Rift Valley fever
Aetiology Epidemiology Diagnosis Prevention and control References
AETIOLOGY
Classification of the causative agent
Virus family Bunyaviridae, genus Phlebovirus
Resistance to physical and chemical action
Temperature: Survives several months at 4°C. In serum, inactivated by 56°C for 120 minutes pH: Resistant to alkaline pH but inactivated by pH <6.8 Chemicals: Inactivated by ether and chloroform Disinfectants: Inactivated by strong solutions of sodium or calcium hypochlorite (residual chlorine should exceed 5000 ppm) Survival: Survives in dried discharges and multiplies in some arthropod vectors. Can survive contact with 0.5% phenol at 4°C for 6 months EPIDEMIOLOGY
- High mortality rate in young animals
- High abortion rate in ruminants
Hosts
- Cattle, sheep, goats, dromedaries, several rodents
- Wild ruminants, buffaloes, antelopes, wildebeest, etc.
- Humans are very susceptible (major zoonosis)
- African monkeys and domestic carnivores present a transitory viraemia
Transmission
- Haematophagous mosquitoes of many genera (Aedes, Anopheles, Culex, Eretmapodites, Mansonia, etc.) can transmit fever as biological, competent vectors. Mosquitoes (Aedes) are the reservoir host
- Direct contamination: occurs in humans when handling infected animals and meat
Sources of virus
- For animals: wild fauna and vectors
- For humans: nasal discharge, blood, vaginal secretions after abortion in animals, mosquitoes, and infected meat. Possibly also by aerosols and consumption of raw milk
Occurrence
RVF has been recognised exclusively in African countries, with an underlying association with high rainfall and dense populations of vector mosquitoes. The only epizootic outbreaks of RVF outside sub-Saharan Africa were recorded in animals and humans in Egypt in 1977-78, Mauritania in 1987 and again in Egypt in 1993. Laboratory infections have been recorded in other parts of the world
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Bulletin
DIAGNOSIS
Incubation period varies from 1 to 6 days
Clinical diagnosis
Cattle
- Calves: fever (40-41°C), depression. Mortality rate: 10-70%
- Adults: fever (40-41°C), excessive salivation, anorexia, weakness, fetid diarrhoea, fall in milk yield. Abortion may reach 85% in the herd. Mortality rate is usually less than 10%
Sheep, goats and pigs
- Lambs: fever (40-42°C), anorexia, weakness, death within 36 hours after inoculation. Mortality rate: for animals under 1 week of age - up to 90%; for animals over 1 week of age - up to 20%
- Adults: fever (40-41°C), mucopurulent nasal discharge, vomiting; in pregnant ewes, abortion may reach 100% and mortality may reach 20-30%
Inapparent infections are quite frequent in other species than sheep
Humans
- Influenza-like syndrome: fever (37.8-40°C), headache, muscular pain, weakness, nausea and epigastric discomfort, photophobiae. Recovery occurs within 4-7 days
Complications: retinopathy, blindness, meningo-encephalitis, haemorrhagic syndrome with jaundice, petechiae and death
Lesions
- Focal or generalised hepatic necrosis (white necrotic foci of about 1 mm in diameter)
- Congestion, enlargement, and discoloration of liver with subcapsular haemorrhages
- Brown-yellowish colour of liver in aborted fetuses
- Widespread cutaneous haemorrhages, petechial to ecchymotic haemorrhages on parietal and visceral serosal membranes
- Enlargement, oedema, haemorrhages and necrosis of lymph nodes
- Congestion and cortical haemorrhages of kidneys and gallbladder
- Haemorrhagic enteritis
- Icterus (low percentage)
Differential diagnosis
- Bluetongue
- Wesselsbron disease
- Enterotoxemia of sheep
- Ephemeral fever
- Brucellosis
- Vibriosis
- Trichomonosis
- Nairobi sheep disease
- Heartwater
- Ovine enzootic abortion
- Towic plants
- Bacterial septicaemias
- Rinderpest and Peste des petits ruminants
Laboratory diagnosis
Procedures
Identification of the agent
- Virus isolation:
- inoculation of mice or hamsters - preferred method
- inoculation of 1-2-day-old lambs
- inoculation of embryonated chicken eggs
- tissue culture inoculation (Vero, CER, BHK-21, mosquito line cells or primary calf, lamb and goat kidney and testis cells) in combination with immunofluorescence
- Viral antigen identification by immunofluorescence in cryostat sections or in impression smears of liver, spleen and brain. Also by complement fixation and immunodiffusion on tissue suspensions
- Antigen detection in blood: immunodiffusion, enzyme immunoassay
Serological tests
- Enyzme-linked immunosorbent assay - IgG and IgM
- Virus neutralisation
- Fluorescent antibody test
- Haemagglutination inhibition
- Plaque reduction neutralisation
- Complement fixation
- Immunodiffusion
Samples
- Heparinised or clotted blood
- Plasma or serum
- Tissue samples of liver, spleen, kidney, lymph node, heart blood and brain from aborted fetus. Specimens should be submitted preserved in 10% buffered formalin and in glycerol/saline and transported at 4°C
PREVENTION AND CONTROL
No specific treatment. Symptomatic treatment in severe human cases
Sanitary prophylaxis
Hygiene and vector control have had little effect
Medical prophylaxis
- Attenuated virus vaccine (Smithburn strain)
- one inoculation confers immunity lasting 3 years
- residual pathogenicity for pregnant ewes (abortion)
- pathogenic for humans
- Inactivated - virus vaccine
- requires two inoculations and annual revaccination
REFERENCES AND OTHER INFORMATION
- Reference experts and laboratories
- Classified as an OIE List A disease (A080)
- Chapter 2.1.8. in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.
- Terrestrial Animal Health Code
- Other references - see the Index
- World Animal Health .
- Current Animal Health Status (Disease Information)
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Contact : scientific.dept@oie.int