Distribution of Rift Valley Fever in Africa. Blue, countries with endemic disease and substantial outbreaks of RVF; green, countries known to have some cases, periodic isolation of virus, or serologic evidence of RVF.
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Rift Valley fever

Rift Valley fever (RVF) is a viral zoonosis (affects primarily domestic livestock, but can be passed to humans) causing fever. It is spread by the bite of infected mosquitoes. The disease is caused by the RVF virus, a member of the genus Phlebovirus (family Bunyaviridae). more...

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The disease was first reported among livestock in Kenya around 1915, but the virus was not isolated until 1931. RVF outbreaks occur across sub-Saharan Africa, with outbreaks occurring elsewhere infrequntly (but sometimes severely - in Egypt in 1977-78, several million people were infected and thousands died during a violent epidemic; in September 2000 an outbreak was confirmed in Saudi Arabia and Yemen).

In humans the virus can cause several different syndromes. Usually sufferers have either no symptoms or only a mild illness with fever, headache, myalgia and liver abnormalities. In a small percentage of cases (< 2%) the illness can progress to hemorrhagic fever syndrome, meningo-encephalitis (inflammation of the brain), or affecting the eye. Patients who become ill usually experience fever, generalized weakness, back pain, dizziness, and weight loss at the onset of the illness. Typically, patients recover within 2-7 days after onset.

Approximately 1% of human sufferers die of the disease. Amongst livestock the fatality level is significantly higher. In pregnant livestock infected with RVF there is the abortion of virtually 100% of fetuses. An epizootic (animal disease epidemic) of RVF is usually first indicated by a wave of unexplained abortions.

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Update: Outbreak of Rift Valley Fever — Saudi Arabia, August-November 2000 - Statistical Data Included
From Morbidity and Mortality Weekly Report, 11/3/00

On September 10, 2000, the Ministry of Health (MOH), Kingdom of Saudi Arabia and subsequently, the MOH of Yemen began receiving reports of unexplained hemorrhagic fever in humans and associated animal deaths and abortions from the far western Saudi-Yemeni border region. These cases subsequently were confirmed as Rift Valley fever (RVF), the first such cases on the Arabian peninsula [1]. This report updates the findings of the ongoing investigation conducted by the Saudi Arabian MOH in collaboration with CDC and the National Institute of Virology, South Africa.

As of November 1 in Saudi Arabia, 516 persons with suspected severe RVF [*] requiring hospitalization have been reported from primary health-care centers and hospitals (Figure 1);87 (17%) have died. Suspected cases have been identified through an elaborate pre-existing system of primary health-care centers that refer acutely ill persons to district hospitals for assessment of hepatitis and other criteria for admission as RVF case-patients. Of the 216 suspected severe case-patients with appropriate serum samples, 206 (95%) have been laboratory confirmed by either viral antigen or IgM antibody testing. Of the 516 case-patients, 407 (79%) were male; the median age was 46 years (range: 1-95 years); the youngest confirmed patient was aged 14 years; and 424 (82%) were Saudi citizens, 80 (16%) were Yemeni citizens, and 12 (2%) were of other nationalities. The largest number of cases have been reported from the southwestern province of Jazan (365 [77%]), and 122 (24%) cases have been reported from the contiguous Asi r region. Except for one case-patient in Al Quenfadah, northwest of Jazan, all other case-patients had traveled recently to Jazan or Asir.

The mean duration from disease onset to hospitalization was 3.3 days (standard deviation [SD] = [+ or -] 3.2 days), and the average time from disease onset to death among the 87 fatalities was 6.3 days (SD = [+ or -] 5.3 days). Of 148 case-patients at King Fahad Central Hospital in Jazan, 57 (39%) with mild to moderate RVF disease had reversible acute renal failure, requiring only supportive care for 2-14 days; 27 (18%) with severe disease required hemodialysis.

Based on preliminary data from the ongoing epidemiologic investigation, 125 (76%) of 165 case-patients reported close contact with animals, especially sheep and goats, and 91 (64%) of 143 case-patients reported a history of exposure to dead, and/or aborted animals. Nearly all persons reported having had mosquito bites and that the mosquitoes were present at their place of residence.

Entomologic studies found large numbers of two species of mosquitoes, Culex tritaeniorrhynchus and Aedes caspius, in the flood irrigation farming areas at the foot of the mountains and the foothills of Al Ardah district in Jazan, where the first and most human cases were reported. Preliminary laboratory studies have already yielded isolates of RVF virus from both of these species. Further laboratory identification of the collected mosquitoes suggests the presence of additional Aedes species; definitive species typing is pending. A regional survey for RVF antibody prevalence in domestic ungulates, primarily goats and sheep, was conducted in Jazan and Asir provinces. RVF antibody prevalence [greater than or equal to]90% was found in Al Ardah district. RVF antibodies also were found among ungulates in other surveyed areas. A correlation was found between areas where human cases were reported and the same flood irrigation farming areas in the upper reaches of the wadis identified by the entomologists.

