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	WHO = > Programmes and = projects=20 > Media=20 centre > Fact=20 sheets =20

Main content

pr= intable=20 version

Fact sheet N=C2=B0207
Revised = September=20 2007

Rift Valley fever

Overview

Rift Valley Fever (RVF) is a viral zoonosis that = primarily=20 affects animals but also has the capacity to infect humans.=20 Infection can cause severe disease in both animals and = humans,=20 leading to high rates of disease and death. The disease also = results=20 in significant economic losses due to death and abortion = among=20 RVF-infected livestock.

RVF virus is a member of the Phlebovirus genus, one of = the five=20 genera in the family Bunyaviridae. The virus was first = identified in=20 1931 during an investigation into an epidemic among sheep on = a farm=20 in the Rift Valley of Kenya. Since then, outbreaks have been = reported in sub-Saharan and North Africa. In 1997-98, a = major=20 outbreak occurred in Kenya, Somalia and Tanzania and in = September=20 2000, RVF cases were confirmed in Saudi Arabia and Yemen, = marking=20 the first reported occurrence of the disease outside the = African=20 continent and raising concerns that it could extend to other = parts=20 of Asia and Europe.

TRANSMISSION TO HUMANS

The vast majority of human infections result from = direct or=20 indirect contact with the blood or organs of infected = animals. The=20 virus can be transmitted to humans through the handling of = animal=20 tissue during slaughtering or butchering, assisting with = animal=20 births, conducting veterinary procedures, or from the = disposal of=20 carcasses or fetuses. Certain occupational groups such as = herders,=20 farmers, slaughterhouse workers and veterinarians are = therefore at=20 higher risk of infection. The virus infects humans through = inoculation, for example via a wound from an infected = knife or=20 through contact with broken skin, or through inhalation of = aerosols produced during the slaughter of infected = animals. The=20 aerosol mode of transmission has also led to infection in=20 laboratory workers.=20
There is some evidence that humans may also become = infected=20 with RVF by ingesting the unpasteurized or uncooked milk = of=20 infected animals.=20
Human infections have also resulted from the bites of = infected=20 mosquitoes, most commonly the Aedes mosquito.=20
Transmission of RVF virus by hematophagous = (blood-feeding)=20 flies is also possible.=20
To date, no human-to-human transmission of RVF has = been=20 documented, and no transmission of RVF to health care = workers has=20 been reported when standard infection control precautions = have=20 been put in place.=20
There has been no evidence of outbreaks of RVF in = urban areas.=20

CLINICAL FEATURES IN HUMANS

Mild form of RVF in humans

The incubation period (interval from infection to = onset of=20 symptoms) for RVF varies from two to six days.=20
Those infected either experience no detectable = symptoms or=20 develop a mild form of the disease characterized by a = feverish=20 syndrome with sudden onset of flu-like fever, muscle pain, = joint=20 pain and headache.=20
Some patients develop neck stiffness, sensitivity to = light,=20 loss of appetite and vomiting; in these patients the = disease, in=20 its early stages, may be mistaken for meningitis.=20
The symptoms of RVF usually last from four to seven = days,=20 after which time the immune response becomes detectable = with the=20 appearance of antibodies and the virus gradually = disappears from=20 the blood.

Severe form of RVF in humans

While most human cases are relatively mild, a small = percentage=20 of patients develop a much more severe form of the = disease. This=20 usually appears as one or more of three distinct = syndromes: ocular=20 (eye) disease (0.5-2% of patients), meningoencephalitis = (less than=20 1%) or haemorrhagic fever (less than 1%).=20
- Ocular form: In this form of the disease, the = usual=20 symptoms associated with the mild form of the disease = are=20 accompanied by retinal lesions. The onset of the lesions = in the=20 eyes is usually one to three weeks after appearance of = the first=20 symptoms. Patients usually report blurred or decreased = vision.=20 The disease may resolve itself with no lasting effects = within 10=20 to 12 weeks. However, when the lesions occur in the = macula, 50%=20 of patients will experience a permanent loss of vision. = Death in=20 patients with only the ocular form of the disease is = uncommon.=20
- Meningoencephalitis form: The onset of the=20 meningoencephalitis form of the disease usually occurs = one to=20 four weeks after the first symptoms of RVF appear. = Clinical=20 features include intense headache, loss of memory,=20 hallucinations, confusion, disorientation, vertigo, = convulsions,=20 lethargy and coma. Neurological complications can appear = later=20 (> 60 days). The death rate in patients who = experience only=20 this form of the disease is low, although residual = neurological=20 deficit, which may be severe, is common.=20
- Haemorrhagic fever form: The symptoms of this = form of=20 the disease appear two to four days after the onset of = illness,=20 and begin with evidence of severe liver impairment, such = as=20 jaundice. Subsequently signs of haemorrhage then appear = such as=20 vomiting blood, passing blood in the faeces, a purpuric = rash or=20 ecchymoses (caused by bleeding in the skin), bleeding = from the=20 nose or gums, menorrhagia and bleeding from venepuncture = sites.=20 The case-fatality ratio for patients developing the = haemorrhagic=20 form of the disease is high at approximately 50%. Death = usually=20 occurs three to six days after the onset of symptoms. = The virus=20 may be detectable in the blood for up to 10 days, in = patients=20 with the hemorrhagic icterus form of RVF. =

