Rift Valley Fever
|Common Name(s)||Rift Valley Fever|
|Scientific Name(s)||Genus Phlebovirus in the family Bunyaviridae|
|Physical Attributes||Tripartite negative strand; enveloped|
|Geography||Eastern and southern Africa where sheep and cattle are raised, but also most countries of sub-Saharan Africa and Madagascar|
|Mode(s) of Transmission||Bites from infected Aedes mosquitoes and other biting insects that have virus-contaminated mouthparts; blood, body fluids, or tissues of infected animals; aerosol from laboratory|
|Likely BW Form(s)||Aerosol or droplets|
|Pathology||Influenza-like illness, with sudden onset of fever, headache, myalgia (muscle pain) and backache; some patients develop neck stiffness, photophobia (the patient finds exposure to light uncomfortable) and vomiting; mild disease for 4-7 days|
|Vector/Dormant Form||Aedes and other mosquitoes|
|Incubation Period||2-5 days|
|Fatality||Less than 1%; viral hemorrhagic fever syndrome ~50% fatality|
|Vaccine||Live attenuated vaccine (MP-12) has been developed for human use but not licensed; killed and live attenuated RVFV available for animals|
|Treatment||Supportive therapy; ribavirin in vitro|
Rift Valley Fever (RVF) is a viral disease caused by RVF virus. The virus, which is of genus Phlebovirus, family Bunyaviridae, has a enveloped, tripartite negative genome. The virus circulates mostly in sub-Saharan Africa through a mosquito vector. Epizootics occur when susceptible domestic animals are infected. Domestic animals with large amount of virus in their serum amplify the RVF infection and transmit the amplified infection to biting arthropods. Deaths and abortions among susceptible species such as cattle and sheep constitute a major economic consequence of these epizootics, as well as providing a diagnostic clue and a method of surveillance. Humans become infected by the bite of mosquitoes or by exposure to virus-laden aerosols or droplets.
The human disease appears to be similar whether acquired by aerosol or by mosquito bite. A biological warfare attack, most likely delivered by aerosol, would be expected to elicit the rather specific spectrum of human clinical manifestations and to cause disease in sheep and cattle in the exposed area. If disease occurred in the absence of heavy vector populations or without domestic animals as amplifiers of mosquito infection, a BW attack would also be a likely cause.
The incubation is two to five days and is usually followed by an incapacitating febrile illness of similar duration. The typical physical findings are fever, conjunctival injection, and sometimes abdominal tenderness. A few petechiae (small flat round spot of pinpoint size under the skin) or epistaxis (nosebleds) may occur. A small proportion of cases (approximately one percent) will progress to a viral hemorrhagic fever syndrome; mortality in this group is roughly 50 percent. A small number of infections will lead to a late encephalitis. After apparent recovery from a typical febrile illness, the patient develops fever, meningeal signs, obtundation, and focal defects. Approximately 1% - 10% of affected patients may have some permanent vision loss. These patients may die or often suffer serious sequelae.
Approximately 1% of humans that become infected with RVF die of the disease. Case-fatality proportions are significantly higher for infected animals. The most severe impact is observed in pregnant livestock infected with RVF, which results in abortion of virtually 100% of fetuses.
The occurrence of an epidemic with febrile disease, hemorrhagic fever, eye lesions, and encephalitis in different patients would be characteristic of RVF. Demonstration of viral antigen in blood by ELISA is rapid and successful in a high proportion of acute cases of uncomplicated disease or hemorrhagic fever.
In hemorrhagic fever, supportive therapy may be indicated for hepatic and renal failure, as well as replacement of coagulation factors. Studies in monkeys and other animals have shown promise for ribavirin, an antiviral drug, for future use in humans. The virus is sensitive to ribavirin in vitro and in rodent models. Additional studies suggest that interferon, immune modulators, and convalescent-phase plasma may also help in the treatment of patients with RVF.
An effective inactivated vaccine is available in limited quantities, but prevention typically involves avoiding mosquitoes, contact with fresh blood from dead domestic animals, and inhaling small particle aerosols.
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