Reported by: H Arishi, MD, A Ageel, MD, M Abdu Rahman, MD, A Al Hazmi, MD, AR Arishi, MD, B Ayoola, MD, C Menon, MD, J Ashraf, MD, O Frogusin, MD, L Ochia, F Sawwan, M Al Hazmi, MD, Medical Svcs, King Fahad Central Hospital, Jazan; M Almaradni, MD, Medical Svcs, Al Ardah Hospital, Jazan; M Yasim Shah, MD, Medical Svcs, Samta General Hospital, Jazan; A As-Sharif, MS, M Al Sayed, Preventive Medicine, A Raheem Ageel, MSD, Regional Health Affairs, Jazan; A Shihry. MD, Al Khobar, Eastern Province; A Abudahish, PharmD, A Al Sharif, MD, Abha, Asir Province; I Al Hazmi, Al Ouenfadah; An A Alrajhi, MD, King Faisal Specialist Hospital and Research Center, Riyadh; MA AI-Hedaithy, MD, College of Medicine, King Khalid Univ Hospital, Riyadh; A Fatani, MD, A Sahaly, MD, A Ghelani, MD, TAI Basam, MD, A Turkistani, DDS, AM Al Rabeah, N Al Hamdan, MD, Saudi Arabia Field Epidemiology Training Program, Riyadh; A Mishkas, MBBS, Infectious Diseases; MH Al Jeifri, MD, Parasitic and Infectious Diseases; VYAI Mazrou, MD, M MA Alamri, MM Al-Qahtani, MD, A Al Drees, Laboratories and Blood Banks, Riyadh; T Madani, MD, G Al Gasabi, MD, OA Shubokshi, MD, Ministry of Health, Saudi Arabia; M Al Khamees, DVM, D Al Mujalli, DVM, A Aziz lbn Moamar, PhD, Ministry of Agriculture and Water, Riyadh, Saudi Arabia. World Health Organization, Geneva, Switzerland. P Jupp, PhD, A Kemp, MS. F Bur PhD, R Swanepoel, PhD, Special Pathogens Unit, National Institute of Virology, Johannesburg, South Africa. Infectious Disease Pathology Activity, Special Pathogens Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note: RVF is a mosquito-borne zoonotic disease affecting domestic ungulates (especially goats and sheep) characterized by large epizootics during periods of heavy rainfall with associated outbreaks in humans. Most human infection is associated with an uncomplicated febrile illness or is inapparent. More severe complications include retinitis, hepatitis, renal failure, hemorrhagic fever, encephalitis, and death. This outbreak extends the geographic distribution of known infection outside of Africa and indicates this virus may be able to establish itself almost anywhere in the world based on the availability of potential permissive vectors and animal reservoirs.

Official reports from Yemen suggest ongoing transmission over a large area, compared with the outbreak in Saudi Arabia, which is more circumscribed and is now mainly focused in Asir province. However, the differing case definitions and surveillance methodologies preclude a direct comparison of the Saudi Arabian and Yemeni outbreaks. Nevertheless, these outbreaks demonstrate disease transmission in an approximately 600 km area, including the flood plains of the wadis extending from the Sarawat mountains to the Red Sea coastal plain and extending from the Hodediah governate in Yemen to the Al Quendafah health region in Saudi Arabia. Epidemiologic data suggest the simultaneous, extensive, and multicentric nature of the outbreaks rather than radiation of disease from a single focus in Saudi Arabia or Yemen.

Control and prevention measures are ongoing in these countries as are preparations for studies to better define risk factors for infection and severe disease, examine the risk for nosocomial infection, gauge the magnitude and scope of the outbreak, characterize viral sequences from isolates, test the efficacy of intravenous ribavirin, and determine the prevalence of infection among captured vector species. The abundance of A. caspius (a floodwater breeding aedine mosquito) breeding in the flooded agricultural fields suggests that this species can act as an interepidemic (reservoir) host for the virus and an epidemic vector when heavy rains promote mosquito population explosions; C. tritaeniorrhynchus is probably an epidemic vector. Continued surveillance will be necessary to determine if these infected "floodwater" Aedes, the major vector for persistence of the virus in Africa attributed to transovarial transmission, supports establishment of RVF on the Arabian Peninsula.

(*.) Suspected severe RVF is defined as unexplained illness [greater than]48 hours in duration associated with threefold elevation in transaminases (alanine aminotransferase, aspartate aminotransferase, and gamma glutamyl transpeptidase) or clinical jaundice; or unexplained illness [greater than]48 hours in duration associated with abortion or bleeding manifestations (e.g., from puncture sites, ecchymosis, petechiae, purpura, epistaxis, gastrointestinal bleeding, or menorrhagia); or unexplained illness [greater than]48 hours in duration associated with neurologic manifestations (e.g., vertigo, confusion, disorientation, amnesia, lethargy, hallucination, meningismus, choreiform movements, ataxia, tremor, convulsions, hemiparesis, decerebrate posturing, locked-in syndrome, or coma); or unexplained illness [greater than]48 hours in duration associated with fever, diarrhea, nausea, vomiting, or abdominal pain and any one of the following laboratory values: 1) hemoglobin [less than]8 gm/dL; 2) platelets [less than ]100,000 [mm.sup.3] ([less than]10 x [10.sup.10]/ L); 3) LDH 2 x upper limit of normal; 4) creatinine [greater than]150 mol/L; or 5) CPK 2 x upper limit of normal; or unexplained death with recent history of fever during the preceding 2 weeks; and if a specimen is available, evidence of RVF-specific antigen or IgM antibody. Specimens must be obtained at least 7 days after illness onset before they can be considered negative.


(1.) CDC. Outbreak of Rift Valley fever--Saudi Arabia, August-October, 2000. MMWR 2000;49:905-8.

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