Efficacy of simultaneous administration of Marek's disease and viral tenosynovitis vaccines to day-old broiler chickens.

Experiments were conducted to determine the efficacy of simultaneous administration at 1 day of age of the herpes virus of turkeys vaccine (HVT) and viral tenosynovitis (VT) vaccine. Day-old broilers from commercial breeders, which were immunized against both MD and VT, were used. The vaccines were injected at full dosage or diluted 1 to 4. Challenge with either Marek's disease virus (MDV; intraperitoneal) or VT virus (footpad) was at 7 days of age. At 7 wk of age, birds were weighed, killed, and examined for gross lesions. Either vaccine given at full dosage alone, or in combination, rendered birds resistant to homologous viral challenge. However, when either vaccine was diluted and administered alone, the efficacy of the VT but not the HVT vaccine was reduced, resulting in increased lesions and mortality and decreased weight gain after VT challenge. When the HVT vaccine was diluted and combined with undiluted VT vaccine, interference occurred, resulting in reduced efficacy of the HVT vaccine (increased tumors and mortality and reduced body weight). In contrast, combining the HVT at full dosage and the diluted VT vaccine given alone. The data indicate that caution should be maintained when mixing HVT and VT vaccines. For maximum efficacy, both vaccines should be mixed undiluted when administered simultaneously.