Reduced pathogenicity associated with a small plaque variant of the Egyptian strain of Rift Valley fever virus (ZH501)

Variants of Rift Valley fever virus producing plaques in CER cells of four different sizes are described. A plaque-forming unit (PFU) variant forming minute plaques was isolated and purified. Virus derived from this variant was not pathogenic to adult Swiss albino mice by the intraperitoneal (i.p.) route and was less pathogenic than the parent strain (ZH501) to adult Sprague Dawley rats by i.p. route, but produced typical severe liver necrosis in adult Syrian hamsters with intranuclear and intracytoplasmic eosinophilic inclusions. Antigen and antiserum to the minute variant prepared in mice reciprocally cross-reacted with antisera and antigens of the original strain (ZH501) in the complement fixation test. Plaque size of the minute variant remained constant after serial passages in cell culture and in suckling mouse brain. When the minute plaque variant was passaged i.p. in hamsters, virus which formed large plaques in CER cells was recovered from the hamster sera.