Efficacy of Marek's disease vaccines: protection and viraemia studies with turkey herpes virus vaccines.
Two freeze dried turkey herpes virus vaccines A and C, of similar virus content were administered in graded doses to groups of day old chicks which were genetically susceptible to Marek's disease. Immediately after vaccination the chicks were challenged by contact with 4-weeks-old birds that had been infected at one day of age with virulent Marek's disease virus strain HPRS 16. Assessment of protection against Marek's disease was based on the absence of gross lesions after a 15 week observation period and a significant difference in the protective capacity of the 2 products was found. From the relationship between protection and vaccine dose that was obtained with vaccine C it was calculated that 0.48 of a field dose protected 50% of the birds. Vaccine A provided no significant protection at any dose level. Isolation of vaccinal virus 3 weeks after administration of graded doses of vaccines A and B revealed that a significantly higher dose of vaccine A was required to produce viraemia in vaccinated birds. Determination of viraemia at intervals following inoculation of one field dose of the vaccines demonstrated that Vaccine A also induced a less rapid response than the other 2 vaccines. These effects were not related to the virus content of the vaccines.
Thornton
D H
DH
Central Veterinary Laboratory, New Haw, Weybridge, Surrey, England.
Hinton
M H
MH
Muskett
J C
JC