Disease Information
Viral Hemorrhagic
Fevers: Fact SheetsRift Valley Fever
What
is Rift Valley Fever?
Rift Valley fever (RVF) is an acute, fever-causing viral disease that affects domestic
animals (such as cattle, buffalo, sheep, goats, and camels) and humans. RVF is most
commonly associated with mosquito-borne epidemics during years of heavy rainfall.
The disease is
caused by the RVF virus, a member of the genus Phlebovirus in the family Bunyaviridae. The
disease was first reported among livestock by veterinary officers in Kenya in the early
1900s.
Where is
the disease found?
RVF is generally found in regions of eastern and southern Africa where sheep and cattle
are raised. However, RVF virus also exists in most countries of sub-Saharan Africa and
Madagascar.
RVF virus primarily
affects livestock and can cause disease in a large number of domestic animals (this
situation is referred to as an "epizootic"). The presence of an RVF epizootic
can lead to an epidemic among humans who are exposed to diseased animals. The most notable
epizootic of RVF, which occurred in Kenya in 1950-1951, resulted in the death of an
estimated 100,000 sheep. In 1977, the virus was detected in Egypt (probably exported there
in infected domestic animals from Sudan) and caused a large outbreak of RVF among animals
and humans. The first epidemic of RVF in West Africa was reported in 1987 and was linked
to construction of the Senegal River Project. The project caused flooding in the lower
Senegal River area and altered interactions between animals and humans resulting in
transmission of the RVF virus to humans.
How is RVF
virus spread among animals?
An epizootic of RVF is generally observed during years in which heavy rainfall and
localized flooding occur. The excessive rainfall allows mosquito eggs, usually of the
genus Aedes, to hatch. The mosquito eggs are naturally infected with the RVF
virus, and the resulting mosquitoes transfer the virus to the livestock on which they
feed. Once the livestock is infected, other species of mosquitoes can become infected from
the animals and can spread the disease. In addition, it is possible that the virus
can be transmitted by other biting insects.
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Go to map showing RVF distribution |
How
do humans get RVF?
Humans can get RVF as a result of bites from mosquitoes and possibly other blood-sucking
insects that serve as vectors. Humans can also get the disease if they are exposed to
either the blood or other body fluids of infected animals. This exposure can result from
the slaughtering or handling of infected animals or by touching contaminated meat during
the preparation of food. Infection through aerosol transmission of RVF virus has resulted
from contact with laboratory specimens containing the virus.
What are
the symptoms of RVF?
RVF virus can cause several different disease syndromes. People with RVF typically have
either no symptoms or a mild illness associated with fever and liver abnormalities.
However, in some patients the illness can progress to hemorrhagic fever (which can lead to
shock or hemorrhage), encephalitis (inflammation of the brain, which can lead to
headaches, coma, or seizures), or ocular disease (diseases affecting the eye). Patients
who become ill usually experience fever, generalized weakness, back pain, dizziness, and
extreme weight loss at the onset of the illness. Typically, patients recover within two
days to one week after onset of illness.
Are there
complications after recovery?
The most common complication associated with RVF is inflammation of the retina (a
structure connecting the nerves of the eye to the brain). As a result, approximately 1% -
10% of affected patients may have some permanent vision loss.
Is the disease
ever fatal?
Approximately 1% of humans that become infected with RVF die of the disease. Case-fatality
proportions are significantly higher for infected animals. The most severe impact is
observed in pregnant livestock infected with RVF, which results in abortion of
virtually 100% of fetuses.
How is RVF
treated?
There is no established course of treatment for patients infected with RVF virus. However,
studies in monkeys and other animals have shown promise for ribavirin, an antiviral drug,
for future use in humans. Additional studies suggest that interferon, immune modulators,
and convalescent-phase plasma may also help in the treatment of patients with RVF.
Who is at risk
for the illness?
Studies have shown that sleeping outdoors at night in geographical regions where outbreaks
occur could be a risk factor for exposure to mosquito and other insect vectors. Animal
herdsmen, abattoir workers, and other individuals who work with animals in RVF-endemic
areas (areas where the virus is present) have an increased risk for infection. Persons in
high-risk professions, such as veterinarians and slaughterhouse workers, have an increased
chance of contracting the virus from an infected animal. International travelers increase
their chances of getting the disease when they visit RVF-endemic locations during periods
when sporadic cases or epidemics are occurring.
How is RVF
prevented?
A person’s chances of becoming infected can be reduced by taking measures to decrease
contact with mosquitoes and other blood-sucking insects through the use of mosquito
repellents and bednets. Avoiding exposure to blood or tissues of animals that may
potentially be infected is an important protective measure for persons working with
animals in RVF-endemic areas.
What needs to
be done to address the threat of RVF?
A number of challenges remain for the control and prevention of RVF. Knowledge regarding
how the virus is transmitted among mosquitoes and the role of vertebrates in propagating
the virus must be answered to predict and control future outbreaks of RVF. Vaccines for
veterinary use are available, but they can cause birth defects and abortions in sheep and
induce only low-level protection in cattle. The human live attenuated vaccine, MP-12, has
demonstrated promising results in laboratory trials in domestic animals, but more research
will be needed before the vaccine can be used in the field. In addition, surveillance
(close monitoring for RVF infection in animal and human populations) is essential to
learning more about how RVF virus infection is transmitted and to formulate effective
measures for reducing the number of infections.
Unsure about some of
the terms used above? Visit our glossary of terms
for help.
For
information on Rift Valley Fever outbreak in Saudi Arabia and Yemen, see SPB
Main Index
Special Pathogens Branch
Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases
Centers for Disease Control and Prevention
Public Health Service, U.S. Department of Health and Human Services
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