Fact Sheet No 207
Revised September 2000
RIFT VALLEY FEVER
Rift Valley Fever (RVF), is a
zoonosis (a disease which primarily affects animals, but occasionally causes disease in
humans). It may cause severe disease in both animals and humans leading to high morbidity
and mortality. The death of RVF-infected livestock often leads to substantial economic
Since 1930, when the virus was first isolated during an investigation
into an epidemic amongst sheep on a farm in the Rift Valley of Kenya, there have been
outbreaks in sub-Saharan and North Africa. In 1997-98, there was a major outbreak in Kenya
and Somalia. In September 2000, RVF was for the first time reported outside of the African
Continent. Cases were confirmed in Saudi Arabia and Yemen. This virgin-soil epidemic in
the Arabian Peninsula raises the threat of expansion into other parts of Asia and Europe.
Many different species of mosquitoes are vectors for the RVF virus.
There is, therefore, a potential for epizootics (epidemics amongst animals) and associated
human epidemics following the introduction of the virus into a new area where these
vectors are present. This has been demonstrated in the past and remains a concern.
The virus, which causes RVF, is a
member of the Phlebovirus genus, one of the five genera in the family Bunyaviridae.
RVF virus is primarily spread amongst animals by the bite of infected mosquitoes.
- A wide variety of mosquito species may act as the vector for transmission of the RVF
virus; in different regions a different species of mosquito may prove to be the
predominant vector. In addition, the various vector species play differing roles in
sustaining transmission of the virus.
- Aedes mosquitoes, for example, may acquire the virus from feeding on infected
animals, and are capable of transovarial transmission (transmission of the virus from
infected female mosquitoes to offspring via eggs), so new generations of infected
mosquitoes may hatch from their eggs.
This provides a durable mechanism for maintaining the virus in nature,
as the eggs of these mosquitoes may survive for periods of up to several years in dry
conditions. During periods of inundation of larval habitats by rainfall, for example, in
the rainy season, the eggs will hatch, and the mosquito population will increase and
spread the virus to the animals on which they feed.
Previously uninfected Aedes and other species of mosquitoes will
feed on infected, viraemic (virus circulating in the bloodstream) animals and thus amplify
and perpetuate the outbreak by transmitting the virus to the animals on which they
RVF Virus Non-human Hosts
Many types of animals may be infected with RVF, and disease may be severe in many
domesticated animals including cattle, sheep, camels and goats. Sheep appear to be more
susceptible than cattle and goats are less susceptible.
- Exotic breeds, which have been recently introduced into an endemic area, fare worse than
breeds long adapted to local conditions.
- Animals of different ages also differ in their susceptibility to severe illness: over
90% of lambs infected with RVF die, whereas mortality amongst adult sheep can be as low as
- The abortion rate amongst pregnant, infected ewes is almost 100%. An epizootic (epidemic
animal disease) of RVF is usually first manifested as a wave of unexplained abortions
amongst livestock. This may signal the start of an epidemic.
Transmission to Humans
During epizootics, people may become infected with RVF either by being bitten by
infected mosquitoes, or through contact with the blood, other body fluids or organs of
- Such contact may occur during the care or slaughtering of infected animals, or possibly
from the ingestion of raw milk.
- The virus may infect humans through inoculation (e.g., if the skin is broken, or through
a wound from an infected knife), or through inhalation as an aerosol. The aerosol mode of
transmission has also led to infection in laboratory workers.
The incubation period (interval from infection to onset of symptoms) of RVF varies from
two to six days.
- There then follows an influenza-like illness, with sudden onset of fever, headache,
myalgia (muscle pain) and backache. Some patients also develop neck stiffness, photophobia
(the patient finds exposure to light uncomfortable) and vomiting; in these patients the
disease, in the early stages, may be mistaken for meningitis.
- The symptoms of RVF usually last from four to seven days, after which time the immune
response to infection becomes detectable with the appearance of IgM and IgG antibodies,
and the disappearance of circulating virus from the bloodstream.
