Rift Valley fever: a mosquito-borne exotic disease of sheep, cattle, and humans

Historic, Arcliive Document

Do not assume content reflects current
scientific knowledge, policies, or practices.

' U.S. Department of Agriculture

Animal and Plant Health Inspection Service
Program Aid No. 1430

Rift Valley Fever

A Mosquito-Borne Exotic Disease
of Sheep, Cattle, and Humans

Rift

Valley

Fever

A Mosquito-Borne Exotic Disease
of Sheep, Cattle, and Humans

Rift Valley fever is a viral disease spread by mosquitoes. It
has not been reported outside Africa, but officials of the
U.S. Department of Agriculture (USDA) see the disease as
an ever-present threat to the United States.

USDA's Animal and Plant Health Inspection Service is
alerting people to the dangers of this disease, in particular
those people connected with the livestock industry and
their veterinarians. They should know its clinical signs,
especially the symptoms that differentiate this exotic
disease from other, better known scourges of livestock.

Rift Valley Fever Virus Cycle of Transmission

Man

Mosquito*

*Culex spp., Aedes spp., Mansonia spp.
Ereatmapodites spp.

Rift Valley fever is transmitted between animals (and
humans) by mosquitoes. Infection in humans also can occur
through contact with blood or tissues.

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Rift

Valley

Fever

A Mosquito-Borne Exotic Disease
of Sheep, Cattle, and Humans

Rift Valley fever is a viral disease spread by mosquitoes. It
has not been reporled outside Africa, but officials of the
U.S. Department of Agriculture (USDA) see the disease as
an ever-present threat to the United States.

USDA's Animal and Plant Health Inspection Service is
alerting people to the dangers of this disease, in particular
those people connected with the livestock industry and
their veterinarians. They should know its clinical signs,
especially the symptoms that differentiate this exotic
disease from other, better known scourges of livestock.

History

Rift Valley fever got its name from the place of its discovery
in the Rift Valley of Kenya in 1931. From the first, those who
experienced the disease have reported serious losses in
lambs as well as infections in cattle, game animals, and
humans.

Outbreaks of Rift Valley fever regularly recur in years of
high rainfall when there is a large buildup in the number of
mosquitoes that spread the virus. The disease is
widespread in Africa (see map), but has not spread outside
that continent.

An outbreak in Egypt in 1977 shows how disruptive the
disease can be when it strikes a new area. The infection
spread widely among domestic animals and humans,
resulting in more than 700 human deaths.

Signs

Rift Valley fever causes high abortion rates in pregnant
animals and high death losses in the young. The younger
the animal, the higher the death rate. Mortality in newborn
lambs, kids, and calves may exceed 95 percent.

Rift Valley Fever Virus Cycle of Transmission

Man

Mosquito

*Culex spp., Aedes spp., Mansonia spp.
Ereatmapodites spp.

Rift Valley fever is transmitted between animals (and
humans) by mosquitoes. Infection in humans also can occur
through contact with blood or tissues.

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Documented Outbreaks of R'lft Valley Fever

Rift Valley fever was first diagnosed along
the Rift Valley in Kenya. Shaded areas
indicate countries where outbreaks have
occurred. None have been reported outside
Africa.

Jersey cow with a swarm of biting insects. Large swarms
are needed to cause an outbreak.

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Rift Valley fever has its heaviest impact on sheep. Pregnant
ewes abort in 95 to 100 percent of the cases. Signs
develop suddenly, especially in young animals. In lambs
under 1 week of age, the incubation period can be as short
as 12 hours, followed by collapse and death within 16
hours. The incubation period in older lambs and adult
sheep varies from 24 to 96 hours.

The usual progression of symptoms include a rapid rise in
temperature, rapid pulse, muscle spasms, and unsteady
gait. Pregnant ewes frequently abort. Infected animals may
vomit and have a discharge from the nose.

Death usually takes place 24 to 48 hours after the onset of
symptoms. In adult sheep, the death rate is usually about
20 percent, but it can be as high as 45 percent with some
virulent types of virus. In some cases, no symptoms may be
observed and animals may be found dead without previous
signs of illness.

Rift Valley fever is less common in cattle than in sheep.
However, in South Africa, cattle became infected on almost
every farm where there were infected sheep.

The death rate in cattle varies from 10 to 30 percent.
Infected cows often abort. Symptoms in adult cattle can be
indefinite but usually include a period of high temperature,
lack of appetite, excessive salivation, diarrhea, abdominal
pain, rough hair coat, and cessation of milk production.
Other symptoms can include erosions in the mouth, sore
feet, and dead skin on the udder or scrotum.

Young goats with Rift Valley fever have signs similar to
lambs. Adult goats are variably affected, but abortion rates
can be high.

Humans who contract Rift Valley fever experience an
abrupt rise in body temperature to 102-104 degrees
Fahrenheit (39-40 degrees Centigrade) after a 4- to 6-day
incubation period. Other symptoms resemble influenza and
include chills, headache, muscular pain, weakness, vertigo,
sensitivity to light, and a feeling of abdominal fullness,
tenderness, and pain. Nausea and vomiting sometimes
occur. The eyes also may be affected, causing eye pain
and obscured vision. In serious cases, patients develop
typical signs of hemorrhagic fever, such as bleeding from
the nose, vomiting of blood, and passing of blood in the
stools and urine. Death may result.

Patients who survive usually experience a rapid and
complete recovery, although inflammation of some veins
and eye damage occasionally persist for some time.
Recovered patients have at least a 20-year immunity.

