Equine rhinopneumonitis virus (herpesvirus type 1): attenuation in stable monkey cell line

An isolate of virulent equine herpesvirus (EHV) type 1 was adapted to Vero stable cell line by 13 serial passages at 37 C and 50 serial passages at 26 C. Characteristics of the attenuated EHV-1 were found to be avirulent, but immunogenic in horses if injected intramuscularly. The attenuated virus was regularly isolated from peripheral leukocytes in inoculated horses, but was not recovered from nasal turbinate tissues. A mild leukopenia was noticed. The attenuated virus produced characteristic large syncytia on primary isolation in rabbit kidney (RK13) or Vero cells at 37 C in contrast to cell rounding observed with virulent EHV-1. The syncytial marker was stable through 20 serial passages in Vero cells at 37 C. New application of double immunodiffusion test for distinguishing between EHV-1 and EHV-2 also is described.