LARYNGOTRACHEITIS VACCINATION" 7 



reinfection at this time because the forces of infection and resistance which were 

 being studied moved so rapidly. 



However, further investigation after the relapse had cleared up revealed two 

 chronic carriers among the birds which had not shown any symptoms of in- 

 fectious laryngotracheitis, except the results of the vaccination. These two 

 birds remained carriers for a period of two months after all coughing and sneezing 

 had ceased in the rest of the flock, and at necropsy showed lesions of infectious 

 laryngotracheitis in the respiratory tract. One had a diphtheritic ulcer in the 

 mucous membrane of the trachea and the other had a similar lesion in the antero- 

 ventral rima glottis. The remaining birds showed no lesions of disease. During 

 this experiment, the birds did not go off feed, or appear seriously affected, although 

 considerable coughing and sneezing occurred during the period of reinfection. 



This experiment indicates that the percentage of takes computed from Tables 

 1 and 3 insured a satisfactory degree of immunity for the flock as a whole, although 

 a mild outbreak occurred later in those birds showing poor takes. However, 

 the susceptible birds contracted the disease from the carriers in a mild form, thus 

 completing their immunity. 



Cloacal and Bursal Carriers in Laboratory Experiments 



In a previous experiment (Gibbs, 1933b) neither cloacal nor bursal carriers 

 were found in 52 birds five months after vaccination. Such carriers would be 

 important if they exist, for the virus would be located in a region from which 

 it could be most readily transmitted to other birds in the same flock. However, 

 since the cloaca and bursa of Fabricius are subject to comparatively rapid growth 

 and development, it may be that it is impossible for the virus of infectious laryn- 

 gotracheitis to maintain a foothold in the mucous membrane of these organs to 

 constitute chronic carriers. 



Table 5. Length of Time in Which Virus Was Recovered 

 from Visible Takes. 



Days after Number of Number of Birds from Which Virus 



Vaccination Visible Was Recovered from — 



