38 



MASS. EXPERIMENT STATION BULLETIN 27:J 



Table 8. — History of birds rsi:i) as controls on the Granby viri^s 



EXPERIMENTS. 



Virus from MC 60840 used. 



e.\])erinients, showing tliat the vini.ses were not esjtecially virulent. At- 

 tein))ts to enhance tlie virulence of these viruses by continuous passages 

 through fowls and chickens failed. For reasons not well understood, 

 these viruses tended to weaken in continuous passage under laboratory 

 conditions. The results of tyjiica! experiments with Leverett, Westwood, 

 Gardner, South Lincoln, and Watertown chronic field cases of infectious 

 trachitis are recorded in Tables 9 and 10. 



The Leverett virus killed four of the seven pullets wiiich contracted 



