Johne's Disease: Frequently Asked Questions
What is Johne's disease, and what kind of animals get Johne's disease?
Johne's (pronounced "Yo-nees") disease is a contagious bacterial disease of the intestinal tract. A German veterinarian, Dr. H. A. Johne, first described the disease in a dairy cow in 1895; his name is used as the common name for the disease. The disease is also called paratuberculosis.
Animals most commonly affected are cattle, sheep and goats. Johne's disease has also been reported in several species of wild ruminants, both captive and free-ranging. In addition, a few reports of isolated cases in non ruminants including nonhuman primates have occurred, but none of these species are believed to be sources for Johne's disease in cattle.
What causes Johne's disease?
The bacterium that causes Johne's disease is named Mycobacterium paratuberculosis. In some literature, it is referred to as Mycobacterium avium subspecies paratuberculosis, and is a distant relative of the bacterium that causes tuberculosis (TB) in humans and animals. M. paratuberculosis does not multiply outside the animals body, but is able to survive in soil and water up to one year, thus representing a danger to other animals that might ingest the organism.
What are the signs of Johne's disease, and how can I tell if my herd has Johne's disease?
Animals infected with M. paratuberculosis usually develop diarrhea and rapidly lose weight. However, in some animals, like sheep, goats, and deer, diarrhea is less common. In general, Johne's disease is a wasting disease although infected animals continue to eat well. They appear unthrifty, are often weak, and usually do not have a fever. The signs of Johne's disease can be confused with the signs of several other diseases. Because of the slowly progressive nature of the infection, signs of Johne's disease are usually not seen until the animals are adults.
What causes the signs of Johne's disease?
Young animals, which are most susceptible to infection, usually ingest the M. partuberculosis bacterium with colostrum, milk, or from a feces contaminated udder, feed, or water. Specialized cells in the wall of the intestine take up the bacteria. Normally, an invading bacterium would be killed, priming the immune system to strengthen itself against future invasion. However, some of the organisms which cause Johne's disease are able to survive this process. As time passes, more and more specialized cells are recruited to try and kill the bacteria, causing a thickening of the intestinal wall, resulting in poor absorption of nutrients, and eventually diarrhea. Infected animals can't be cured of the disease, and most cases of Johne's disease progress to a terminal condition, when the animal is 2-6 years of age.
How can I tell if my herd is infected?
Some animals may be infected and still appear normal until some stressful event, such as calving, triggers them to break with clinical illness. Because Johne's disease can take so long to run its cycle, the owner may not realize the herd is infected until years down the road, when more and more animals are being culled due to chronic diarrhea. For every animal that is visibly ill, there may be as many as 15 to 25 animals infected with the bacteria, but not yet showing signs.
How do I test for Johne's Disease?
There are five known ways to test animals for Johne's Disease: culture of fecal samples, DNA probe on fecal samples, blood tests for antibodies, blood and skin tests for cellular immune response, and microscopic examination of certain target tissues. The two most common tests utilized are the culture of fecal samples, and an ELISA to detect antibodies in the blood.
How can I prevent my animals from getting Johne's Disease?
The best way to avoid introducing this disease into your herd is to be as certain as possible that animals brought into the herd are not infected with M. paratuberculosis . Using the laboratory tests mentioned above for pre-purchase screening of animals is a good start. However, it is important to understand the limitations of the tests: Those done on individual animals are not able to detect every infected animal. It is much more reliable to buy from herds in which all adults have been tested negative, or in which at least 30 adults have been tested.
What can I do if my herd is already has Johne's Disease?
Consult your veterinarian for advice on the best approach to deal with the problem in your herd. The most effective method is most often a combination of testing and management changes. This two-pronged attack focuses on removing dangerous infected animals as soon as possible, and protecting the young, susceptible animals.