Rift Valley Virus

Rift Valley fever (RVF) is a viral zoonotic disease caused by Rift Valley fever virus (RVFV), a member of the Phlebovirus genus. It primarily affects domestic animals such as sheep, goats, cattle, and camels, but it can also infect humans. RVF is important from both a public health and veterinary perspective, as outbreaks can cause severe disease in livestock, including high abortion rates and mortality, together with major economic losses. The virus was first identified in 1931 in Kenya’s Rift Valley, and since then outbreaks have been reported in several parts of sub-Saharan Africa, as well as in Egypt, Saudi Arabia, and Yemen. 

Transmission and animal hosts. Rift Valley fever virus is mainly transmitted through the bite of infected mosquitoes, especially species from the Aedes and Culex genera, although other vectors may also play a role depending on the region. In animals, the virus spreads mainly among ruminants through mosquito bites, and can persist in nature through vertical transmission in Aedes mosquitoes, passing from infected females to their offspring through eggs. In humans, infection often occurs through direct or indirect contact with blood, organs, tissues, or body fluids from infected animals, for example during slaughtering, butchering, veterinary procedures, or animal deliveries. Human infection can also occur through infected mosquito bites. To date, person-to-person transmission has not been documented. 

Clinical features. The incubation period of Rift Valley fever in humans is usually 2 to 6 days. Most human infections are either asymptomatic or present as a mild self-limiting febrile illness, typically with sudden fever, headache, muscle pain, joint pain, weakness, and general malaise. Some patients may also develop neck stiffness, photophobia, loss of appetite, and vomiting, so early disease may be confused with other acute infections or even meningitis. In most cases, symptoms last around 4 to 7 days, after which antibodies appear and the virus disappears from the bloodstream. 

A small proportion of patients develop more severe forms of Rift Valley fever virus infection. The main severe presentations are three. The ocular form occurs in approximately 0.5–2% of cases and may cause retinal lesions, blurred vision, or permanent vision loss if the macula is affected. The meningoencephalitic form is less common but may present with severe headache, confusion, disorientation, hallucinations, seizures, lethargy, or coma, and may leave important neurological sequelae. The haemorrhagic form is the most severe and usually begins with signs of serious liver involvement, followed by haematemesis, blood in stools, purpura, epistaxis, gingival bleeding, or bleeding from venepuncture sites. In these cases, the fatality rate can be around 50%. 

Diagnosis. The clinical diagnosis of Rift Valley fever can be difficult, especially early in the course of disease, because symptoms may resemble those of malaria, typhoid fever, yellow fever, shigellosis, and other viral haemorrhagic fevers. Laboratory confirmation is therefore essential. Recommended methods include RT-PCR for the detection of RVFV RNA, as well as serological assays such as IgM and IgG ELISA. Virus isolation in cell culture is also possible, but it requires high-containment laboratory conditions. Non-inactivated clinical samples are considered high-risk biological material and should be handled and transported under strict biosafety measures. 

Treatment and vaccination. There is no specific antiviral treatment routinely approved for most cases of Rift Valley fever virus infection. Patients with mild disease generally require only symptomatic management and supportive care. In severe cases, treatment is based on early intensive supportive care, including rehydration, complication management, and treatment of specific clinical manifestations. Prevention remains focused on vector control and reducing exposure to infected animals and contaminated animal materials. An inactivated human vaccine has been developed and used experimentally in people at high occupational risk, but it is not licensed for general commercial use. Additional vaccine candidates are still under investigation.