Long-term field trials with the herpesvirus of turkeys vaccine against Marek's disease

In long-term field trials lasting approximately 18 months, the herpesvirus of turkeys proved safe and highly effective in preventing Marek's disease (MD). The average MD mortality was reduced from 19.2 to 2.8%, and the degree of protection averaged 80.8% in the 11 experiments in which detailed necropsies were performed. Protection lasted the entire experimental period and there was a reduction in leukosis condemnation at slaughter. The degree of protection varied from flock to flock but was greatest in flocks that had the highest incidence of MD in the unvaccinated controls. There was no apparent difference in protection between the low, intermediate, or high (respectively about 500, 2,000, or 10,000 PFU per chick) doses of vaccine. Vaccination reduced the number of birds from which MD virus could be isolated and also reduced the level of virus in the positive birds. Lymphoid leukosis mortality exceeded 10% on 2 of the farms and the occurrence of this disease was not affected by the vaccine. The incidence of other neoplasms is recorded. Vaccinated birds produced 4.0 and 14.7% more eggs than unvaccinated birds on a hen-day and hen-housed basis, respectively. In addition, vaccinated birds reached 50% egg production 2 to 8 days earlier than unvaccinated controls.