Physical and biological properties of Nairobi sheep disease virus.

Thermal and pH stability of Nairobi sheep disease (NSD) virus were studied. The 180th mouse brain passage lost infectivity at a higher rate than "wild" virus at 4 degrees C. At 37 degrees C and neutral pH, "wild" virus again was more stable than cell culture and mouse brain attenuated strains with half-life periods of 104, 87 and 51 min, respectively. At 0 degrees C the cell culture attenuated virus was most stable at pH 7.4 with an estimated half-life of 164 h. The density of the virus in sucrose gradients came to 1.195 g cm -3. Metabolic growth inhibition studies using a halogenated nucleoside, and staining of RNase and DNase-treated infected cell cultures with acridine orange, indicated that NSD virus has a single stranded RNA genome. The growth of the cell culture adapted virus was assayed in monolayers of BHK21/13 cells at low multiplicity of infection. Cell-associated virus (CAV) was first detected at 6 h post-inoculation (PI). The titre increased rapidly until CPE appeared at 48 h and declined after 72 h PI. Cell-free virus (CFV) was first detected at 10 h PI. The titre of CFV increased up to 72 h, but on average was two log units less than the CAV titre.