Characteristics of CVI988/Rispens and R2/23, two prototype vaccine strains of serotype 1 Marek's disease virus.

Studies were focused on two attenuated serotype 1 Marek's disease (MD) vaccine viruses, CVI988/Rispens (passage 42) and R2/23 (passage 105). Both serotype 1 vaccine viruses provided much higher levels of protection than the prototype MD vaccine, turkey herpesvirus (HVT); the best protection was generally provided by CVI988/Rispens when compared with other vaccines. The efficacy of neither serotype 1 vaccine was improved by mixture with viruses of other serotypes (synergism). No differences between the two serotype 1 vaccines were revealed by cross-neutralization tests, thus excluding preferential in vivo neutralization by maternal antibodies as an explanation for differences in protective efficacy. Neither vaccine strain induced MD lesions or reduced growth rates in 8- or 18-week trials. Neither virus depressed humoral or cellular immune responses to antigenic challenge at 3 or 15 days after vaccination. Both virus strains exhibited altered characteristics during serial backpassage; R2/23 acquired increased oncogenic potential, and CVI988/Rispens acquired the potential for increased viremia titers, accompanied by an increased frequency of both histologic nerve lesions and gross thymic atrophy. During backpassage trials, contact spread was not observed for R2/23 and, surprisingly, seemed relatively limited for CVI988/Rispens. Studies on these two serotype 1 strains generally support the safety and efficacy of the serotype 1 class of MD vaccines.