Evaluation of the Safety and Efficacy of a Live Attenuated Thermostable Rift Valley Fever Vaccine in Sheep, Goats and Cattle

Abstract

Rift valley fever (RVF) is a highly significant vector-borne disease causing huge economic loses in livestock (ruminants and camels) and also human fatalities. The disease is endemic in most Sub-Saharan African countries, including West Africa, and has been present in the Middle East since 2010. Vaccination is considered to be the most effective way to prevent and control the expansion of the disease. Currently available attenuated live vaccines for RVF have significant limitations in that they are either thermolabile (CL13 strain vaccine) or causes abortion and teratogenic effects (Smithburn strain vaccine). This study therefore set out to develop a safe and effective thermostable live attenuated RVF vaccine. The existing CL13 vaccine, which is a naturally attenuated strain, was made thermostable through three cycles of heating (56°C) and selection. The resulting candidate vaccine (CL13T) was stable at 4°C for 20 months and shows significantly improved levels of thermostability over the existing CL13 vaccine. A pilot batch of the CL13T vaccine was produced and tested for safety and efficacy in cattle, sheep and goats. The vaccine was found to be safe, with no clinical signs or side effects observed in vaccinated animals, and there was no evidence for circulation of the virus in the blood of animals post-vaccination. On testing for efficacy in cattle, sheep and goats, through the detection of neutralizing antibodies post-vaccination, good levels of neutralizing antibodies were detected for a minimum of one year in sheep and goats, and neutralizing antibodies were detected for least 4 months in cattle. This new thermostable vaccine could represent an efficient tool for the control of rift valley fever in endemic countries. The vaccine also has the potential to be used, along with an appropriate diagnostic test, to differentiate vaccinated from infected animals (DIVA).