Mycobacterium avium

Also know as: Avian tuberculosis — Avian mycobacteriosis — Mycobacterium avium complex (MAC) disease — Mycobacterium avium-intercellulare complex (MAIC) disease

Introduction

Tuberculosis occurs worldwide in birds as a contagious, chronic, bacterial disease caused by members of the Mycobacterium avium complex which currently consists of four subtypes:

M. avium subsp avium is fully virulent for birds and small mammals.

M. avium subsp hominissuis is found in the environment but some are virulent in birds.

M. avium subsp paratuberculosis affects ruminants and other animals.

M. avium subsp silvaticum is rarely found and can be virulent in birds.

Avian tuberculosis is a slowly developing disease that leads to anorexia, emaciation, dyspnoea and death after a few months. After being established it can persist in the environment and bird populations for years.

Hosts

All species of bird can be affected, but susceptibility amongst the domestic species is in the following order: chickens, ducks, geese and turkeys.

The disease is most commonly seen in older poultry due to the greater opportunity for infection with age.

Game birds such as pheasants are also susceptible.

Cage birds can also be affected by avian tuberculosis but parrots and canaries can also contract tuberculosis from Mycobacterium bovis and Mycobacterium tuberculosis species.

Many species of wild birds also become infected.

Amongst mammals, M. avium can cause progressive disease in pigs, rabbits and mink and cause sensitivity to the skin tuberculin test in cattle.

The disease is zoonotic, although infectivity for humans is very low. The disease is progressive and often fatal and particularly severe in children and immunocompromised individuals.

Spread of the organism

The most important source of the organism is the infected host. Also infectious are the items contaminated with the droppings of infected birds, such as litter, pastures and pens, equipment and implements, and also the hands, feet and clothing of workers.

Eggs are of a minor importance in the epidemiology of the disease, and chicks hatched from eggs laid by tuberculous hens are not infected.

Lack of hygiene influences the appearance of the disease due to the organism being highly resistant in the environment. It is relatively resistant to many antimicrobials and disinfectants, but sensitive to ionic detergents.

Clinical signs

Signs can occur for a period of weeks of months before death.

There is a generally progressive loss of condition and lethargy, although appetite remains good.

Affected birds will show marked emaciation and atrophy of the sternal muscles with a prominent keel.

The face and comb are pale and sometimes jaundiced.

There is persistent diarrhoea with soiling of the tail feathers.

Sometimes birds will show a hopping motion, usually unilateral and thought to be associated with tuberculosis lesions in the bone marrow of leg bones and joints.

Sudden death can also occur due to rupture of the liver or spleen and internal haemorrhage.

Diagnosis

The clinical signs are strongly suggestive.

Gross lesions include: granulomatous lesions in the intestine, liver, spleen and bone marrow present as irregular grey-white nodules, firm and caseous, with a pale yellow centre.

The lungs are more commonly affected in waterfowl.

Ziehl-Neelson staining of smears or sections and demonstration of acid-fast bacilli also supports the diagnosis.

Immunological tests are useful in recognising infected live birds, and include:

Tuberculin test: In chickens, avian tuberculin is injected into the wattle, and the test read 48 hours later. The two wattles are palpated simultaneously and a positive reaction is recognised by a hot, soft and oedematous swelling in the injected wattle. Sensitivity is 80% in chickens. In other species it is not as reliable and the agglutination test is preferred.

Agglutination test: a drop of antigen is mixed with a drop of blood from the bird and a positive reaction corresponds to agglutination within 1 minute. It has a poor specificity.

ELISA: antibodies against M. avium can be detected in serum. A positive result can indicate previous exposure to the organism, latent infection or active infection.

Control

Antimicrobials are generally ineffective against the organism, and drug therapy is usually not economically feasible.

In commercial poultry flocks, relatively rapid turnover of populations, together with improved general sanitation, has largely eliminated the incidence of avian tuberculosis.

In an outbreak, to eradicate and maintain freedom from infection:

All infected material must be removed by destroying infected birds and thoroughly cleaning the housing and equipment.

Stock should only be introduced if it is free from infection: negative serological tests or absence of clinical signs.

Infection should be prevented from entering the stock by preventing contact with wild birds and maintaining high biosecurity levels.

Freedom from infection should be monitored with serological testing or post-mortem investigation.

In circumstances where it is not practicable to eliminate infection, control measure can be put in place to live with a reduced level of infection. The weight of infection can be reduced by only keeping stock in their first laying season and monitoring for infection and disposing of any positive cases.

Avian tuberculosis in zoos is difficult to eradicate. New additions to the aviary should be quarantined for 2-3 months. The movement of ratites through sales and the long life of these animals have made tuberculosis a major concern for ratite producers. Isolation of ratites purchased at sales is essential to prevent the introduction of tuberculosis into established flocks.

Mycobacterium avium Learning Resources
Flashcards Test your knowledge using flashcard type questions	Avian Medicine Q&A 05

References

Hagan, W. (1988) Hagan and Bruner's microbiology and infectious disease of domestic animals Cornell University Press

Pattison, M. (2008) Poultry diseases Elsevier Health Sciences

Thomas, N. (2007) Infectious diseases of wild birds John Wiley and Sons

Kahn, C. (2005) Merck Veterinary Manual Merck and co