DISEASES OF POULTRY. 213 



miUed by some authorities and denied b}- others. It 

 is certain that poultry and pigeons are not easily in- 

 fected experimentally with the tul^erculosis of people, 

 cattle and other animals which are classed together as 

 mammals. This may sometimes be accomplished; 

 but after one bird is infected in this way it is equally 

 difficult to transmit the disease from this bird to other 



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Fi;/. 64.— Growth of horu from region of head affected with tuberculosis. 



birds. It appears, therefore, out of the question to 

 produce a rapidly spreading outbreak of disease 

 among fowls or pigeons by the use of human virus. 



The bird or avian tuberculosis spreads rapidly from 

 bird to bird and is easil}^ transmitted experimentally 

 to birds, but it has little effect upon most mammals 

 which are very susceptible to human tuberculosis. 

 There is, consequently, a marked difference between 

 avian and mammalian tuberculosis. The disease in 

 the two cases does not appear to be absolutely 

 distinct, but should rather be regarded as two varieties 

 of the same maladj'. These varieties have been de- 

 veloped because the bacilli have grown for a long 

 series of years under different conditions. They may 

 still possibly be changed from mammalian to avian 



