This group of Mycobacterium spp. is widespread in a variety of species of birds and the organisms are ubiquitously present in soil and water in the envirionment. These mycobacteria occasionally spill over into domestic animals or humans, most commonly because of inhalation from the environment rather than direct contact with infected birds.

The M. avium organisms are of increasing concern as a cause of TB in immunocompromised patients- they cause ~ 40% of the cases of disseminated TB in immunocompromised patients, and they are of special concern because of their inherent resistance to anti-TB therapy.

Mycobacterium avium-intracellulare infections in animals:

• Birds:
  • granulomas in a variety of organs (GI, liver, spleen)
• Swine:
  • This is a relatively common infection that swine contract when rooting in soil. Lesions are present in 1-3% of slaughtered hogs, with granulomas in the lymph nodes of the head and GI tract.
• Cattle:
  • M. avium-intracellulare infections are relatively uncommon in cattle and there is no associated pathology, but infection does cause (+) caudal fold skin test reactivity.
• Dogs and cats:
  • These animals are thought to be inherently resistant to M. avium-intracellulare infection, but occasional cases are reported, e.g., recent case of disseminated infection in an immunosuppressed renal transplant cat patient.

Mycobacterium avium subsp. paratuberculosis, the cause of Johne's disease in cattle, has been incriminated as a possible cause of Crohn's disease in humans. (See http://johnes.org/zoonotic/index.html for more information on this very interesting story.)

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