We describe a type-specific ELISA, which distinguishes antibody to equine herpesvirus 4 (EHV4; equine rhinopneumonitis) and EHV1 (equine abortion virus) thereby identifying horses that have been infected with either or both of these antigenically related viruses. The antigens used are parts of the EHV4 and EHV1 glycoprotein G (gG) homologues expressed in E. coli as fusion proteins [Crabb and Studdert, 1993: J Virol 67: 6332-6338). The expressed proteins comprise corresponding regions of the gG molecules that are highly divergent and encompass strong, typespecific epitopes. Plasma samples from 97 Thoroughbred and 174 Standardbred horses were tested, all of which were unvaccinated. All horses were strongly EHV4 ELISA positive while 30% were EHV1 ELISA positive.
The type-specificity of the EHV1 gG antigen was tested in cross-absorption experiments and it was found that 96% (66 of 69) of EHV1 ELISA positive horses were true EHV1 antibody positives. It was also shown that 100% (26 of 26) horses known to have been exposed to EHV1, either by infection or immunisation with EHV1, had significant levels of antibody against the EHV1 gG antigen (i.e., all horses recognised the EHV1 epitope(s) contained within this molecule). Maintenance of EHV1 gG antibody was examined by testing sera obtained from mares four years after confirmed EHV1 abortion. Seven out of 10 of these mares remained EHV1 ELISA positive. In summary, the ELISA is highly specific and is sufficiently sensitive to detect all horses previously infected with EHV4 and most previously infected with EHV1.