What is Johne's disease and what causes it?
Johne's (pronounced "Yoh-nees") disease and paratuberculosis are two names for the same animal disease. Named after a German veterinarian*, this fatal gastrointestinal disease was first clearly described in a dairy cow in 1895.
A bacterium named Mycobacterium avium ss. paratuberculosis (let’s abbreviate that long name to “MAP”) causes Johne’s disease. The infection happens in the first few months of a lamb’s life but the sheep may stay healthy for a very long time. Symptoms of disease may not show up for many months to years later. This infection is contagious, which means it can spread from one sheep to another, and from one species to another (cows to sheep, sheep to goats, etc.
MAP is hardy - while it cannot replicate outside of an infected animal, it is resistant to heat, cold and drying. See "Biology of M. avium ss. paratuberculosis" for more information about this bacterial pathogen.
(*Dr. Heinrich Albert Johne; follow the paths taken by scientists to understand Johne’s disease on the History page)
What kinds of animals can get Johne's disease?
Johne's disease is primarily a health problem for ruminant species (ruminants are hoofed mammals that chew their cud and have a 3-4 chambered stomach) and occurs most frequently in domestic agriculture herds. Some of the more common ruminants are cattle, sheep, goats, deer, and bison. It is particularly common in dairy cattle, not because they are more susceptible to infection but because they are more frequently exposed to the organism that causes Johne’s disease (MAP). Infected ruminants have been reported from all parts of the world. Non-ruminants such as omnivores or carnivores (birds, raccoons, fox, mice, etc.) may become infected, but rarely do they become sick because of the infection.
For more information about a particular species, look at the column at the left: choose the animal under "area of interest" and select a specific topic of interest.
What are the symptoms of Johne's disease and what causes them?
There really are only two clinical signs of Johne’s disease: rapid weight loss and diarrhea. In sheep, diarrhea is less common than in cattle. The infection occurs in lambs in the first months of life, but signs of disease usually do not appear until the animals are adults. Despite continuing to eat well, adult sheep become emaciated and weak. Since the signs of Johne's disease are similar to those for several other diseases, laboratory tests are needed to confirm a diagnosis. If a case of Johne’s disease occurs, it is very likely that other infected sheep (who may still appear healthy) are in the flock.
No one yet understands what causes a clinically normal sheep that has been infected by MAP for months or years to suddenly become sick from the infection. We do know that at some point the MAP that have been lying quiet within cells of the last section of the small intestine (called the ileum) start to replicate and take over more and more of the tissue. The animal’s immune system responds to all these organisms with what is called granulomatous inflammation. This inflammation thickens the intestinal wall, preventing it from functioning normally. This, among other factors, means the animal cannot absorb the nutrition it needs and thus begins to lose body condition, milk production drops off, and diarrhea may occur. In effect, an animal with Johne’s disease is starving in spite of having a good appetite and eating well.
How common is Johne's disease?
Every country that has tested its ruminant domestic agriculture species for Johne’s disease has found cases of infection. The infection is much more prevalent in captive ruminants than in free-ranging ruminants such as deer or elk. In the U.S. it is estimated that 8% of the beef flocks and 68% of the cattle dairy flocks contain at least one animal infected with MAP. No one knows the number of infected sheep flocks in the United States due to lack of testing and reporting. Cases have occurred in many breeds, in sheep raised for meat, milk and fleece, in 4H flocks, etc. It is a common problem in countries with a strong ruminant industry (Spain, Australia, etc.)
How do sheep get infected?
Johne's disease typically enters a flock when an infected, but healthy-looking, sheep is purchased. This infected sheep then sheds the organism onto the premises – perhaps onto pasture or into water shared by its new flockmates. Young animals are far more susceptible to infection than are adults: these lambs swallow the organism along with grass or water. Perhaps they are bottle-fed with MAP-contaminated milk collected from the infected but healthy-appearing new arrival. (The milk may become contaminated from the environment (manure-stained teats) or, in the advanced stages of the infection, the bacterium is shed directly into the milk.) Animals may even have been infected before they are born (in utero transmission) if the ewe is infected. Thus the infection spreads, often without the owner's being aware of it.
How can you protect your flock from Johne's disease?
Do not introduce it! Try to purchase animals from a source flock free of Johne’s disease. Second best is to work with producer who knows the level of Johne’s disease in his or her flock, follows good infection control practices, and then purchase test-negative animals from test-negative ewes. Remember that Johne’s disease is a flock problem, and that knowing the test-status of numerous adults in the source flock will give you a much better sense of the risk of purchasing an infected animal than the one test result you might get on the single sheep you wish to buy. Evaluating a source flock is not always easy but keeping the infection out of your flock is much less cost and trouble than controlling it once it gets in. Laboratory tests are available for any species – you can even test the environment nowadays.
do you test sheep for Johne's disease?
New approaches are now available for testing that are cheaper and more reliable than ever before. The two common ways to test a flock for Johne's disease are:
- culture of fecal samples (individual or pooled; collected directly from the animal or the barn or pasture) to isolate MAP, direct PCR with the same type of samples to detect MAP DNA,
- tests on blood or milk samples for antibodies by the animal in response to MAP infection.
There are many good testing strategies to choose from – more detailed diagnostic testing information can be found by selecting, in the column at the left, “Sheep” as the species of interest and then the topic "Diagnosis".
Consult with your veterinarian to select the best approach for you and your animals. You may also contact our veterinary staff via this website’s Ask An Expert feature if you still have questions after reviewing all the information on this website.
Click here to download a sample submission form.
How do you control Johne's disease in a sheep flock?
The best methods for MAP infection control in your sheep flock depend on the resources available, the goals of your enterprise, and the methods you use to take care of your sheep. All control methods however rely on two core strategies that must be employed at the same time:
1. Lambs must be protected from infection by being born and raised in a clean environment and fed milk and water free of MAP contamination. The primary sources of MAP contamination are manure and/or milk from an infected adult animal.
2. Adult animals infected with MAP must be identified and managed to ensure no lambs are exposed to their milk or manure.
Because twins and triplets are common for sheep flocks, the annual risk of a new case born to an infected ewe is believed to be 2-3x the risk seen in species that produce just one offspring a year.
No vaccines are available in the United States for Johne’s disease in sheep, but a vaccine is in use in other countries to try to control the infection.
No. In the few studies that attempted to treat Johne's disease with antibiotics in cattle, symptoms appeared to subside but animals relapsed after therapy was halted. As with other mycobacterial infections (for instance, human tuberculosis) multiple antibiotics must be injected or given orally daily for months. For most animals, this is cost-prohibitive as well as infeasible. For
more detailed information visit the page on "Antimicrobial
Can Johne's disease be cured with antibiotics?
Can humans get Johne's disease?
The term “Johne’s disease” is used only to describe the clinical illness in ruminants that occurs after MAP infection.
There is a human ailment however called “Crohn's disease” that in several ways resembles Johne's disease. Crohn’s disease is a chronic inflammatory bowel disease (IBD) that has no known cause and no known cure. In some studies MAP has been found in tissues of Crohn’s disease patients more often than controls. Some researchers believe MAP contributes to Crohn's disease for at least a subset of patients. The majority of gastroenterologists, however, do not; they believe that MAP, if found in this subset of patients, is simply a by-stander amongst the many other organisms that are found in a malfunctioning gastrointestinal tract. No connection has been shown between contact with animals with Johne's disease, dairy product or meat consumption and Crohn's disease. This aspect of MAP is a complex and controversial area of scientific investigation. A detailed discussion
of this topic can be found on the "Zoonotic
Potential" page of this website.