Genomic heterogeneity of equine betaherpesviruses.

The genomes of 51 isolates of slowly cytopathic equine herpesviruses were examined by digestion with restriction endonucleases. Forty-seven of the isolates showed considerable fragment pattern heterogeneity although common fragments were evident, especially when any two isolates were compared or when they were digested with SalI. Fifteen of the 47 viruses, selected for their diverse fragment patterns, showed a high degree of homology in Southern blot hybridization. In contrast, four viruses, representing three epidemiologically distinct isolations, shared few, if any, comigrating fragments with the 47 equine herpesvirus 2 (EHV-2) isolates, although they shared comigrating fragments with each other. These four viruses showed reduced homology to a representative EHV-2 isolate by Southern blot hybridization under stringent conditions. Although not sharply delineated from EHV-2, these four viruses grew very slowly and had low yields in vitro, and preliminary data suggested they had a significantly smaller genome than EHV-2 (148 +/- 12 kb compared to 190 kb). These four viruses may be prototypic of a novel equine betaherpesvirus.