Equine Immunoglobulin and Equine Neutralizing F(ab′)(2) Protect Mice from West Nile Virus Infection

West Nile virus (WNV) is prevalent in Africa, Europe, the Middle East, West Asia, and North America, and causes epidemic encephalitis. To date, no effective therapy for WNV infection has been developed; therefore, there is urgent need to find an efficient method to prevent WNV disease. In this study, we prepared and evaluated the protective efficacy of immune serum IgG and pepsin-digested F(ab′)(2) fragments from horses immunized with the WNV virus-like particles (VLP) expressing the WNV M and E proteins. Immune equine F(ab′)(2) fragments and immune horse sera efficiently neutralized WNV infection in tissue culture. The passive transfer of equine immune antibodies significantly accelerated the virus clearance in the spleens and brains of WNV infected mice, and reduced mortality. Thus, equine immunoglobulin or equine neutralizing F(ab′)(2) passive immunotherapy is a potential strategy for the prophylactic or therapeutic treatment of patients infected with WNV.