Identification and Phylogenetic Comparison of Salem Virus, a Novel Paramyxovirus of Horses

A virus that could not be identified as a previously known equine virus was isolated from the mononuclear cells of a horse. Electron microscopy revealed enveloped virions with nucleocapsid structures characteristic of viruses in the Paramyxoviridae family. The virus failed to hemadsorb chicken or guinea pig red blood cells and lacked neuraminidase activity. Two viral genes were isolated from a cDNA expression library. Multiple sequence alignments of one gene indicated an average identity of 45% as compared to Morbillivirus N protein sequences. A weaker relationship was found with Tupaia paramyxovirus (TPMV) and Hendra virus (HeV) N proteins. In the second gene, multiple open reading frames (ORFs) were identified, corresponding to the arrangement of the P, V, and C ORFs in the Morbillivirus and Respirovirus viruses. Short stretches in the C-terminal regions of the P and C proteins showed limited homologies to viruses in the Morbillivirus genus but no obvious relationship to viruses in other genera. The V ORF translation product contained a highly conserved, cysteine-rich domain that is common to most viruses in the Paramyxovirinae subfamily. Sequencing of P gene cDNA clones confirmed the use of a cotranscriptional editing mechanism for the regulation of P/V expression. Based on the location of its origin it has been named Salem virus (SalV).