RESUMO
The safety and immunogenicity of commercial lyophilised herpesvirus of turkeys (HVT) vaccine against Marek's disease (MD) was evaluated in 19 field trials involving 76,848 vaccinated and 87,590 control chickens. In the first series of 11 trials involving one commercial vaccine at a dose of 536 plaque forming units (PFU) there was a significant reduction in the incidence of MD (P less than 0.001) and in total mortality (P less than 0.001) from 0 to 18 weeks due to vaccination and the overall protection was 85.2%. In the second series of eight trials involving another commercial vaccine at a dose range of 1360 to 1900 PFU there was a significant reduction in the incidence of MD (P less than 0.001) from 0 to 18 weeks of age due to vaccination. The overall protection was 77.8%. In both series there was a significant difference in MD mortality between meat breeders and crossbred pullets (P less than 0.005) in response to vaccination. There was also a significant difference in total mortality from 0-6 weeks of age between the two series suggesting either differences in response to the two vaccines or differences in the vaccination technique. In a number of trials the incidence of MD was low in the controls, however, where MD incidence was above 2% in the controls protection usually exceeded 80%. It was concluded the vaccines were safe and efficacious and the degree of protection was comparable to that obtained by HVT vaccines described in other countries.
