Viral determinants of virulence for Rift Valley fever (RVF) in rats.

Rift Valley fever viral strains or variants (RVFV) were compared with respect to (a) virulence for Wistar-Furth rats; (b) in vitro sensitivity to rat and human interferon; (c) ability to form plaques in primary hepatocyte cultures from genetically resistant or susceptible rat strains, and (d) replicative potential in continuous rat cell lines. Egyptian strains were highly virulent for Wistar-Furth rats; relatively resistant to rat interferon-alpha/beta; capable of producing plaques in primary hepatocyte monolayers; and, in general, replicated more rapidly than the low-virulent, sub-Saharan strains. Virtually all strains from sub-Saharan Africa were sensitive to rat interferon and did not form plaques in rat hepatocyte monolayers. An exception was the 2269/74 strain from Zimbabwe, which had characteristics of the Egyptian strains including increased virulence for Wistar-Furth rats. The relative virulence of RVFV strains for rats did not correlate with interferon sensitivity when human recombinant interferon-alpha was tested on A-549 cells. Thus, several in vitro phenotypic characteristics of RVFV strains tend to correlate with virulence for Wistar-Furth rats and with geographical origin of the viral strains.