44 



CONTROL SERIES No. 63 



PART V 



A lot of 129 day-old chicks was divided into two groups. Group A, consisting 

 of 75 chicks, was exposed to artificial infection. Group B, consisting of 54 chicks, 

 served as controls. 



The chicks in Group A did not show clinical symptoms of pullorum disease. 

 On the day following exposure, 1 chick died and S. pullorum was not isolated. 

 This was the only mortality up to 2 weeks of age. A possible explanation of the 

 failure of this group of chicks to react to the exposure in the same manner that 

 chicks in Part IV reacted is that the size of dose was less, the chicks may have 

 been slightly older and more resistant, and the pathogenicity of the strain may 

 have changed. Then 24 chicks from Group A were placed in a separate pen and 

 designated as Group A-1. Each chick in Group A-1 was subjected to a second 

 exposure consisting of 0.2 cc. of a 48-hour broth culture. Beginning at 4 veeks 

 of age, all chicks in Groups A and A-1 were tested by the tube agglutination test, 

 at weekly intervals, except as otherwise noted. A sufficient quantity of 5. pul- 

 lorum antigen was prepared for the duration of the experiment. Dilutions of 

 1:10 and higher, sufficient to determine the titer, were employed. A chick 



Table i6 — Serological and Necropsy Data Concerning Thirtv-Two 

 Artificially Exposed Chicks (Part V) 



Not considered a reactor at any time. 



Agglutination not complete in 1 :20 and higher dilutions. 



