ULTRASTRUCTURAL STUDIES ON THE REPLICATION AND MORPHOGENESIS OF NAIROBI SHEEP DISEASE VIRUS, A NAIROVIRUS

The Nairovirus Nairobi sheep disease virus (NSDV) affects sheep and go ats causing severe hemorrhagic gastroenteritis and high mortality. Rep lication and morphogenesis of NSDV was determined by electron microsco pic examination of ultra-thin sections of 143B and BHK-21 cells at var ying times after infection. By 4 h post-infection (p.i.) of 143B cells , virions budding from the luminal side of the bilayer membrane of smo oth membrane vesicles were observed. Morphologically mature virus part icles were electron-dense, spherical and of uniform size (100 nm diame ter) and accumulated in smooth membrane vesicles associated with the G olgi complex. In BHK-21 clone 13 cells, mature virus particles in smoo th membrane vesicles were present by 8 h p.i. The morphogenesis of NSD V was restricted to the smooth membrane vesicles of Golgi complex, and budding of virus from other sites was not detected. Extracellular vir us particles were observed by 10 h p.i., before expression of cytopath ic effects. The cytopathic effects were observed at 24 h p.i. in 143B cells and at 36 h p.i. in BHK-21 cells. The morphology and morphogenes is of NSDV in BHK-21 cells and in 143B cells resembles that of other m embers of the family Bunyaviridae.