RIFT VALLEY FEVER
Overview
Rift Valley Fever (RVF), is a zoonosis . It may cause severe disease in both animals and humans leading to high morbidity and mortality.
The virus isolated in 1930.during epidemic amongst sheep on a farm in the Rift Valley of Kenya, there have been outbreaks in sub-Saharan and North Africa. In 1997-98, there was a major outbreak in Kenya and Somalia. In September 2000, RVF was for the first time reported outside of the African Continent. Cases were confirmed in Saudi Arabia and Yemen.
Many different species of mosquitoes are vectors for the RVF virus. Risk of epidemics in new areas is feared.
RVF Virus
Bunyaviridae Virus of the Phlebovirus genus
RVF Vectors
RVF virus is primarily spread amongst animals by the bite of infected mosquitoes.
Aedes mosquitoes, for example, may acquire the virus from feeding on infected animals, and are capable of transovarial transmission (transmission of the virus from infected female mosquitoes to offspring via eggs), so new generations of infected mosquitoes may hatch from their eggs.
This provides a durable mechanism for maintaining the virus in nature, as the eggs of these mosquitoes may survive for periods of up to several years in dry conditions. During periods of inundation of larval habitats by rainfall, for example, in the rainy season, the eggs will hatch, and the mosquito population will increase and spread the virus to the animals on which they feed.
RVF Virus Non-human Hosts
Sheep appear to be more susceptible than cattle and goats are less susceptible.
Over 90% of lambs infected with RVF die, whereas mortality amongst adult sheep can be as low as 10%.
Increase abortion rates in sheep, may signal the start of an epidemic.
Transmission to Humans
People may become infected with RVF either by being bitten by infected mosquitoes, or through contact with the blood, other body fluids or organs of infected animals ( farmers- butchers-handling raw meat).
Clinical Features
The incubation period varies from two to six days.
Early symptoms of an influenza-like illness, with sudden onset of fever, headache, myalgia and backache, neck stiffness, photophobia and vomiting; (often mistaken for meningitis).
The symptoms of RVF usually last from four to seven days, after which one sees the appearance of IgM and IgG antibodies.
Most cases are mild, some go one to a much more severe disease
Clinical Features of Severe Cases
Several recognizable syndromes:
1-Eye disease, characteristically manifests itself in retinal lesions. The onset of eye disease is usually one to three weeks after the first symptoms appear. When the lesions are in the macula, some degree of permanent visual loss will result. Death in patients with only ocular disease is uncommon
2-Meningoencephalitis: The onset of this syndrome is also usually one to three weeks after the first symptoms appear. Classic symptoms of severa neurological follow. Death in patients with only meningoencephalitis is uncommon
3-Haemorrhagic fever.. Two to four days after the onset of illness, the patient shows evidence of severe liver disease, with jaundice and haemorrhagic phenomena, vomiting blood, passing blood in the faeces, developing a purpuric rash (a rash caused by bleeding in the skin), and bleeding from the gums. The case-fatality rate for patients developing haemorrhagic disease is high at approximately 50%.
Total case fatality rate has varied widely in the various documented epidemics, but, overall, is less than 1%.
Diagnosis and Treatment
1-Serological tests such as enzyme-linked immunoassay (the "ELISA" or "EIA" methods) may demonstrate the presence of specific IgM antibodies to the virus.
2-The virus identification:
Virus propagation (in cell cultures or inoculated animals), antigen detection tests,
PCR testing.
Antiviral drug ribavirin has been shown to inhibit viral growth in experimental systems, but has not been evaluated in the clinical setting. Most human cases of RVF are relatively mild and of short duration, so will not require any specific treatment. For the more severe cases, the mainstay of treatment is general supportive therapy.
Prevention and Control
RVF can be prevented by a sustained program of animal vaccination. Both live, attenuated, and killed vaccines have been developed for veterinary use.
An inactivated vaccine has been developed for human use. This vaccine is not licensed and is not commercially available, but has been used experimentally to protect veterinary and laboratory personnel at high risk of exposure to RVF.
The risk of transmission from infected blood or tissues exists for the following people.
Gloves and other appropriate protective clothing should be worn, and care taken when handling sick animals or their tissues.
control of the mosquito vectors. Personal insect bite protection Measures to control mosquitoes during outbreaks, e.g., use of insecticides, are effective if conditions allow access to mosquito breeding sites.
New satellite systems that monitor variations in climatic conditions are being applied to give advance warning of impending outbreaks by signaling events which may lead to increases in mosquito numbers (rains).
Dr Assad