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. 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. By the time the agent has spread outside the intestinal tract, the feces contain large numbers of M. paratuberculosis which can serve as a source of infection for many animals in the herd.
Transmission can occur in several ways.
The first to consider is prenatal or in utero transmission of the agent. In this instance, the agent passes from the infected cow to the calf before birth and the calf is born infected. These cows will have too high a chance of infecting their calf prenatally and will also shed high numbers of the organism in the environment. The most common time of infection of calves occurs soon after birth. The young calves are most susceptible to the disease.