A study of vaccination against Marek's disease with an attenuated Marek's disease virus

Approximately 2,000 day-old white leghorn chickens were distributed into 10 pens and half the pens vaccinated with the attenuated strain of HPRS-16. At 72 weeks of age, mortality from non-specific death and Marek's disease was 17.6% and 15.6% respectively, in vaccinated chickens compared with 14.2% and 51.7% in unvaccinated chickens. Body weights of vaccinated chickens were 5.6% higher at 8 weeks of age than unvaccinated chickens. Vaccinated chickens consumed 2.3% more feed and the hen housed egg production and hen day egg production were 58.7% and 7.0% greater than unvaccinated chickens. Vaccination resulted in a 3.8 fold increase in net margin over food and bird costs per bird housed at 16 weeks. Active antibody production to 'A' antigen occurred later and in a smaller proportion of vaccinated chickens than unvaccinated chickens. Viraemia due to vaccination was detected at 2 weeks of age and viraemia due to field virus was detected at 5 weeks of age. The incidence and titres of viraemia due to field virus were higher in unvaccinated chickens compared with vaccinated chickens. No evidence of spread of vaccine virus to unvaccinated chickens could be found. Acute, classical and apathogenic strains of field virus were isolated from vaccinated chickens and strains of field virus were found to persist throughout the life of the vaccinated chickens. Mild classical and/or apathogenic strains were first apparent at 18 weeks of age and increased in proportion thereafter, forming the majority of isolates from 52 weeks of age. Data on individual birds suggested a direct relationship between virus titre and lesion (or Marek's disease) frequency.