Rift Valley Fever
Rift Valley Fever (RVF) is a zoonotic viral disease caused by RVF virus (genus Phlebovirus, family Bunyaviridae). RVF virus is closely related to the Lunyo virus discovered in Uganda and the Zinga virus originally identified in the Central African Republic. The virus is inactivated by heat at 56 degrees Celsius for 120 minutes, low pH levels (less than 6.2), treatment with ether and chloroform, and solutions with sodium or calcium hypochlorite. The virus can survive for months at low temperatures (less than 4 degrees Celsius).
An epizootic of RVF is generally observed during years in which unusually heavy rainfall and localized flooding occur. The excessive rainfall allows mosquito eggs, usually of the genus Aedes, to hatch. The mosquito eggs are naturally infected with the RVF virus. Once they hatch, the mosquitoes transfer the RVF virus to the livestock on which they feed. Livestock experience spontaneous abortions and the virus has a 90% fatality rate among young livestock. Once the livestock is infected, other species of mosquitoes can become infected from the animals and can spread the disease. Wind can potentially spread the mosquitoes and the epidemic a long distance. It is possible that the virus can be transmitted by other biting insects as well as mosquitoes. The diseases typically occur in 5-20 year cycles in south and east Africa, but 15-30 year cycles in the semi-arid regions of east Africa.
History of Rift Valley Fever
Since the beginning of the 20th Century, Rift Valley Fever had induced deadly epidemics among sheep in northern and sub-Saharan Africa. The infection was first identified in 1931 when a farm in the Rift Valley in Kenya experienced an epidemic of RVF among its sheep population. The disease is named after its endemic location in Africa, the Great Rift Valley, which stretches 6,000 miles along the earth's crust from Lebanon to Mozambique through East Africa. Along the Great Rift Valley, a geographic feature known as dambos, shallow areas near rivers that fill with water during the rainy season, allows the mosquito population to thrive.
Between 1950 and 1951, an outbreak of RVF in Egypt killed over 200,000 sheep. In 1977, a large outbreak spread from Sudan to Egypt along the Nile affecting 25%-50% of all sheep and cattle. This was the first documented outbreak of RVF outside of sub-Saharan Africa. The outbreak caused hundreds of thousands of human infections with 18,000 confirmed clinical cases and roughly 600 deaths.
Following the opening of the Diama Dam in 1987, an outbreak of RVF occurred in Mauritania not associated with heavy rainfall as all previous outbreaks were. Another outbreak occurred following the opening of the Aswan dam in Egypt. The relation between major irrigation infrastructure and dams suggests that such projects increase the risk of RVF outbreaks through the increase of mosquito breeding grounds.
In 1997-98, another outbreak occurred in Kenya and Somalia starting in December 2007 when the World Health Organization (WHO) and the Kenyan Health Ministry received reports of 478 deaths in northeast Kenya and southern Somalia. Reports suggested that close to 89,000 people were affected. During this outbreak, an embargo was placed on East Africa beef exports for a year and a half. In 2000, cases of RVF were discovered in Saudi Arabia and Yemen marking the spread of the disease outside of Africa and the Rift Valley. By November 2000, over 500 cases of serious RVF were hospitalized in Saudi Arabia, and 87 died. In Yemen, between 7 August and 7 November 2000, there were over 1,000 suspected occurrences of the disease among humans. The result was 121 deaths.
Rift Valley Fever as a Biological Weapon
As potential biological agent, Rift Valley Fever can be transmitted through an initial aerosol release and subsequent transmission through the mosquito vector. Like the West Nile Virus, RVF can spread and become endemic outside its initial geographical region. If introduced to the United States, the virus can easily spread across North America by way of the mosquito vector (Aedes, Anopheles, Culex). The virus can lay dormant in mosquito eggs in dry soil for years. Though rarely fatal, if released as a biological weapon, the RVF virus can potentially cause serious damages to humans (such as blindness) and livestock populations resulting in long-term health effects and economic destruction.
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