Disease Information (4) (Provided by Emerging Exotic Diseases Lab, Foreign Animal Diseases Division) ( 07/05/11 ) | |||||
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Part | Charger | leeyh | date | 07/05/11 | |
□ Rift Valley fever
1. Definition
- Vector (Mosquito) borne zoonotic viral disease
2. Etiology
- Bunyaviridae Family, Phlebovirus Genus, Rift valley fever virus
3. Host range
- 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
4. Geographical distribution
- 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
5. 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
6. Incubation period
- Incubation period varies from 1 to 6 days
7. Gross 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)
8. Diagnosis
- 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
- Laboratory diagnosis
Identification of the agent
Virus isolation: inoculation of mice or hamsters - preferred method
inoculation of 1-2-day-old lambs, embryonated chicken eggs, or Cell culture (Vero, CER, BHK-21, mosquito line cells or primary calf, lamb and goat kidney and testis cells)
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
9. Control and eradication
- No specific treatment. Symptomatic treatment in severe human cases
- Hygiene and vector control have had little effect
10. Public health aspects
- Zoonotic disease
11. Further infomation : http://www.oie.int/eng/maladies/en_fiches.htm?e1d7
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