Rift Valley Fever Virus

Rift Valley fever virus (RVFV) is a mosquito-borne phlebovirus that infects domestic ruminants and occasionally humans, leading to an acute febrile illness known as Rift Valley fever.

Biology and Epidemiology

RVFV is a member of the genus Phlebovirus within the family Phenuiviridae. Its genome consists of three segments of negative-sense RNA that encode structural and non-structural proteins. The virus was first recognized in the Rift Valley region of Kenya in 1931 during an outbreak that caused high mortality in lambs. RVFV is maintained in nature through transovarial transmission in certain Aedes species; infected female mosquitoes pass the virus to their eggs, which can remain dormant in dry environments for long periods. After periods of heavy rain, large populations of infected mosquitoes can emerge and transmit the virus to livestock. Humans typically become infected by handling blood or tissues of viraemic animals or through bites from infected mosquitoes. In ruminants the infection causes fever, abortion and high mortality in young animals, particularly sheep and goats. In humans the majority of infections are asymptomatic or cause influenza-like illness with fever, weakness and back pain. A small proportion of patients develop complications such as retinal lesions, encephalitis or haemorrhagic manifestations. RVFV is endemic in sub-Saharan Africa and has caused significant outbreaks in Egypt, Saudi Arabia and Yemen. Climate variability influences the dynamics of mosquito populations and hence the risk of outbreaks. Control measures focus on vaccination of livestock, vector control and avoiding contact with potentially infected animal tissues. There is no licensed vaccine for human use, and treatment is supportive.

Notable Outbreaks and Facts

Major outbreaks have occurred outside Africa, including the 1977 epidemic in Egypt that led to thousands of human cases and the 2000 outbreak in Saudi Arabia and Yemen, the first recorded outside Africa. In East Africa during 1997-1998 heavy rainfall linked to El Nino triggered an epidemic with large numbers of livestock deaths and human disease. RVFV does not transmit efficiently from person to person, limiting its spread, but it poses a significant risk to veterinary and abattoir workers through exposure to animal blood or aerosols. Research has shown that the virus can be experimentally aerosolized, which is why it is handled in high-containment laboratories. A range of vaccine candidates for animals exist, including live attenuated and inactivated formulations, with vaccination of livestock being the most effective preventive measure in endemic regions.

Rift Valley fever virus is a segmented negative-sense RNA virus maintained in mosquito populations and livestock. It causes a zoonotic disease that leads to economic losses in agriculture and sporadic severe disease in humans. Effective surveillance, vector control and vaccination of animals are vital to mitigate its impact.

Related Terms: Bunyavirales, Phlebovirus, Arboviruses, Viral Hemorrhagic Fever, Mosquito-borne Diseases