Wesselsbron virus infection in West African Dwarf Sheep: Effect of pre-infection serum flavivirus antibodies on severity of disease

West African Dwarf Sheep inoculated sequentially with two flaviviruses: yellow fever and Dakar bat or West Nile and yellow fever viruses, developed neutralizing (N) and haemagglutination inhibiting (HI) antibodies to several flaviviruses; dengue type 1, dengue type 2, yellow fever, West Nile and Wesselsbron viruses. Experimental infection of these animals with the indigenous strain of Wesselsbron virus resulted in the modification of the clinical disease. The disease was reduced in severity and there was complete absence of, or marked reduction in the level and duration of circulating virus and fever. Seronegative animals infected with the same strain of Wesselsbron virus developed the classical disease characterized by a high fever, anorexia, leucopenia and a high level of viraemia. The absence of clinical reports of Wesselsbron disease and lack of other isolations of the virus from Nigeria could possibly be explained by the presence of the flavivirus group antibodies in domestic animal sera.