Avian Tuberculosis in Chickens: Symptoms, Zoonotic Risk and Control

Mycobacterium avium subsp. avium (MAA) is a slow-growing mycobacterium that forms granulomas in organs throughout the body. Its extreme environmental persistence makes eradication from contaminated premises very difficult.

• Fecal-oral transmission: Infected birds shed MAA in feces for months to years before showing clinical signs. Other birds in the flock ingest the organism while pecking at contaminated litter or soil.
• Soil persistence: MAA survives in soil for 4 years or longer under typical outdoor conditions. This is the key challenge in managing avian TB outbreaks: even after all infected birds are removed, the environment remains infectious for years.
• Slow progression: Avian TB typically takes 1-2 years from initial infection to clinical signs. By the time one bird shows signs, the entire flock has likely been exposed for months.
• Wild bird reservoir: Wild birds, especially crows, starlings, and sparrows, carry M. avium and can introduce it to free-range flocks. Long-lived backyard flocks with outdoor access are at highest risk.

Avian tuberculosis is usually diagnosed at necropsy when characteristic granulomas are found in the liver and intestines of a wasting bird.

• Necropsy: Multiple gray-white to yellow nodules (granulomas) throughout the liver, spleen, intestines, and bone marrow are classic. Intestinal granulomas may be palpable as hard nodules along the intestinal wall.
• Acid-fast smear: Impression smear from a granuloma, stained with Ziehl-Neelsen (acid-fast stain), reveals red-staining acid-fast bacilli. This is a rapid, inexpensive confirmatory test.
• Culture: MAA grows very slowly (4-8 weeks) on specialized Lowenstein-Jensen media. Required for definitive species confirmation but not practical for field diagnosis.
• Tuberculin skin test: Intradermal injection of avian tuberculin into the wattle shows swelling at 48-72 hours in sensitized birds. Less reliable than necropsy for flock-level diagnosis.

There is no practical or approved treatment for avian tuberculosis in poultry. Treatment options used in humans and pet birds are not applicable to production or backyard flocks.

Recommended action: Cull the entire flock as soon as avian TB is confirmed. This is the only reliable way to eliminate the infection source. Partial culling leaves subclinical carriers that continue to shed and reinfect.

Environmental decontamination: Remove all litter and topsoil to a depth of 6 inches from soil areas. Dispose as directed by your state environmental guidelines. Treat housing surfaces with phenolic disinfectants. M. avium in soil cannot be practically disinfected; the most effective approach is removing the top layer of contaminated soil.

Minimum downtime before restocking: 12 months minimum for housed birds. Outdoor runs contaminated with M. avium should be rested for 2-4 years or have topsoil completely removed and replaced before safe use.

• Limit the lifespan of your flock if possible. Avian TB rarely affects birds under 2 years. Rotating flocks every 18-24 months greatly reduces the risk of clinical disease expression.
• Prevent wild bird access to feed, water, and housing. Crows and starlings are M. avium carriers and shed the organism in their feces.
• Test any birds with unexplained chronic wasting before integrating them into a new flock. The tuberculin skin test can identify infected individuals before they seed a clean environment.
• After a confirmed avian TB outbreak, do not use the same soil run for 2-4 years. Consider replacing the top 6 inches of soil in high-density areas before restocking.