What is Johne's disease?
Johne’s disease (JD) is a contagious, progressive bacterial infection that causes abnormal thickening of the lining of the intestine restricting the absorption of nutrients. Clinical signs of animals infected with JD are long-lasting diarrhea and extreme weight loss despite maintaining some appetite. JD is caused by a bacterium (Mycobacterium avium subspecies paratuberculosis; MAP), a distant relative to the bacteria that cause tuberculosis. Animals usually become infected with MAP as calves. No signs of disease are seen for years after infection and most infected cows never show signs. Infected animals may appear normal and spread the disease to other animals in the herd before showing signs themselves. During this time, infected animals may be sold, culled or die without owners being aware that these cows are infected.
How is it spread?
JD is spread through the ingestion of MAP-containing manure from udders, feed or water contaminated by infected animals. Calves are the most susceptible to new infection, but adults may also be susceptible if there is exposure to high levels of MAP bacteria. While MAP does not multiply in the environment, it can survive in manure, water and pastures for up to one year, depending on conditions. Calves may also be infected through colostrum and milk from infected cows. Occasionally calves can be born already infected. With increasing age, calves become resistant to infection and by about one year of age their resistance equals that of an adult. JD usually enters a herd through the purchase of an infected animal that sheds the bacteria in its manure.
Why should dairy farmers be concerned?
JD can have a significant financial impact in a dairy herd through reduced milk production, increased involuntary culling, loss of heifer sales, and reduced beef production. JD may also be associated with an increased incidence of other diseases. There may be an association with milk and meat safety. JD may be linked with Crohn’s Disease, an incurable, chronic, intestinal disorder in humans. If this link is proven, beef and milk may be viewed as a route of disease transmission to humans. The presence of JD in our herds will increasingly affect our ability to market cattle internationally as countries have begun to demand certification of JD-free status before permitting imports.
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