Stages of Johne s disease progression. For each animal in Stage IV, the hypothetical animals in other Stages are in parentheses. In herd with established Johne s disease, animals are present in all four stages of disease. Common sources of infection The most common source of infection is feces or manure. Except under unusual circumstances infection begins in the first few months of life.
Animals become more resistant with age, though resistance is never complete.
If given a sufficiently large dose of the bacteria adults can become infected. Nevertheless, under normal animal husbandry conditions, exposure to such large doses probably rarely occurs. This means, for example, that an infected adult dairy cow in a stanchion or tie-stall barn is not going to infect the cow standing next to her. Infected cows are primarily the main source of infection for calves. In a beef herd, large doses of pathogens may be deposited on ground-fed forages thus increasing the risk for other animals and especially youngstock.