IMMUNOLOGY OF LARYNGOTRACHEITIS 5 



2. Intermit lent Inoculation of Small Doses of ]'uus. 



Chickens, six weeks of age, were divided into four groups of 12 each, and inoc- 

 ulated intravenously with tracheal exudate which had been prepared by triturat- 

 ing 1 gram of desiccated tracheal exudate with 25 cc. of saline solution and 

 filtering through a Seitz filter. The chickens in Group 1 were inoculated with 

 1/4 cc. of the filtered tracheal exudate; those in Group 2, with 1/4 cc. and 1/2 cc. 

 at an interval of seven days; those in Group 3, with 1/4 cc, 1/2 cc, and 3/4 cc. 

 of the same preparation in the same manner; and those in Group 4, with 1/4 cc, 

 1/2 cc, 1 cc, and 2 cc. Twenty-two controls were swabbed intratracheally and 

 contracted infectious laryngotracheitis and died, while the chickens inoculated 

 intravenously remained unaffected. 



Seven days after the final intravenous inoculation the 48 unaffected chickens 

 and two new controls were treated intratracheally with fresh laryngotracheitis 

 virus. The results are shown in Table 3. 



Table 3. — Tests of Immunity Imparted to Chickens by Intermittent 

 Intravenous Inoculations with Small Doses of Virus 



Table 3 indicates that the chickens in Groups 1 and 2 receiving one and two 

 intravenous inoculations of Seitz filtered tracheal exudates, respectively, were not 

 protected against intratracheal swabbings of virulent virus, while the chickens in 

 Groups 3 and 4 receiving increasingly larger doses of the filtrate over longer 

 periods of time were made immune to infectious laryngotracheitis. 



The Duration of Immunity 



Another Hock of chickens as nearly like the preceding as it was possible to secure 

 were treated in a similar manner, except that they were not inoculated intratrache- 

 ally imtil a period of four months had passed, in order to allow the immunity time 

 to wear off if it were inclined to do so. The treatments were as follows: Group 

 1 received 1/4 cc. of the Seitz filtered tracheal exudate diluted 1-25; Group 2, 1/4 

 and 1/2 cc. ; Group 3, 1/4, 1/2, 3/4, and 2 cc. ; and Group 4, 1/4, 1/2, 3/4, 2, 2, 3/4, 

 1/2, and 1/4 cc. The doses were administered at weekly intervals, and the fluid 

 was warmed to body temperature each time. The results are shown in Table 4. 



This experiment indicates that chickens may be immunized to infectious 

 laryngotracheitis by inoculation with small doses of properly diluted infectious 

 laryngotracheal exudate in physiological saline solution at seven-day intervals 

 until four or five doses have been given, over a period of twenty-eight days. 



Success or failure to secure an enduring immunity by this method depends upon 

 the virulence of the Seitz filtered virus. This virulence may be determined by 

 inoculating controls of the same age and resistance as the birds it is desired to 

 immunize. If at any time the filtrate fails to cause active symptoms of infectious 

 laryngotracheitis in the controls, its value as an immunizing agent should be 

 questioned. 