Clinical Features of Severe Cases
While most human cases are relatively mild, a small proportion of patients develops a
much more severe disease. This generally appears as one of several recognizable syndromes:
eye disease, meningoencephalitis (inflammation of the brain and surrounding tissue) or
haemorrhagic fever. The proportion of patients developing these three types of
complications is about 0.5-2% for eye disease, and less than 1% for meningoencephalitis
and haemorrhagic fever syndrome.
The fever and other symptoms described in the preceding section, Clinical Features, may
appear in association with eye disease, which characteristically manifests itself in
retinal lesions. The onset of eye disease is usually one to three weeks after the first
symptoms appear. When the lesions are in the macula, some degree of permanent visual loss
will result. Death in patients with only ocular disease is uncommon.
Another syndrome manifests itself with acute neurological disease, meningo-encephalitis.
The onset of this syndrome is also usually one to three weeks after the first symptoms
appear. Death in patients with only meningoencephalitis is uncommon.
- RVF may also manifest itself as haemorrhagic fever. Two to four days after the onset of
illness, the patient shows evidence of severe liver disease, with jaundice and
haemorrhagic phenomena, such as vomiting blood, passing blood in the faeces, developing a
purpuric rash (a rash caused by bleeding in the skin), and bleeding from the gums.
Patients with the RVF-haemorrhagic fever syndrome may remain viraemic for up to 10 days.
The case-fatality rate for patients developing haemorrhagic disease is high at
- Most fatalities occur in patients who have developed haemorrhagic fever. The total case
fatality rate has varied widely in the various documented epidemics, but, overall, is less
Diagnosis and Treatment
Several approaches may be used in diagnosing acute RVF. Serological tests such as
enzyme-linked immunoassay (the "ELISA" or "EIA" methods) may
demonstrate the presence of specific IgM antibodies to the virus. The virus itself may be
detected in blood during the viremia phase of illness or post-mortem tissues by a variety
of techniques including virus propagation (in cell cultures or inoculated animals),
antigen detection tests, and PCR, a molecular method for detecting the viral genome.
- The antiviral drug ribavirin has been shown to inhibit viral growth in experimental
systems, but has not been evaluated in the clinical setting. Most human cases of RVF are
relatively mild and of short duration, so will not require any specific treatment. For the
more severe cases, the mainstay of treatment is general supportive therapy.
Prevention and Control
RVF can be prevented by a sustained program of animal vaccination. Both live,
attenuated, and killed vaccines have been developed for veterinary use. The live vaccine
requires only one dose and produces long-lived immunity, but the presently-available
vaccine may cause abortion if given to pregnant animals. The killed vaccines do not cause
these unwanted effects, but multiple doses must be given to produce protective immunity.
This may prove problematic in endemic areas.
protect veterinary and laboratory
personnel at high risk of exposure to RVF. Other candidate vaccines are under
The risk of transmission from infected blood or tissues exists for people working with
infected animals or people during an outbreak. Gloves and other appropriate protective
clothing should be worn, and care taken when handling sick animals or their tissues.
Healthcare workers looking after patients with suspected or confirmed RVF should employ
universal precautions when taking and processing specimens from patients. Hospitalized
patients should be nursed using barrier techniques. As noted above, laboratory workers are
at risk, so samples taken for diagnosis from suspected human and animal cases of RVF
should be handled by trained staff and processed in suitably equipped laboratories.
Other approaches to the control of disease involve protection from and control of the
mosquito vectors. Personal protection is important and effective. Where appropriate,
individuals should wear protective clothing, such as long shirts and trousers, use bednets
and insect repellent, and avoid outdoor activity at peak biting times of the vector
species. Measures to control mosquitoes during outbreaks, e.g., use of insecticides, are
effective if conditions allow access to mosquito breeding sites.
- An inactivated vaccine has been developed for human use. This vaccine is not licensed
and is not commercially available, but has been used experimentally to
New systems that monitor variations in climatic conditions are being
applied to give advance warning of impending outbreaks by signalling events which may lead
to increases in mosquito numbers. Such warnings will allow authorities to implement
measures to avert an impending epidemic.
For further information, journalists can contact :
WHO Press Spokesperson and Coordinator, Spokesperson's Office,
WHO HQ, Geneva, Switzerland / Tel +41 22 791
4458/2599 / Fax +41 22 791 4858 / e-Mail: firstname.lastname@example.org