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Short
answers to common
What
is Johne's disease and can bison get it?
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Johne's (pronounced "Yo-nees") disease is a contagious, slowly developing and usually fatal bacterial disease of the intestinal tract. A German veterinarian first described it in a dairy cow in 1895 and the disease was named after him. The disease is also called paratuberculosis.
The bacterium that causes Johne's disease is named Mycobacterium avium subspecies paratuberculosis. The name is abbreviated M. a. paratuberculosis or M. paratuberculosis. It is in the same family as the bacterium that causes tuberculosis in humans and animals and the type of disease it causes shares some characteristics with tuberculosis: it is slow to develop, it is resistant to treatment with antibiotics and is contagious. M. paratuberculosis can replicate only when within the animal; it cannot multiply in the environment. However, the bacterium reportedly can survive, still infectious, in the soil or water for over a year because of its resistance to heat, cold and drying.
This infection is hard to detect. The antemortem signs of Johne’s disease are vague and months can pass after infection before they appear. In cattle, weight loss and diarrhea in an animal with a good appetite are common indications. Weight loss in animals with a good appetite may be the only indication in some species such as sheep, goats and perhaps bison as diarrhea may not apppear. The animal may appear unthrifty and its coat may be in poor condition. The signs of Johne's disease can be confused with the clinical indications of other conditions such as parasitism, primary or secondary malnutrition or other chronic diseases. Of course, multiple health problems may be occurring and treatment for one condition may produce some improvement in the animal’s condition. Because of the slowly progressive nature of the infection, signs of Johne's disease are usually not seen until animals are adults. The infection is easy to miss at necropsy as well. The gastrointestinal tract of cases in early stages of infection may look completely normal both grossly and microscopically. This means that you may not find evidence at necropsy that supports an antemortem (fecal culture isolation, high antibody) diagnosis. Bison may present minimal signs of pathology even in later phases of infection.
Data on the prevalence of this disease is limited due both to lack of testing and reporting. Confirmed cases have been reported in several farmed and free-ranging herds in the United States.
Johne's
disease typically enters a herd when an apparently healthy, but
infected, animal is introduced. This animal contaminates the premises
(feed, ground, water) by shedding the organism in its manure. The
organism is then ingested by other animals. Calves less than 6 months
of age are thought to be most susceptible to infection and their
suckling manure-soiled teats is another route of infection. It is
also possible that a cow in the later phases of the infection can
shed M. paratuberculosis into her milk and colostrum, thus transmitting
it to her offspring.
The best way to avoid this chronic infectious disease of course is to be as certain as possible that newly introduced animals are not infected with M. paratuberculosis. Johne's disease test-negative herds are the best sources of these animals. Assess the body condition of all adult animals (not just the animals you intend to buy), ask about the cull rate and common cause for culling - how many animals were thin/unthrifty for no confirmed reason? Check the body condition of the dam of young hoofstock you intend to purchase and have her tested for the infection. Following general rules of sanitation are helpful in blocking transmission of the infection. Feeders and waterers should be designed to minimize the likelihood of fecal contamination. Provide clean, dry areas for calving. Animals with weight loss or diarrhea should be segregated and tested.
The currently recommended testing methods for bison are culture and/or PCR of fecal samples to detect the M. paratuberculosis organism. At least one of the strains of M. paratuberculosis infecting bison is particularly slow-growing (the samples need to incubated for at least 20 weeks). The biology of the disease can make it difficult to detect infected animals, especially those in an early stage of infection. A truly infected animal in this "sub-clinical" phase may not be shedding the organism consistently (thus the fecal culture may be negative). Animals in later stages of the infection shed the organism more frequently and in greater numbers, thus the culture assay is more sensitive in clinically affected animals. Blood tests for antibodies made in response to M. paratuberculosis infection are also available but their use is recommended for domestic hoofstock only (cattle, sheep, goats) at this time as the assays have not yet been validated for bison.
Although few studies have been reported, Johne's disease is not considered a curable disease. As with most diseases caused by mycobacteria, the course of therapy is very long (months) and expensive. For most animals, treatment is cost prohibitive and the best that can be expected is an improvement in clinical signs while under treatment, not a cure.
This is a very controversial subject. There is a human disease called Crohn's disease that in some ways resembles Johne's disease. The cause of this chronic inflammatory bowel disease is not known and there is no known cure. A few laboratories have grown M. paratuberculosis from a few Crohn's patients’ specimens but there is no evidence that the organism caused the disease. No connection has been shown between contact with animals with Johne's disease or milk/meat
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