Disease: Avian tuberculosis

This page provides general information about this condition. Text can be revealed by clicking on the green headers. Links to press releases, results from DWHC investigations as well as other useful documents and relevant literature available on the DWHC website can be found at the bottom of the page.

Pathogen

Avian tuberculosis is a disease of birds caused by infection with bacteria belonging to the genus Mycobacterium. There are many sorts of Mycobacteria and they are grouped into different complexes and clades [7]. Not all sorts are pathogenic. Medically speaking, of particular importance are those belonging to the M. tuberculosis-complex which typically infect mammals, and those belonging to the M. avium complex which tend to be the cause of avian tubrculosis [2].

Over the period 1975-1985 wild birds in the Netherlands underwent post-mortem Investigation [6] and from the 82 Mycobacterium-positive cases, different species were cultured: 76 M. avium, 4 M. fortuitium, 1 M. terrae, 1 M. nonchromogenicum, 1 M. flavescens and one species (found in two cases that were co-infected with multiple sorts of Mycobacteria) that could not be further identified.

Susceptible species

In addition to causing disease in birds, M. avium complex bacteria can also establish infection in people, particularly those with immune suppression. Additionally it has been associated with disease in various species of wild and domestic mammals including pigs, sheep, rabbits, mink and deer [1].
Avian tuberculosis is occurs worldwide and all bird species appear to be susceptible to some degree. It is most commonly encountered in private aviaries, zoos and wild birds.
In the population studied between 1975 and 1985, Mycobacterium spp. were found in 82/11,664 (0,7%) birds, from 25 different species and 10 different orders [6]. In some cases researchers believe that the infection was an incidental finding with no obvious clinical significance. Similar prevalence rates were observed in US and Canadian surveys which reported Mycobacterium spp. infection in 0.3% of 3000 water birds and 0.6% of 600 wild birds, respectively [1]. The bacteria are spread in the feces of birds with advanced infections. Additionally, cadavers and slaughterhouse waste may be sources of infection for prey animals and carnivorous birds. Recent molecular studies demonstrate heterogenicity within the M. avium complex which may explain differences in epidemiology and pathogenesis between the different species [5].

Signs in animals

Avian tuberculosis is a slowly-developing, granulomatous disease which presents with gradual weight loss in infected birds. It is often associated with systemic infection in which the liver, spleen, intestines and bone marrow are typically involved and contain variably-sized granulomas [4, 8]. In contrast to the classic granulomas caused by M. tuberculosis complex species of Mycobacteria, the granulomas seen in avian tuberculosis are rarely calcified [4]. Onset of signs tends to indicate advanced disease. Provisional diagnosis is based on the presence of large numbers of acid-fast bacteria in impression smears of organs and lesions; this must confirmed by laboratory investigation [7].

Preventative measures

In the poultry industry this previously common disease has been largely eliminated due to changes in husbandry practices whereby bird lifetimes are short, and biosecurity/hygiene has been improved. If infection is detected in any number of birds, the entire flock must be culled as treatment is not effective and current tests do not allow the accurate detection of every individual case [1]. In addition, the housing must be thoroughly disinfected and, as M. avium is very resistant and can survive for years in the environment, several centimeters of ground must be removed from infected premises [2].
It is very important to minimize the spread of avian tuberculosis associated with human activities including conservation programs. For example, wild bird carcasses fed to birds of prey in sanctuaries should be thoroughly checked for signs of infection [1]. This is important as signs in infected birds are minimal and diagnostic tests in live birds are typically unreliable. Eradication of the disease in wild birds is impossible. However, in some situations where recurrent outbreaks in wild bird populations have been reported, habitat manipulation (e.g. bird-scarers and drainage) is employed to deter birds from entering these areas.
By minimizing access to such environments that are likely to have been contaminated during previous outbreaks, it may possible to reduce the number of healthy birds that become infected. To be effective, such measures should be in place for at least two years due to the persistence of Mycobacterium in the environment [1].
The difficulty of eradicating avian tuberculosis from zoo collections is evidenced by the regularly occurring cases [1]. Due to the often absent or slow-to-develop clinical signs, new birds should be quarantined for 2-3 months prior to being released into existing collections [3, 5].

References:
1. 1999. Tuberculosis. 93-98. In Field manual of wildlife diseases: general field procedures and diseases of birds. Geological Survey (U.S.). Biological Resources Division.
2. 2010. Merck Veterinary Manual. 10th ed. Publisher, Whitehouse Station, N.J.: Merck & Co. Editor: Cynthia M. Kahn. 2945 pp.
3. Cooper J.E. 2007. Birds of prey: health & disease. 3rd ed. Wiley-Blackwell. 364 pp.
4. Fulton R.M. & Thoen C.O. 2003. In: Diseases of poultry. 11th ed. Saif Y.M. Blackwell Press. 1233 pp.
5. Schrenzel M. Nicolas M. Witte C. Papendick R. Tucker T. Keener L. Sutherland-Smith M. Lamberski N. Orndorff D. Heckard D. Witman P. Mace M. Rimlinger D. Reed S. and Rideout B. 2008. Molecular epidemiology of Mycobacterium avium subsp. avium and Mycobacterium intracellulare in captive birds. Veterinary microbiology, 126, 1-3: 122-131.
6. Smit T. Eger A. Haagsma J. and Bakhuizen T. 1987. http://www.jwildlifedis.org/doi/pdf/10.7589/0090-3558-23.3.485 Avian Tuberculosis in Wild Birds in the Netherlands. Journal of Wildlife Diseases, 23, 3: 485-487.
7. Turenne C.Y. Wallace R. and Behr M.A. 2007. Mycobacterium avium in the Postgenomic Era. Clinical microbiology reviews, 20, 2: 205-229.
8. Whiteman C.E. & Bickford AA. 1989. Avian disease manual. 3rd ed. Dubuque, Iowa: Kendall/Hunt. 242 pp.

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