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Rift Valley Fever
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| What
is Rift Valley fever? |
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RVF
virus electron
micrograph. Image
courtesy, C. S.
Goldsmith, M. D.
Bowen and T. G. Ksiazek |
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
unusually 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? |
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RVF is generally
found in regions of eastern and southern Africa where sheep and cattle
are raised, but the virus also exists in most countries of sub-Saharan
Africa and in Madagascar. In September 2000, a RVF outbreak was reported
in Saudi Arabia and subsequently Yemen. These cases represent the first
Rift Valley fever cases identified outside Africa.
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? |
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| RVF
is most commonly associated with mosquito-borne epidemics during years
of unusually heavy rainfall |
An epizootic of RVF
is generally observed during years in which unusually 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.
| How
do humans get RVF? |
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Humans can get RVF
as a result of bites from mosquitoes and possibly other bloodsucking 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? |
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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? |
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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? |
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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? |
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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? |
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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? |
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A person's chances
of becoming infected can be reduced by taking measures to decrease contact
with mosquitoes and other bloodsucking 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? |
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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.
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