6 .MASS. EXPERIMENT STATION BULLETIN 311 



What Is a Satisfactory Degree of Immunity 

 for a Flock as a Whole? 



From the studies reported in Tables 1 and 3, the percentages of takes that 

 would insure a satisfactory degree of immunity for a flock as a whole were com- 

 puted from the visible lesions as follows: 



Number Age Satisfactory 



of in Percentage 



Birds Months of Takes 



225 1-3 93.8 



225 4-7 97.3 



125 8-12 . 90.0 



It was tentatively concluded from this computation that flocks ranging in age 

 from one. to three months could be considered satisfactorily vaccinated against 

 infectious laryngotracheitis if 94 percent of takes were secured for the flock as 

 a whole, 97 percent for those four to seven months old, and 90 percent for hens 

 and roosters. 



While these conclusions were logical and backed by scientific results, there 

 were other factors such as the duration of immunity and the production of laryn- 

 gotracheal carriers in vaccinated flocks that might operate under field conditions 

 and have some influence on the final results. The reason for believing that this 

 experiment should not be considered as final is based on a previous study (Gibbs, 

 1933b) in which it was found that immunity was not lasting in birds receiving 

 small doses of filtered virus intravenously, and that laryngotracheal carriers were 

 produced in flocks by vaccination Just what result would ensue from two such 

 diametrically opposed forces as these operating in a vaccinated flock could only 

 be imagined, since no experimental work had ever been done with such a flock. 



With the object in view of determining just what would happen when the 

 immunity began to wear off in some of the birds in a vaccinated flock in which 

 there were carriers, the 70 birds showing poor and fair takes in Table 2 were 

 placed in a colony house with five known infectious laryngotracheitis carriers. 



After two months the immunity began to wear off in the vaccinated birds. 

 As soon as any of the birds were observed coughing, sneezing or gasping, the 

 larynx and trachea were swabbed, and the exudate inoculated intratracheal!) 

 into susceptible chickens. In this way, 17 of the 25 birds observed coughing 

 and sneezing were found to be carriers for indefinite periods of time as shown in 

 Table 4. It was impossible to test the birds not showing acute symptoms of 



Table 4. Laryngotracheal Carriers Following a Reinfection of 

 Infectious Laryngotracheitis in a Vaccinated Flock. 



