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.