م ي ح ر ل ا ن م ح ر ل ه ا ل ل م ا س بUniversity of Bahri College of Veterinary Medicine Department of Preventive Veterinary Medicine (Viral Diseases)
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