Abstract: [Objective] To determine the etiologic agent causing the suspected cases of contagious ecthyma （orf） in large-scale sheep farms in Inner Mongolia. [Method] A total of 7 clinical samples were aseptically collected from the diseased sheep with suspected orf symptoms, and they were subsequently inoculated with goat skin fibroblasts （GSFs） for virus isolation after cutting, grinding, and centrifugation. The obtained viral strains were morphologically observed by electron microscope. B2L gene primers were used for specific PCR identification, and the product was then sequenced. The phylogenetic tree based on B2L gene was constructed, and homology analysis was performed as well. [Result] There were 3 clinical samples with capacity to cause evident cytopathic alterations in GSFs after inoculation for 48 hours. Morphological observation with electron microscope demonstrated that the isolated virus was oval in shape, and the viral particles were 220-250 nm long and 125-200 nm wide, which were consistent with the morphological characteristics of orf virus （ORFV）. The 3 obtained ORFV strains were designated as NM-ORFV-1, NM-ORFV-2, and NM-ORFV-3, respectively. For each isolated viral strain, a product of 1 137 bp was amplified by PCR assay targeting B2L gene, which was consistent with the expecting size. Phylogenetic tree and homology analysis revealed that NM-ORFV-2 strain was closely genetically related to NM-ORFV-3 strain, and they clustered in the same branch; NM-ORFV-2 strain shared the highest homology （99.4%） with ORFV KP336704（China）wild strain; NM-ORFV-1 strain clustered in different branch from NM-ORFV-2 strain and NM-ORFV-3 strain, and the homologies of NM-ORFV-1 strain shared with NM-ORFV-2 strain and NM-ORFV-3 strain were 99.1% and 99.0%, respectively; NM-ORFV-1 strain shared the highest homology （99.6%） with ORFV JQ904789（China）vaccine strain. [Conclusion] ORFV is identified as the pathogenic agent that causes the suspected cases of orf in large-scale sheep farms in Inner Mongolia.

Key words: orf virus, isolation, identification