B303 - TUBERCULOSIS (AVIAN)

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B303 - TUBERCULOSIS (AVIAN)

Nature of the disease
Avian tuberculosis (avian TB) is a chronic wasting disease caused by infection with Mycobacterium avium that can affect a wide range of bird species and may infect a number of mammalian species.
Classification
OIE List B disease
Susceptible species
All species of birds are susceptible with the domestic fowl the most frequently infected. Turkeys are rarely infected.

Among mammals, rabbits and pigs are the most susceptible species. Localised infections can occur in cattle and are of concern because they induce sensitivity to mammalian tuberculin. 

Infections have been recorded in humans but the serotypes have generally been different to those isolated from poultry.

Distribution
Avian TB occurs worldwide, with it being most frequently recognised in the Northern Hemisphere temperate zone, however the disease is present in New Zealand and in Australia.
Clinical signs 
Birds of all ages may be affected but usually the disease occurs after 3 weeks:
  • Gradual wasting especially evident as atrophy of the pectoral muscles
  • Tiredness
  • Dull and ruffled plumage
  • Jaundice (if liver is involved)
  • Lameness or dropping of wing (with bone or joint lesions)
  • Diarrhoea
Post-mortem findings 
  • Characteristic greyish-white or greyish-yellow nodules of varying size are almost invariably present in spleen, liver and intestines. Almost any tissue may be affected. 
  • Affected livers and spleens are enlarged and may eventually rupture
  • Involvement of the lungs is less common
Differential diagnosis 
  • Pseudotuberculosis (more common in ducks and turkeys than the domestic fowl)
  • Coli granuloma (Hjarre’s disease)
Specimens required for diagnosis 
Characteristic lesions at post-mortem, together with the presence of acid-fast bacilli in stained smears or sections are generally sufficient to make a diagnosis. Samples of nodules should be collected for histopathology.
Transmission   
Infection results from ingestion of material contaminated with faeces of infected birds or other animals. The faeces of infected birds can contain very large numbers of tubercle bacilli.

M. avium can persist in the environment for many years. Spread from contaminated environments can take place mechanically on shoes and equipment. Ingestion of infected carcase material is another source of infection. 

Infected free-flying birds and infected pigs may also spread infection. 

Eggs are not considered to have a significant role in spreading avian TB.

Risk of introduction   
Avian TB could be introduced into a previously free country with importations of infected live birds (including companion and zoological species). Contaminated crates, equipment and clothing are another risk. 

When importing live poultry, tuberculin testing can be used to test for the presence of the disease.

Control / vaccines  
There is no treatment, to eradicate Avian TB, stringent measures are required:
  • All birds in affected flocks should be slaughtered, all equipment and housing destroyed, preferably by burning or using cresylic compounds. 
  • The flock should be repopulated with day-old chicks that are not allowed any contact with the old fittings and environment. 
  • Any pigs present should be tuberculin-tested and restricted from areas where poultry run.
References
  • Avian Tuberculosis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 1952
  • Office International des Epizooties, 2002