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Focal hemorrhage and severe congestion in the nasal
mucosa of a Karakul ewe.

Colon of a Karakul ewe, showing pinpoint and focal
hemorrhages in the serosa.

Incised gall bladder, showing severe hemorrhage
between the serosal covering and mucosa.

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Liver from a mature cow with greatly enlarged gall
bladder, probably due to congestion with blood.

Abomasum of a Karakul ewe. Note edema and severe
pinpoint and focal hemorrhages.

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Post-Mortem Lesions

In sheep, severe hemorrhages and edema can occur
throughout the body in all organs and membranes.

One of the most striking post-mortem changes occurs in
the liver and distinguishes Rift Valley fever from other
diseases. The surface of the liver is mottled with areas of
congestion, hemorrhage, and yellow areas of various sizes,
caused by dead tissue. Dead tissue also may look red if it
fills with blood; this may be quite pronounced in lambs,
where most areas of some liver lobes may be involved.
Other liver changes also are most severe in lambs.

The gall bladder often is swollen, congested, and
hemorrhagic. Hemorrhages are common in the
gastrointestinal tract, particularly the lining of the
abomasum. Hemorrhages also can be found in the
pancreas, kidneys, lungs, and lymph nodes.

The lungs often are swollen and have areas of
hemorrhagic consolidation. The spleen can also be
enlarged with focal areas of dead tissue. The lymph nodes
can be enlarged and moist. In addition, there can be
hemorrhages of the skin of the udder and flanks. Aborted
fetal lambs often are hemorrhagic and show a marked
hemothorax.

Post-mortem changes in cattle generally resemble those in
sheep. Fresh hemorrhage is often found in the abomasum
of cattle, accompanied by large visceral and serosal
hemorrhages. Hemorrhages in the heart are more severe in
cattle than in sheep.

Dead skin is common, especially on the udder and
scrotum. The feet can become inflamed and the hooves
may drop off.

Some investigators also have reported finding cattle with
mouth lesions, typically including erosion of the lips,
tongue, and mucous membranes. These investigators were
undecided, however, whether these lesions were due
solely to Rift Valley fever or possibly were evidence of
concurrent infection with other viral diseases, like
bluetongue or virus diarrhea.

Confusion with Other Diseases

Even an experienced eye finds it hard to differentiate Rift
Valley fever from similar diseases. It resembles
enterotoxemia, bluetongue, ephemeral fever, brucellosis,
vibriosis, and trichomoniasis. In fact, Nairobi sheep disease

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Multifocal necrosis in a lamb liver, a condition common
in neonatal lambs, calves, and aborted fetuses.

Liver section from a Karakul ewe with a more diffuse
hemorrhagic appearance, caused by a confluence of
focal lesions.

and ovine enzootic abortion should also be considered,
since abortion often is the only sign of Rift Valley fever.

The only accurate method to confirm Rift Valley fever is to
arrange a laboratory diagnosis. Submit tissue samples,
especially of the liver, as well as blood samples, for
analysis.

How It Spreads

Rift Valley fever is caused by an arbovirus of the
Bunyaviridae family. It has many subtypes.

Mosquitoes appear to be the only natural means of
transmitting the virus from animal to animal. Entomologists

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have isolated the virus from at least 16 species of
mosquitoes.

Direct transmission between animals is rare; many
researchers say it cannot occur. Even lambs nursing ewes
with clinical symptoms remain uninfected. Occasional
excretion of the virus in feces, urine, and milk is possible;
however, there is no evidence that this plays a major role in
transmission. The virus also can be found in the blood and
other tissues from infected animals.

Direct human contact with the virus can result in infection.
People can become infected directly through the skin or
mucous membranes, by being punctured with infected
needles, or by inhaling the virus. Workers who might be
exposed to the virus should be immunized with human
vaccine.

Susceptible Species

Rift Valley fever primarily affects sheep, cattle, and goats.
The youngest animals are the most susceptible.

Humans, dogs, and cats also are susceptible, while horses
and swine are resistant.

Prevention and Control

The policy of the U.S. Department of Agriculture toward
exotic communicable diseases of livestock has two basic
tenets: (1) Keep the disease out if at all possible; (2) if the
disease breaches our defenses, eradicate it promptly.
Keeping disease out of this country is more effective and
much cheaper than fighting an outbreak here or "living
with" the disease.

To prevent the entry of Rift Valley fever, USDA prohibits the
entry of animal hosts from Africa or requires that they be
isolated, prior to shipment, from insects that could carry the
disease. Such restrictions apply to sheep, goats, cattle, and
susceptible wild animals.

What Can the U.S. Livestock Owner Do?

All people connected with the livestock industry should
know the clinical signs of Rift Valley fever. Early recognition
of the symptoms and prompt notification of animal health
officials are essential if eradication is to be achieved
quickly.

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Livestock owners should observe their flocks or herds
regularly for suspicious symptoms. Report symptoms
immediately to a local veterinarian and State and Federal
disease control officials.

Suspect Rift Valley fever if:

• A fatal disease occurs in lambs, kids, or calves following
an increase in the number of mosquitoes in an area.

• The disease is milder in adult animals than in young
animals.

• The disease causes abortion in pregnant sheep, goats,
and cattle.

• The disease results in extensive necrotic changes of the
liver.

• An influenza-like disease occurs in people working with
infected animals or handling infected materials.

All programs and services are available to anyone without
regard to race, color, sex, age, handicap, religion, or
national origin.

Most photographs provided courtesy of the Armed Forces
Institute of Pathology, Washington, D.C.

May 1989

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