New serotype 2 and attenuated serotype 1 Marek's disease vaccine viruses: selected biological and molecular characteristics
Witter, R.L.; Silva, R.F.; Lee, L.F.
Avian Diseases 31(4): 829-840
Two new Marek's disease vaccine viruses, Md11/75C/R2 (serotype 1) and 301B/1 (serotype 2), were evaluated in chickens with maternal antibodies (ab+) or without maternal antibodies (ab-). Strain Md11/75C/R2 was mildly pathogenic in ab- chickens, but this pathogenicity was markedly reduced in ab+ chickens. Md11/75C/R2 spread less by contact and replicated better, both in vivo and in vitro, than CV1988/C, another serotype 1 vaccine virus. Strain 301B/1 was similar to SB-1, another serotype 2 vaccine virus: both were non-pathogenic for ab- chickens, spread readily by contact, and replicated well in vivo. In vitro, 301B/1 grew more rapidly and produced larger plaques than SB-1. Notable characteristics of strain CV1988/C included absence of pathogenicity, poor replicative ability, and the absence of one epitope detected by a common serotype-1-specific monoclonal antibody. All four viruses could be distinguished from each other by restriction enzyme analysis of viral DNA. We conclude that Md11/75C/R2, although exceptionally protective, may require further attenuation. On the other hand, 301B/1, which in other studies induced higher levels of protection than SB-1, is nonpathogenic and may be considered for use as a commercial vaccine.