Polyvalent Marek's disease vaccines: safety, efficacy and protective synergism in chickens with maternal antibodies.

Three Marek's disease (MD) vaccines were evaluated for safety and protective efficacy in chickens with maternal antibodies against serotype 1, 2 and 3 MD viruses and in chickens with no maternal antibodies. The vaccines were: (1) Md11/75C, an attenuated serotype 1 MD virus, (2) a trivalent mixture of MD virus strains Md11/75C and SB-1, and turkey herpesvirus (HVT) strain FC 126, and (3) bivalent mixtures of these three viruses. These vaccines were compared with HVT or SB-1 vaccines in some trials. None of the vaccines was pathogenic or immunodepressive in susceptible chickens with or without maternal antibodies. No interference with in vivo HVT replication by additional viral components was noted, although some interference was demonstrated in vitro. Md11/75C replicated to limited titres in vivo and did not spread by contact, however, it acquired mild pathogenicity upon serial back-passage. Although Md11/75C provided good protection against highly virulent MD viruses in chickens without maternal antibodies, it was poorly protective in chickens with homologous maternal antibodies, and appeared more susceptible to in vivo neutralisation than did SB-1 or HVT. The trivalent vaccine, in contrast, was highly efficacious against very virulent MD virus challenge, even in chickens with maternal antibodies of all three serotypes, and was significantly more effective than monovalent vaccines. A bivalent vaccine composed of SB-1 and HVT was superior to the other two bivalent combinations of the three viruses. Protective synergism among all three vaccine viruses was confirmed. The efficacy of HVT was enhanced by as little as 80 PFU of SB-1, and even fractional doses of HVT and SB-1 together were superior to full doses of HVT alone.