In most cases of Johne s Disease, symptoms do not appear until the first or second lactation, even if the animal contracted the bacteria as a young calf. Age of the animal, amount of bacteria present, genetics and stresses such as calving or moving to a new environment may be determining factors as to when clinical disease or symptoms are noticed. Cattle may also have subclinical infections during which no symptoms of diarrhea or weight loss are seen. However, they do not produce as much milk and may be more susceptible to other problems, such as infertility. Johne s Disease is spread by feed and water that is contaminated with the bacteria.
Infected animals shed the bacteria in their manure.
Feed troughs, water tanks, maternity pens and other group pens can become contaminated by an infected animal or by equipment that has been used to feed animals or spread manure. Newborn animals and young animals can ingest the bacteria on manure covered teats, or in colostum or milk from infected cows. Infected cows can infect their unborn calves. Newborn animals are most susceptible to infection. All breeds of dairy and beef cattle can be infected.