بسم الله الرحمن الرحيم

University of Bahri

College of Veterinary Medicine

Department of Preventive Veterinary Medicine

(Viral Diseases)

Rift Valley Fever

Rift Valley Fever (RVF) is an arthropod-borne,

acute, fever-causing viral disease of sheep, goats,

cattle, buffalo, camels and people.

Infection with RVF virus may cause abortion of

pregnant animals and high mortality rate in young

livestocks. In humans, it usually causes an influenza-

like disease but occasionally leads to more serious

complications with high morbidity and mortality.

*The RVF virus belongs to the family bunyaviridae, genus phlebovirus.

*There is only one serotype of RVF virus.

*Stable at

*-60oC to 23°C

*50-85% relative humidity

*Inactivated

*Lipid solvents

*Detergents

*Low pH

Etiology

Distribution of RVF

Rift Valley Fever was first

reported at Lake Naivasha in

Kenya (1931).

There were many sheep

abortions and young lambs

were found sick or dead.

*Generally found in eastern and southern Africa where

sheep and cattle are raised.

*Most countries of sub-Saharan Africa.

*Madagascar.

*September 2000 RVF outbreak in Saudi Arabia and

Yemen – first outbreak outside of the African

continent.

Last OutbreaksDate Country 1950-1951 Kenya 1967-1970 Nigeria 1969 Central African Republic 1976-1977 Sudan 1977-1980 Egypt 1987 Mauritania 1990-1991 Madagascar 1993 Egypt – Senegal 1997 Kenya – Somalia 1999 South Africa 2000-2001 Saudi Arabia - Yemen

Rift Valley Fever

in the World in 2004, OIE

Disease reported present

Disease reported absentData unavailable or incomplete

Host Range

Susceptible species are:-

Sheep, goat, cattle, camels, dogs and buffalo

and man.

Reservoir

The reservoir for RVF is:

Mosquitoes – Aedes species

* Transovarial transmission.

* Eggs dormant in soil for long

periods.

* Heavy rainfall, eggs hatch.

Ruminants serve as an amplifying

host once it affected.

Infected livestock (sheep, cattle, goats) can have high

levels of viremia which is sufficient to infect various

mosquito vectors. These amplifying hosts help the

disease become established in the environment and can

lead to large epizootic epidemics.

Transmission

RVF is primarily transmitted from

animal to animal by a mosquitoes.

Other arthropods (Stomoxys, midges

and tabanids) are able to transmit RVF

by mechanical means.

Transmission

Transmission to humans

Village butchers are at risk

Transmission to humans

Veterinarians and Livestock handlers are at risk

Ingestion of raw milk

Transmission to humans

PATHOGENESIS

Hepatocytes are the primary site of viral replication in

lambs and calves.

In very young animals, hepatic lesions progress from

degeneration and necrosis of individual hepatocytes to

extensive necrosis throughout the liver resulting in

hepatic insufficiency and failure.

In young animals, encephalomyelitis may also occur.

Clinical signs Sheep and Goats

Incubation period less than 3 days

Early signsFever 40-41°CLoss of appetite JaundiceWeakness

*High rate of abortion at any stage of gestation

In pregnant ewes, abortion may approach 100%Aborted fetus is usually autolyzed.

*Ewe may also retain the placenta.

*Endometritis is another complication after aborting the fetus.

Encrustation around the muzzle.

Some develop diarrhea.

*Acute death may occur in 20-30% of adults.

Heavy sheep losses occur during epidemic.

Clinical Signs in lambs and kids*Newborn Lambs, Kids: Most severe in young

lambs under 2wks old (mortality has high as

90%).

*Fever (40-42°C).

*Anorexia.

*Weakness.

*Disinclination to move or feed.

*Evidence of abdominal pain, and rapid respiration.

*Death may occur within 24 to 36 hours after the

first signs appear.

Clinical signs in cattle

Calves: Fever (40-41°C).

Depression.

Mortality rate: 10-70%

Death occurs about 2-8 days after the first

signs appear.

Symptoms may be prolonged and will include

jaundice in some calves.

Aborted calves are moderately autolyzed.

Adults:

Fever (40-41°C).

Excessive salivation.

Anorexia and weakness.

Fetid diarrhea.

Fall in milk yield.

Abortion may reach 85% in the herd.