The total case fatality rate has varied widely between = different=20 epidemics but, overall, has been less than 1% in those = documented.=20 Most fatalities occur in patients who develop the = haemorrhagic=20 icterus form.

DIAGNOSIS

Acute RVF can be diagnosed using several different = methods.=20 Serological tests such as enzyme-linked immunoassay (the = "ELISA"=20 or "EIA" methods) may confirm the presence of specific IgM = antibodies to the virus. The virus itself may be detected = in blood=20 during the early phase of illness or in post-mortem tissue = using a=20 variety of techniques including virus propagation (in cell = cultures or inoculated animals), antigen detection tests = and=20 RT-PCR.

TREATMENT AND VACCINE

As most human cases of RVF are relatively mild and of = short=20 duration, no specific treatment is required for these = patients.=20 For the more severe cases, the predominant treatment is = general=20 supportive therapy.=20
An inactivated vaccine has been developed for human = use.=20 However, this vaccine is not licensed and is not = commercially=20 available. It has been used experimentally to protect = veterinary=20 and laboratory personnel at high risk of exposure to RVF. = Other=20 candidate vaccines are under investigation.

RVF VIRUS IN ANIMAL HOSTS

RVF is able to infect many species of animals causing = severe=20 disease in domesticated animals including cattle, sheep, = camels=20 and goats. Sheep appear to be more susceptible than cattle = or=20 camels.=20
Age has also been shown to be a significant factor in = the=20 animal's susceptibility to the severe form of the disease: = over=20 90% of lambs infected with RVF die, whereas mortality = among adult=20 sheep can be as low as 10%.=20
The rate of abortion among pregnant infected ewes is = almost=20 100%. An outbreak of RVF in animals frequently manifests = itself as=20 a wave of unexplained abortions among livestock and may = signal the=20 start of an epidemic.

RVF VECTORS

Several different species of mosquito are able to act = as=20 vectors for transmission of the RVF virus. The dominant = vector=20 species varies between different regions and different = species can=20 play different roles in sustaining the transmission of the = virus.=20
Among animals, the RVF virus is spread primarily by = the bite=20 of infected mosquitoes, mainly the Aedes species, which = can=20 acquire the virus from feeding on infected animals. The = female=20 mosquito is also capable of transmitting the virus = directly to her=20 offspring via eggs leading to new generations of infected=20 mosquitoes hatching from eggs. This accounts for the = continued=20 presence of the RVF virus in enzootic foci and provides = the virus=20 with a sustainable mechanism of existence as the eggs of = these=20 mosquitoes can survive for several years in dry = conditions. During=20 periods of heavy rainfall, larval habitats frequently = become=20 flooded enabling the eggs to hatch and the mosquito = population to=20 rapidly increase, spreading the virus to the animals on = which they=20 feed.=20
There is also a potential for epizootics and = associated human=20 epidemics to spread to areas that were previously = unaffected. This=20 has occurred when infected animals have introduced the = virus into=20 areas where vectors were present and is a particular = concern. When=20 uninfected Aedes and other species of mosquitoes feed on = infected=20 animals, a small outbreak can quickly be amplified through = the=20 transmission of the virus to other animals on which they=20 subsequently feed.

PREVENTION AND CONTROL

Controlling RVF in animals

Outbreaks of RVF in animals can be prevented by a = sustained=20 programme of animal vaccination. Both modified live = attenuated=20 virus and inactivated virus vaccines have been developed = for=20 veterinary use. Only one dose of the live vaccine is = required to=20 provide long-term immunity but the vaccine that is = currently in=20 use may result in spontaneous abortion if given to = pregnant=20 animals. The inactivated virus vaccine does not have this = side=20 effect, but multiple doses are required in order to = provide=20 protection which may prove problematic in endemic areas.=20
Animal immunization must be implemented prior to an = outbreak=20 if an epizootic is to be prevented. Once an outbreak has = occurred=20 animal vaccination should NOT be implemented because there = is a=20 high risk of intensifying the outbreak. During mass animal = vaccination campaigns, animal health workers may, = inadvertently,=20 transmit the virus through the use of multi-dose vials and = the=20 re-use of needles and syringes. If some of the animals in = the herd=20 are already infected and viraemic (although not yet = displaying=20 obvious signs of illness), the virus will be transmitted = among the=20 herd, and the outbreak will be amplified.=20
Restricting or banning the movement of livestock may = be=20 effective in slowing the expansion of the virus from = infected to=20 uninfected areas.=20
As outbreaks of RVF in animals precede human cases, = the=20 establishment of an active animal health surveillance = system to=20 detect new cases is essential in providing early warning = for=20 veterinary and human public health authorities.

