosis. Later, the organism can become disseminated outside the intestinal tract and infect the uterus, mammary lymph nodes, udder, and sexual organs of bulls. Therefore, the agent can be shed in the milk and semen in addition to the feces
