Johne's disease is a bacterial infection of the intestine. It has a very long incubation period in which the cow appears normal - signs of Johne's disease usually do not develop until the cow is 2 or 3 years old. Surface of the thickened intestine of a cow with Johne's disease The cause of Johne's disease, the bacterium Mycobacterium avium subsp. In the long incubation period, there is no accurate diagnostic test. In the infected cow, bacteria are shed into the intestine and leave the animal in feces.
Manure is the major source of infection, often through contaminated soil and water.
This is a hardy bacterium, and it can survive in slurry for up to 9 months. Disease is also spread through contaminated colostrum and milk. There is a long pre-clinical phase in which the animal has no signs of disease and is not shedding bacteria. This phase is followed by a shedding phase during which bacteria are being released in the feces, even though the cow does not have clinical signs of disease.