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Paratuberculosis is a contagious, chronic and sometimes fatal infection that primarily affects the small intestine of . It is caused by the bacterium Mycobacterium avium subspecies paratuberculosis. Infections normally affect ( that have four compartments of their stomachs, of which the rumen is one), but have also been seen in a variety of nonruminant species, including rabbits, foxes, and birds. Horses, dogs, and nonhuman primates have been infected experimentally. Paratuberculosis is found worldwide, with some states in Australia (where it is usually called bovine Johne's disease or BJD) being the only areas proven to be free of the disease.Collins M. and Manning E. "Johne's Information Center" The University of Wisconsin-School of Veterinary Medicine. 13 March 2003. At least in Canada, the signs of BJD usually start when cattle are four to seven years of age, and then usually only are diagnosed in one animal at a time. Cattle "with signs of Johne's disease shed billions of bacteria through their manure and serve as a major source of infection for future calves."
Some sources define "paratuberculosis" by the lack of Mycobacterium tuberculosis, rather than the presence of any specific infectious agent, leaving ambiguous the appropriateness of the term to describe Buruli ulcer or Lady Windermere syndrome.
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.
The clinical signs are similar in other ruminants. In sheep and goats, the wool or hair is often damaged and easily shed, and diarrhea is uncommon. In deer, paratuberculosis can progress rapidly. Intestinal disease has also been reported in rabbits and nonhuman primates.
Unlike cattle and sheep, infections in deer often present with clinical illness in animals under one year of age.
Unfortunately, when M cells bring M. paratuberculosis to the Peyer's patch, the bacteria find an ideal place for growth. Macrophages in Peyer's patches engulf M. paratuberculosis for the purpose of destroying the foreign invader, but for reasons yet unclear, these macrophages fail to do this. Inside a macrophage, M. paratuberculosis multiplies 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 teeming with millions of the mycobacteria. How M. paratuberculosis neutralizes or evades the normally efficient bacterial killing mechanisms of the macrophages is unknown, although the unusually resistant cell wall of mycobacteria likely plays an important role.
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 , in an 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 mycobacteria. 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.
For goats infected with this disease, the most apparent sign of having it is their bodies wasting away, even with a sufficient diet. If a goat develops Johne's and it has diarrhea, it is most likely going to die. When it has diarrhea, the goat is at the last stages of the disease. Herds should be tested once or twice a year to maintain the health and keep out the disease.
As of October 2019, neither the World Health Organization nor any individual nation had declared Johne's disease to be zoonotic.
Clinical similarities are seen between Johne's disease in ruminants and inflammatory bowel disease in humans, and because of this, some researchers contend the organism is causative factor in Crohn's disease.Gitlin L, Borody TJ, Chamberlin W, Campbell J. Mycobacterium avium ss paratuberculosis-associated diseases: piecing the Crohn's puzzle together. J Clin Gastroenterol. 2012 Sep;46(8) 649-55. However, epidemiologic studies have provided variable results; in certain studies, the organism (or an immune response directed against it) has been much more frequently found in patients with Crohn's disease than asymptomatic people.
Action and regulations
Paratuberculosis is a reportable disease in some states of the US.United States Department of Agriculture, "National Animal Health Reporting System - Reportable Diseases" US Federal regulations prohibit culture positive or DNA test-positive animals from being moved across state lines except for slaughter.
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