Cow with Johne's disease. This disease is caused by the bacterium Mycobacterium avium paratuberculosis. Symptoms include weight loss, diarrhoea and deterioration of the coat. Cows may not show symptoms for up to 5 years after infection, although throughout this time they can spread the bacteria to other cows. Johne's diesase causes losses exceeding 200 million US dollars in the USA every year.