Characterization of Mutant Strains of Rift Valley Fever Virus

Abstract

The Zagazig Hospital-501 (ZH501) strain of Rift Valley fever (RVF) virus and three naturally occurring variant strains (Ti, T46, and T59) derived from ZH501 were characterized by plaque size; pathogenicity for hamsters; ability to replicate in cell culture; and ability to replicate in and be transmitted by mosquitoes. In addition to the three natural variants, a mutagenized, attenuated strain of RVF virus (ZH548-M12) was also characterized. The variants of ZH501 all produced small plaques in Vero cells. In contrast, the ZH548-M12 strain produced uniformly large plaques and the parental ZH501 strain produced plaques of various sizes, although 70% were large plaques. Infections with either the ZH501 or T46 strains were uniformly fatal to hamsters. In contrast, nearly all hamsters survived infection with the Ti or ZH548-M12 strains as well as challenge with 105 LID50 of the ZH501 strain. Thus, infection with either of these strains protection against lethal infectious doses. All viral strains tested replicated in and were transmitted by Culex pipiens following intrathoracic inoculation. In general, all strains replicated more efficiently in vertebrate cells (Vero and BHK) than in invertebrate cells (C6/36). Both of the avirulent strains (e.g., ZH548-M12 and Ti) failed to replicate in cells maintained at 41°C while the T59 and ZH501 strains replicated at this temperature.