Mortality rate is usually less than 10%.

Other SpeciesCamels (in Egypt) inapparent disease except abortions.

Dogs

*Abortion up to 100%

*Severe disease and death in puppies

Cats: Death in kittens

Horses: Viremia but resistant

Pigs: Resistant

Birds: Refractory

Necropsy findings

Focal or generalized hepatic necrosis.

Congestion, enlargement, and

discoloration of liver with sub-capsular

hemorrhages.

Brown-yellowish color of liver in aborted

fetuses.

Hemorrhagic enteritis.

Mucosal haemorrhages in the abomasum.

Spleen with subcapsular

haemorrhages.

Haemorrhagic and

oedematous lymph node.

Haemorrhages and oedema of the wall of the gall

bladder are common, and the lumen may contain a

blood coagulum or blood-tinged bile.

Blood coagulum in gall bladder

Haemorrhagic and oedematous gall bladder wall

Gall bladder with blood-tinged bile

DiagnosisSuspect RVF when heavy rains are followed by:

High mosquito populations.

Abortions in pregnant animals.

High mortalities in young animals.

Extensive hepatic necrosis.

Frequently there is also an influenza-like illness

in farm workers and people handling infected

carcasses.

Specimens

Specimens to be submitted for laboratory confirmation of

the diagnosis include:

Heparinized blood from animals with high fever.

Serum of live affected animal.

Tissue samples including:

Liver, spleen, kidney and lymph nodes of dead

animals.

Liver, spleen and brain from aborted foetuses.

Specimens should be securely packaged and

submitted on ice to a suitable laboratory for

isolation of virus or demonstration of antibody.

Where delay in getting specimens to the

laboratory is unavoidable or where material has

to be transported at ambient temperature, tissue

samples can be preserved in glycerol-saline

solution.

Virus isolation

Virus can be isolated readily in a variety of cell cultures,

or in suckling and weaned mice or hamsters inoculated

intracerebrally or intraperitoneally.

Serology

In animals that survive the disease, paired serum samples,

one taken during the acute illness and the other 2 - 3

weeks later, should be submitted for antibody tests by

tests such as CF, ID, serum neutralization or ELISA.

RFV antigen detection

RT-PCR identification of RVF virus.

Virus isolation in tissue cultures or inoculated animals,

Microscopic pathology

Tissue specimens from the liver, spleen, and lymph

nodes should also be collected in 10% buffered-

formalin for histopathological examination.

Note the zoonotic potential of this

disease when handling these

specimens.

Nairobi sheep disease:

No hepatitis, not in newborn lambs.

Bluetongue:

Foot lesions (coronitis) and also no hepatitis.

Heartwater:

Serous fluids in body cavities and neurological

signs.

Ephemeral fever:

Recumbency and rapid recovery.

Differential Diagnosis

Wesselbron:

Less severe than RVF.

Toxoplasmosis, leptospirosis, brucellosis, Q fever

and salmonellosis:

They are not associated with rainfall, nor do they

produce such high neonatal mortality.

Prevention and Control

1. Vaccination Program

• Immunization is the most effective method of

controlling the disease.

• The current vaccine can be abortigenic and

teratogenic so not recommended for pregnant

animals.

The recorded problems include hydrops amnii,

arthrogryposis, hydranencephaly, and

microencephaly.

Hydrops amnii following inoculation with Smithburn vaccine

Ewe. Hydrops amnii.There may be up to 20 litres of fluid

Hydranencephaly Arthrogryposis and anasarca

2. Vector Control

3. Wear personal protective equipment (gloves, coveralls

and boots) when handling aborted fetuses or performing

necropsies.

4. Disinfect animal housing areas that has become

contaminated with RVF virus.

The RVF virus is easily destroyed by

disinfectants.

References:-1) Books:1. Roger W. Blowey and A. David Weaver. Color atlas of diseases and

disorders of cattle, 3rded. PP. 228 – 229..

2. O. M. Radostits, C. C. Gay, K. W. Hinchcliff, P. D. Constable.

VETERINARY MEDICINE A textbook of the diseases of cattle,

horses, sheep, pigs and goats, 10th ed. PP. 1205 – 1207.

3. Recognizing Rift Valley Fever (FAO Animal Health Manual), No. 17.

2) General internet researches.

Prepared by: Ahmed Abdulkadir Hassan