The serology on the infectious bronchitis presented the data indicated 

 in Figures 3 and 4. Individual blood samples were used. 



The results on the graphs show that 74.2 per cent of the samples neu- 

 tralized 100 neutralizing doses or better at day-old: 85.7 per cent at 4 

 weeks, 84.2 per cent at 5-8 weeks, 89.0 per cent at 9-12 weeks, 73.5 per cent 

 at 13-16 weeks, 96.6 per cent at 17-20 weeks, and 96.7 per cent at 20 weeks 

 or more. The reason for the higher percentage of positives at 16 weeks and 

 beyond is the result of revaccinations carried out from 16 to 20 weeks. 



At the time blood samples were withdrawn for serological purposes, 

 birds were also withdrawn for challenge. The ages of the challenge birds 

 ranged from 4 to 16 weeks. The results are shown in Table 5. 



One hundred thirty-eight chicks, found to be susceptible to Newcastle 

 disease at 4 weeks of age by random sampling, were spray-vaccinated at 

 the age of 4 weeks. Two birds died during the period of symptoms. Twelve 

 random samples taken at 6 weeks of age exhibited a geometric mean titre of 

 14.25. confirming previous work (8) that older birds develop higher levels 

 of antibody than young chicks. Six samples run on S. N. tests for infectious 

 bronchitis antibody showed that one sample neutralized 10 neutralizing 

 doses, one sample 100 neutralizing doses, and 4 samples 1000 neutralizing 

 doses. The numbers, while of necessity small, indicate susceptible birds pro- 

 duce a higher titre upon vaccination. 



Discussion 



There is one period in the life of a chicken when carefully controlled 

 factors result in optimum conditions for life and growth. This period, when 

 variables are more constant than at any other time, is when the chicks are 

 being brooded under the hover. It is at this time, when the optimum con- 

 ditions exist, for any early age vaccinations to be carried out. 



With this factor in mind, early field trials of spray vaccination of New- 

 castle vaccine, or Newcastle in combination with infectious bronchitis 

 vaccine, were carried out from 2 to 3 days of age in some cases. Recent 

 work indicates that any age prior to production may be satisfactory for vac- 

 cination, however. 



14 



