14 



MASS. EXPERIMENT STATION BULLETIN 311 



to do with it. The disease was probably due to a bacterium or virus which grew 

 in the cloaca of the fowl but did not grow on the culture media used in this study, 

 and either did not grow or did not maintain its virulence in the respiratory tract 

 of the chickens beyond the first inoculation. 



Table 7. Inoculation Experiments in an Unusual Disease of 

 the Cloaca and Bursa of Fabricius of the Domestic Fowl. 



Since the cloaca and bursa of Fabricius undergo rather radical changes in the 

 growth of the chicken, it is anticipated from this and preceding studies that 

 cloacal and bursal carriers of infectious laryngotracheitis may not exist for any 

 great length of time. As the chicken grows, the mucous membrane of the cloaca 

 is subject to the passage of excretions in both sexes, and the laying of eggs in 

 pullets. These functions tend to thicken the mucous membrane and make it 

 firmer and more resistant to mechanical injury, as well as to the attacks of micro- 

 organisms and viruses. At five or six months of age, the bursa of Fabricius 

 undergoes involution in the course of which this organ disappears entirely and is 

 replaced by a bursal fold. The mucous membrane of the bursal fold is firmer 

 in consistency and more resistant to the virus of infectious laryngotracheitis 

 than that of the bursa of Fabricius. 



DISEASES SOMETIMES MISTAKEN FOR INFECTIOUS 

 LARYNGOTRACHEITIS 



During the course of four years and a half of field investigations, representative 

 cases from 131 epizootics have been studied, and some diseases simulating in- 

 fectious laryngotracheitis clinically have been found. Laboratory studies 

 revealed that these diseases are etiologically and immunologically different. 

 Infectious laryngotracheitis was not only the most important from a mortality 

 point of view, but it appeared to be the most common, as the following summary 

 shows: 



Coryza and colds 14 



Conjunctivitis 5 



Rhinosinusitis 3 



Infectious bronchitis 2 



Chickenpox and roup 4 



Infectious laryngotracheitis 103 



Total 131 



