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Johne's disease (pronounced "yo-knees") is a contagious, chronic and usually fatal infection that affects primarily the small intestine of ruminants. All ruminants are susceptible to Johne's disease. Johne's disease is caused by Mycobacterium paratuberculosis, a hardy bacteria related to the agents of leprosy and tuberculosis. The disease is worldwide in distribution. Signs of Johne's disease include weight loss and diarrhea with a normal appetite. Several weeks after the onset of diarrhea, a soft swelling may occur under the jaw (bottle jaw). Bottle jaw or intermandibular edema is due to protein loss from the bloodstream into the digestive tract. Animals at this stage of the disease will not live very long, perhaps a few weeks at most. Signs are rarely evident until two or more years after the initial infection, which usually occurs shortly after birth. Animals are most susceptible to the infection in the first year of life. Newborns most often become infected by swallowing small amounts of infected manure from the birthing environment or udder of the mother. In addition, newborns may become infected while in the uterus or by swallowing bacteria passed in milk and colostrum. Animals exposed at an older age, or exposed to a very small dose of bacteria at a young age, are not likely to develop clinical disease until they are much older than two years. A national study of US dairies, Dairy NAHMS 96, found that approximately 22 percent of US dairy farms have at least 10% of the herd infected with Johne's disease. The study determined that infected herds experience an average loss of $75 per cow inventory annually. Small herds (<50 cows) lost an average of $178 per cow, while large herds (>500 cows) lost $181 per cow. This loss was due to reduced milk production, early culling, and poor conditioning at culling . The cost of Johne's disease in beef herds still need to be determined.
What is the cause of Johne's disease? Johne's disease is caused by a bacterium named Mycobacterium avium subspecies paratuberculosis; often the name is abbreviated as M. paratuberculosis. M. paratuberculosis is akin to, but not genetically related to Mycobacterium tuberculosis and Mycobacterium bovis, the bacterium that cause tuberculosis in humans. M. paratuberculosis is 99 percent genetically related to Mycobacterium avium, but has different phenotypic characteristics such as 1) slower growth, 2) requires the addition of an iron transport chemical known as mycobactin when grown in vitro (outside the body) 3) forms a rough colony when grown on solid agar media, and 4) infects mammals instead of birds. Also, the environmental distribution of M. paratuberculosis is markedly different from that of M. avium, which can produce mycobactin and therefore grow and multiple outside the body. M. paratuberculosis is a small (0.5 x 1.5 microns) rod-shaped bacterium that has a rough waxy cell wall with a trilaminar structure. This wall is composed of a thick waxy mixture of unique lipids and polysaccharides but lacks glycolipid antigens on its surface. This kind of cell wall facilitates the mycobacterium's resistance to physical factor's (e.g. heat, cold, sunlight, drying. etc.) and common disinfectants. If M. paratuberculosis is found in soil or water samples, it can survive (but not grow and multiple) for over a year after fecal contamination via an infected animal.
The primary site targeted by Johne's disease is the lower part of the intestine know as the ileum. The wall of the ileum contains a large number of pockets of lymphoid tissue known as Peyer's patches that lie just beneath the interior surface of the intestine. Peyer's patches are clusters of macrophages and lymphocytes that are organized much like lymph nodes. Covering Peyer's patches are a layer of cells called M cells. These cells function to circulate into the lumen of the intestines where they ingest antigens (bacteria) before returning to the Peyer's patch to "show" these antigens to the macrophages and lymphocytes. This is a means of "educating" the cells in a young animal about its environment and is a protective mechanism designed to help the animal become immune to pathogens in its environment. Unfortunately, when M cells bring M. paratuberculosis to the Peyer's patch, the bacteria finds an ideal place for growth. Macrophages in Peyer's patches engulf M. paratuberculosis with the intention of destroying the foreign invader, but for reason that are unclear, these macrophages fail to do this. Inside a macrophage M. paratuberculosis multiples until it eventually kills the cell, spreads and infects other nearby cells. In time, other parts of the ileum and other regions of the body are teaming with millions of the mycobacteria. How M. paratuberculosis neutralizes or evades the normally efficient bacterial killing mechanisms of the macrophages is unknown. The animal's immune system reacts to the M. paratuberculosis invasion by recruiting more macrophages and lymphocytes to the site of the infection. The lymphocytes release a variety of chemicals signals, called cytokines, in attempt to increase the bacterial killing power of the macrophages. Macrophages fuse together forming large cells, called multinucleated giant cells, in an apparent attempt to kill the mycobacterium. Infiltration of infected tissues with millions of lymphocytes and macrophages leads to visible thickening of the intestines. This prevents nutrient absorption and diarrhea results. Late in the infection, antibody production by the animal occurs to M. paratuberculosis in serum of animals and is an indicator that clinical signs of disease and death from the infection will soon follow. Preventing Introduction of Johne's Disease
National Johne's Working Group Johne's disease is a growing concern to US cattle industries because the disease impacts international marketing of cattle and cattle products, and causes economic losses to producers. It is also a concern for small ruminant herds (sheep and goats), and non domestic hoof stock (zoos and wild game parks). Concern also exists regarding the zoonotic potential of M. paratuberculosis. In the fall of 1995, the United States Animal Health Association (USAHA) appointed the National Johne's Working Group (NJWG) to assist the Johne's Committee of USAHA in developing a national, coordinated Johne's disease effort in conjunction with the States and cattle industries. The NJWG has developed a strategic plan designed to reduce the prevalence of Johne's disease in US cattle. This plan includes a national educational campaign, the Voluntary Johne's Disease Herd Status Program for Cattle, and guidelines for states to assist infected herds. This national program is designed to be producer driven and voluntary. Other US livestock industries are examining potential certification and control programs for their industries. The American Zoological Association has prepared Johne's disease guidelines as well. For more information on the NJWG's activities, visit USAHA's web site at: http://www.usaha.org USDA-APHIS-VS and Johne's Disease USDA-APHIS-VS role in the design and implementation of a national Johne's strategic plan has been to provide expertise and help coordinate implementation of Johne's disease programs by the States. Some activities include:
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