The ORF37 (UL24) is a neuropathogenicity determinant of equine herpesvirus 1 (EHV-1) in the mouse encephalitis model

Equine herpesvirus 1 (EHV-1) bacterial artificial chromosome clone (Ab4p BAC) was established based on neuropathogenic strain Ab4p. ORF37 encoding UL24 was replaced with a selection cassette, rpsL-neo gene, to produce an ORF37 deletion mutant, Ab4p∆ORF37. Transfection of RK-13 cells with Ab4p∆ORF37 genome DNA produced infectious virus, indicating that ORF37 is not essential for EHV-1 replication in cell culture. Deletion of ORF37 had no effect on the transcript expression of neighboring genes, ORF36 and ORF38, and the growth activity in MDBK cells. Ab4pΔORF37 lost neuropathogenicity in CBA/N1 mice as indicated by the absence of any neurological disorders and death. The growth of Ab4pΔORF37 in cultivated neural cells was one order of magnitude lower than that of parental and revertant viruses. These results indicated that the ORF37 is a neuropathogenicity determinant of EHV-1 in the mouse encephalitis model.