Public health education and risk=20 reduction

During an outbreak of RVF, close contact with animals, = particularly with their body fluids, either directly or = via=20 aerosols, has been identified as the most significant risk = factor=20 for RVF virus infection. In the absence of specific = treatment and=20 an effective human vaccine, raising awareness of the risk = factors=20 of RVF infection as well as the protective measures = individuals=20 can take to prevent mosquito bites, is the only way to = reduce=20 human infection and deaths.=20
Public health messages for risk reduction should focus = on:=20
- reducing the risk of animal-to-human transmission as = a=20 result of unsafe animal husbandry and slaughtering = practices.=20 Gloves and other appropriate protective clothing should = be worn=20 and care taken when handling sick animals or their = tissues or=20 when slaughtering animals.=20
- reducing the risk of animal-to-human transmission = arising=20 from the unsafe consumption of fresh blood, raw milk or = animal=20 tissue. In the epizootic regions, all animal products = (blood,=20 meat and milk) should be thoroughly cooked before = eating.=20
- the importance of personal and community protection = against=20 mosquito bites through the use of impregnated mosquito = nets,=20 personal insect repellent if available, by wearing light = coloured clothing (long-sleeved shirts and trousers) and = by=20 avoiding outdoor activity at peak biting times of the = vector=20 species.

Infection control in health care=20 settings

Although no human-to-human transmission of RVF has = been=20 demonstrated, there is still a theoretical risk of = transmission of=20 the virus from infected patients to healthcare workers = through=20 contact with infected blood or tissues. Healthcare workers = caring=20 for patients with suspected or confirmed RVF should = implement=20 Standard Precautions when handling specimens from = patients.=20
Standard Precautions define the work practices that = are=20 required to ensure a basic level of infection control. = Standard=20 Precautions are recommended in the care and treatment of = all=20 patients regardless of their perceived or confirmed = infectious=20 status. They cover the handling of blood (including dried = blood),=20 all other body fluids, secretions and excretions = (excluding=20 sweat), regardless of whether they contain visible blood, = and=20 contact with non-intact skin and mucous membranes. A WHO=20 Aide=E2=80=93memoire on Standard Precautions in health = care is available=20 at: http://www.who.int/csr/resources/publications/standardpre= cautions/en/index.html=20
As noted above, laboratory workers are also at risk. = Samples=20 taken from suspected human and animal cases of RVF for = diagnosis=20 should be handled by trained staff and processed in = suitably=20 equipped laboratories.

Vector control

Other ways in which to control the spread of RVF = involve=20 control of the vector and protection against their bites.=20
Larviciding measures at mosquito breeding sites are = the most=20 effective form of vector control if breeding sites can be = clearly=20 identified and are limited in size and extent. During = periods of=20 flooding, however, the number and extent of breeding sites = is=20 usually too high for larviciding measures to be feasible. =

RVF FORESCASTING AND CLIMATIC = MODELS

Forecasting can predict climatic conditions that are = frequently=20 associated with an increased risk of outbreaks, and may = improve=20 disease control. In Africa, Saudi Arabia and Yemen RVF = outbreaks are=20 closely associated with periods of above-average rainfall. = The=20 response of vegetation to increased levels of rainfall can = be easily=20 measured and monitored by Remote Sensing Satellite Imagery. = In=20 addition RVF outbreaks in East Africa are closely associated = with=20 the heavy rainfall that occurs during the warm phase of the = El=20 Ni=C3=B1o/Southern Oscillation (ENSO) phenomenon.

These findings have enabled the successful development of = forecasting models and early warning systems for RVF using = satellite=20 images and weather/climate forecasting data. Early warning = systems,=20 such as these, could be used to detect animal cases at an = early=20 stage of an outbreak enabling authorities to implement = measures to=20 avert impending epidemics.

Within the framework of the new International Health = Regulations=20 (2005), the forecasting and early detection of RVF = outbreaks,=20 together with a comprehensive assessment of the risk of = diffusion to=20 new areas, are essential to enable effective and timely = control=20 measures to be implemented.

For more information=20 contact:

WHO Media centre
Telephone: +41=20 22 791 2222
E-mail: mediainquiries@who.int=20

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