Rift Valley fever (RVF) is a viral zoonosis caused by the Rift Valley fever virus (RVFV), a member of the genus Phlebovirus. It primarily affects domestic animals such as sheep, goats, cattle and camels, although it can also infect humans. The disease is of major public health and veterinary importance, as it can cause severe outbreaks in livestock, with high rates of abortion and mortality, as well as significant economic losses. The virus was first described in 1931 in the Rift Valley in Kenya, and since then outbreaks have been reported in several countries in sub-Saharan Africa, Egypt, Saudi Arabia and Yemen.
Transmission and reservoir. The Rift Valley fever virus is transmitted mainly through the bite of infected mosquitoes, particularly those belonging to the genera Aedes and Culex, although other species may also be involved in transmission depending on the region. In animals, the virus circulates mainly among ruminants through mosquito vectors and can persist in nature through vertical transmission in Aedes mosquitoes, passing from infected females to their offspring through their eggs. In humans, infection frequently occurs through direct or indirect contact with the blood, organs, tissues or bodily fluids of infected animals, for example during slaughtering, butchering, assistance during animal births or veterinary procedures. Infection can also occur through the bite of an infected mosquito. To date, person-to-person transmission has not been documented.
Clinical features. The incubation period of Rift Valley fever usually ranges from 2 to 6 days. Most human infections are asymptomatic or present as a mild, self-limiting illness with flu-like symptoms, including sudden-onset fever, headache, myalgia, arthralgia, weakness and general malaise. Some patients may also experience neck stiffness, photophobia, loss of appetite and vomiting, meaning that the disease may initially be confused with other acute infections or even meningitis. In most cases, symptoms last between 4 and 7 days, coinciding with the disappearance of viraemia and the development of antibodies.
A small percentage of patients develop severe forms of Rift Valley fever in humans. There are three main clinical complications. The ocular form occurs in approximately 0.5–2% of cases and may cause retinal lesions, blurred vision or vision loss, which can sometimes be permanent when the macula is affected. The meningoencephalitic form, which is less common, may present with severe headache, confusion, disorientation, seizures, lethargy or coma, and may result in significant neurological sequelae. The haemorrhagic form is the most severe: it usually begins with significant liver involvement and may progress to haematemesis, gastrointestinal bleeding, purpura, epistaxis, gingival bleeding or bleeding from puncture sites. In these cases, the case fatality rate may approach 50%.
Diagnosis. The clinical diagnosis of Rift Valley fever can be difficult, particularly in the early stages, because its symptoms resemble those of other infectious diseases such as malaria, typhoid fever, yellow fever, shigellosis and other viral haemorrhagic fevers. Laboratory confirmation is therefore essential. Recommended techniques include RT-PCR (reverse transcription polymerase chain reaction) for the detection of RVFV RNA, as well as serological methods such as IgM and IgG ELISA. Virus isolation in cell culture may also be performed, although this technique requires the highest levels of biosafety. Non-inactivated clinical samples are considered a high biological risk and must be handled and transported under strict containment measures.
Treatment and prevention. There is no specific antiviral treatment approved for most cases of Rift Valley fever virus infection. In patients with mild disease, general supportive measures and symptomatic treatment are usually sufficient. In severe cases, management is based on early intensive supportive care, including rehydration, management of complications and treatment of specific symptoms. In terms of prevention, vector control and reducing exposure to infected animals remain fundamental measures. An inactivated vaccine for human use has been used experimentally in personnel at high risk of exposure, but it is not licensed for general commercial use. New vaccine candidates are currently under investigation.