Active and passive immunization against Rift Valley fever virus infection in Syrian hamsters.

The protection by active and passive immunization of Golden Syrian hamsters challenged with Rift Valley Fever (RVF) virus intraperitoneally was studied. Six groups of 10 hamsters were inoculated with different amounts of immune serum while five groups were actively immunized with different doses of a formalin-inactivated RVF vaccine. One day after passive and three weeks after active immunization all hamsters were challenged with 5000 plaque-forming units of RVF virus. Plaque reduction neutralization tests (PRNT) were performed with serum samples collected immediately before challenge. A PRNT titer of 10-20 or more was required for full protection against the RVF challenge in both actively and passively immunized hamsters. Actively immunized hamsters and control animals that succumbed to the infection died within the first 4 days after virus challenge, whereas some passively immunized hamsters died as late as 11 days post virus challenge. Animals that died early had developed massive liver necrosis, whereas late death was caused by encephalitis.