A quantitative sequential study of viraemia and neutralising antibody to HVT and MDV in a commercial flock vaccinated with HVT

Abstract

Levels of viraemia due to the herpesvirus of turkeys (HVT) and neutralising antibodies to HVT and Marek's disease virus (MDV) were followed in a flock of commercial broiler breeders held in commercial premises and vaccinated at 1-day-old with HVT. The flock was sampled at 3, 5 and 8 weeks of age and then at 4-weekly intervals until 32 weeks of age. The mean viraemia titre was highest at 3 and 5 weeks and then fell to a low level by 16 weeks where it remained until 32 weeks of age. At all sampling periods there was great variability in individual viraemia titres and a significant proportion with no detectable viraemia. Neutralising antibody titres to both HVT and MDV were highest at 1 day old. The geometric median titres fell rapidly and only started to rise slowly between 8 and 12 weeks of age. The titre of individuals within a sample varied greatly and from 16 weeks of age between 3% and 43% had no neutralising antibody to MDV. An examination of the factors affecting the accuracy of the assay for HVT viraemia indicated that the bird effect was greatest suggesting the variability in viraemia was not due to errors in the assay. Results similar to the field study were obtained when the same strain of chicken was vaccinated under laboratory conditions indicating that faulty vaccination was not responsible for the variable response to vaccination. There was a positive correlation between HVT viraemia and neutralising antibody to HVT and MDV. Groups of 1-day-old progeny of HVT vaccinated parents and grandparents of three laying strains and one broiler strain derived from a number of farms were examined for levels of neutralising antibody to HVT. Both the geometric mean titres of each group and the titres of individuals within a group varied greatly. The strain of chicken had no effect on the neutralising antibody response to vaccination with HVT and probable field exposure to MDV but the farm of origin affected both the geometric mean titre and the variability in titre of neutralising antibody in individuals. It is suggested that the exceptionally high levels of neutralising antibody to HVT noted in a proportion of the 1-day-old chicks of the field study could be responsible for a poor response of some individuals to vaccination with HVT.