A comparison of titration methods for live avian encephalomyelitis virus vaccines

Factors associated with the indirect fluorescent antibody test used for the titration of avian encephalomyelitis virus (AEV) in chick embryo brain cell cultures were examined for their influence on virus replication. It was found that virus should be inoculated onto semi-confluent cell cultures and adsorbed for two hours at room temperature. The cells should then be examined for fluorescence after five days' incubation. Using these conditions, the cell culture assay was compared with the embryo and chick assays for its ability to estimate the virus content of live commercial AEV vaccines. In most cases titres obtained by the chick assay were slightly, but not significantly, higher than those obtained in the cell culture assay, although the reliability of the chick assay was, at times, questionable. In all cases titres obtained in the embryo assay were low. It is recommended that the cell culture assay be adopted as the method of choice for titrating AEV vaccines because it is rapid, reproducible, specific and greatly reduces the requirement for experimental animals.