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Rift Valley fever virus (RVFV) is a member of the family Bunyaviridae, genus Phlebovirus (Bishop et al. 1980). It was first described in Kenya by Daubney et al. (1931), having resulted in a fatal epizootic of sheep on a farm north of Lake Naivasha in the heart of the Rift Valley. The virus may have emerged as early as 1880-1890 based upon RVF-like animal disease reports from Kenya in the early 1900s and analyses of complete genome data from 60 naturally occurring RVFV isolates (Pepin et al. 2010). Since that time, periodic epizootics of RVF have been recorded in Kenya in particular. Hardest hit have been domestic sheep and cattle populations, in particular those that are exotic breeds. In the past, human cases have been identified in laboratory workers and people who live in close proximity to affected animals. RVFV is a zoonosis, that is, it primarily affects animals, but occasionally causes disease in humans. In humans, it predominantly affects pastoral people who inhabit the Rift Valley plains and the high plateau grazing lands of Kenya that mostly border the Rift Valley system.

Although most of the documented historical outbreaks have occurred in Kenya, its range of endemicity is widespread. Outbreaks have occurred in other dry lands of Africa including Tanzania, Somalia, Sudan, and Egypt in the north, Mauritania and Senegal in the West, and in Southern Africa, especially South Africa, Zimbabwe, Zambia, Namibia, and even offshore on the island of Mayotte and the large island of Madagascar (Fig. 1).

Figure 1: Record of recent georeferenced RVF outbreaks: 1977-2016.

Lesser outbreaks or the presence of the virus have been identified in most of the remaining countries of Africa including those of the central Sahel and equatorial central Africa. In totality, most countries of Africa have a certain level of risk of RVF outbreaks. In 2000-2001, RVFV caused a large outbreak for the first time outside of Africa in the Arabian Peninsula along the Red Sea. In general, studies indicate that RVF outbreaks are shown to follow periods of widespread and persistent rainfall in dryland areas (Davies et al. 1985). Where outbreaks reach epizootic/epidemic levels, RVF results in widespread livestock losses and human morbidity and mortality.