[Potency tests with HVT-, MHV- and JMV vaccines against Marek's disease]

Turkey herpesvirus (HVT)(FC 126) vaccine made in W. Germany, Marek's herpesvirus (MHV) (CVI 988) vaccine used in the Netherlands and experimental JMV vaccine were tested in laboratory and field trials for protection against Marek's disease. The tests were carried out with 780 SPF chicks and 3200 commercial white Leghorn chicks (with maternally derived antibodies to HVT and MHV). There were no differences in potency of HVT- and MHV vaccines. Both vaccines showed increased protection with an increased interval between vaccination and challenge. A significant protection (> 80%) resulted with both vaccine viruses after the 8th day following vaccination. JMV vaccine (embryo-adapted strain JMV-A-164) provided a 40 to 60% protection against Marek's disease depending on the time of challenge. However, there was a complete protection against the highly pathogenic JMV tumour cell inoculum. Hyperimmunisation of adult birds with JMV vaccine did not reduce the mortality in their progeny from Marek's disease.