Rift Valley fever (RVF) is an acute disease of domestic ruminants in Africa and the Arabian Peninsula. This disease is caused by a mosquito-borne virus of the family Bunyaviridae and genus Phlebovirus. Large outbreaks occur at irregular intervals when heavy rains favor breeding of mosquito vectors of the virus and are characterized by deaths of newborn animals and abortion in pregnant sheep, goats, and cattle. Recent outbreaks outside Africa have led to rediscover the human disease but it remains poorly known. The wide spectrum of acute and delayed manifestations with potential unfavorable outcome complicate the management of suspected cases and prediction of morbidity and mortality during an outbreak. Although RVF often causes severe disease in animals, most people with RVF have either no symptoms or a mild illness with fever, weakness, back pain, and dizziness. However, a small percentage of about 10% of people with RVF develop much more severe symptoms, including hemorrhage, eye disease, and encephalitis.
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Areas with the most recent RVF outbreaks include Somalia (2006-7), Kenya (2006-7), Tanzania (2007), Sudan (2007-8), Mayotte (2007-8), Madagascar (2008), Swaziland (2008) and South Africa (2008, 2009, and 2010). In affected areas, RVFV epizootics cause catastrophic livestock losses. From November 2018 through July 2019, an outbreak of Rift Valley fever in humans occurred in Mayotte, France; 142 cases were confirmed. Exposure to animals or their biological fluid was reported by 73% of patients. As other arboviral infections including dengue, chikungunya and zika, RVF is emerging worldwide, due to the globalization of arthropod vectors, mainly mosquitoes, which efficiently transmit an increasing number of old, unrecognized and new viruses. Arboviruses pose a major threat of introduction to several continents, including Europe and North America, with the possibility of co-circulation.
At present, Rift Valley Fever is one of the WHO priority diseases. Public health and animal health agencies agree that it is now a priority to develop RVFV vaccines (whether for humans, animals, or both) that will yield highly effective, long-term protective immunity. Past experience with killed inactivated RVFV vaccines has shown them to be protective but expensive to produce and inconvenient to administer, often requiring multiple booster immunizations to achieve and maintain protection.