Infections normally affect ruminants (mammals 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.
[3] 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.
"[4] Some sources define "paratuberculosis" by the lack of Mycobacterium tuberculosis, rather than the presence of any specific infectious agent,[5] leaving ambiguous the appropriateness of the term to describe Buruli ulcer or Lady Windermere syndrome.
The disease, discovered by Heinrich A. Johne, a German bacteriologist and veterinarian, in 1905, is caused by a bacterium named Mycobacterium avium subspecies paratuberculosis, an acid-fast bacillus, often abbreviated MAP.
[citation needed] Pasteurization is used to kill the causal agent, M. paratuberculosis, by heating cow's milk for a short time and then immediately cooling it.
The initial signs can be subtle, and may be limited to weight loss, decreased milk production, or roughening of the hair coat.
[citation needed] Unlike cattle and sheep, infections in deer often present with clinical illness in animals under one year of age.
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.
[citation needed] Unfortunately, when M cells bring M. paratuberculosis to the Peyer's patch, the bacteria find an ideal place for growth.
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.
[citation needed] 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 an attempt to increase the bacterial killing power of the macrophages.