250 



MASS. EXPERIMENT STATION BULLETIN 257 



Interpretation. — To determine duration of immunity, inoculations with 

 virulent virus were made on the conlbs of cutaneously vaccinated birds, on five 

 different dates from May 28 to Novemlber 9, 1928. At each of these tests one 

 or two unvaccinated birds were inoculated to serve as checks on the virulence 

 of the virus used. The data show that all control birds developed a moder- 

 ate to severe comb infection following the inoculation with the viriUent virus. 

 Without exception lesions were absent in the cutaneously vaccinated birds. 



The results indicate that Rhode Island Reds, when cutaneously vaccinated 

 early in their pullet and cockerel year, are completely immune to subsequent 

 fowl pox infection for at least 371 days after vaccination, except for a period 

 of 29 to 31 days immediately after the treatment, during which the immunity 

 is developing. Additional birds were not available for imn)unity tests beyond 

 this period of tinie. It is probable that the duration of inmmnity exceeds 

 371 days. 



The Action of the Glycerol in the Diluent on the Virus Content of the 



Cutaneous Vaccine 



It was shown in a previous report (3) "that when the cutaneous vaccine 

 is 25 days or less in age at the time it is administered, it produces a complete 

 immunity. When older it does not confer absolute protection against the ex- 

 perimental infection". It was suggested that this might have been due to an 

 actual aging of the virus, the action of the glycerol upon it, or some other 

 unsuspected factor. Beach (9) in experimenting with his subcutaneously ad^ 

 ministered lesion tissue vaccine, found that there was a change in the virus 

 contained in ihe vaccine as a result of aging. Older vaccines did not produce 

 as extensive and severe lesions as did the younger vaccines, when tested for 

 their virulence. However, the lesions produced by the older vaccines showed 

 a persistency and prominence comparable to those produced by the younger 



