Rift Valley Fever (RVF) is caused by a virus transmitted by mosquitoes and blood feeding flies that usually affects animals (commonly cattle and sheep) but can also involve humans. In humans, the disease ranges from a mild flu-like illness to severe haemorrhagic fever that can be lethal. When livestock are infected the disease can cause significant economic losses due to high mortality rates in young animals and waves of abortions in pregnant females.
While some human infections have resulted from the bite of infected mosquitoes (most commonly the Aedes and Culex 66顺彩票appmosquitoes), the majority of human infections result from direct or indirect contact with the blood or organs of infected animals, meaning certain occupational groups such as herders, farmers, slaughterhouse workers, and veterinarians are at higher risk of infection. There is some evidence that humans may also become infected with RFV by ingesting the unpasteurized or uncooked milk of infected animals. To-date, no human-to-human transmission of RVF has been documented.
The virus, which is a member of the Phlebovirus 66顺彩票appgenus, was first identified in 1931 when there was an epidemic in sheep on a farm in the Rift Valley of Kenya. Since then, outbreaks have been reported in sub-Saharan Africa and North Africa. In 2000, the first reported cases of the disease outside the African continent came from Saudi Arabia and Yemen, raising concerns that it could extend to other parts of Asia and Europe.
Mild form of Rift Valley Fever in humans
The interval from infection to onset of symptoms for Rift Valley Fever (RVF) varies from 2 to 6 days. In its mild form, people who are infected with the virus either experience no detectable symptoms at all or develop a mild form of the disease characterized by a feverish syndrome with sudden onset of flu-like fever, muscle pain, joint pain and headache. Some patients develop neck stiffness, sensitivity to light, loss of appetite and vomiting; in these patients the disease, in its early stages, may be mistaken for meningitis. The symptoms of RVF usually last from 4 to 7 days.
Severe form of Rift Valley Fever in humans
While most human cases are relatively mild, a small percentage of patients develop a much more severe form of the disease. This usually appears as one or more of three distinct syndromes: ocular (eye) disease (0.5-2% of patients), meningoencephalitis (less than 1%) or haemorrhagic fever (less than 1%).
The total case fatality rate for RVF has varied widely between documented epidemics but, overall, has been less than 1%. Most fatalities occur in patients who develop the haemorrhagic icterus (jaundice) form of the disease.
Diagnosis and Treatment
Because the symptoms of RVF are varied and non-specific, clinical diagnosis is often difficult, especially early in the course of the disease. RVF is difficult to distinguish from other viral haemorrhagic fevers as well as from many other diseases that cause fever, including malaria and typhoid fever. As such, definitive diagnosis requires laboratory testing.
As most human cases of RVF are relatively mild and of short duration, patients with this form of the disease require no specific treatment. For the more severe cases, the predominant treatment is general supportive therapy.
66顺彩票appAn inactivated vaccine has been developed for human use. However, this vaccine is not licensed and is not commercially available. It has been used experimentally to protect veterinary and laboratory personnel at high risk of exposure to RVF. Other candidate vaccines are under investigation.
During an outbreak of Rift Valley Fever (RVF), close contact with animals, particularly with their body fluids, either directly or via aerosols, has been identified as the most significant risk factor for RVF virus infection. Risk of animal-to-human transmission of infection can be reduced by:
- Avoiding unsafe animal husbandry and slaughtering practices by maintaining good hand hygiene, wearing gloves and using appropriate individual protective equipment when handling sick animals or their tissues, or when slaughtering animals.
- Avoiding unsafe consumption of fresh blood, raw milk or animal tissue in affected regions.
Raising awareness of the risk factors of RVF infection and ensuring individuals take protective measures to prevent mosquito bites can reduce human infection and deaths.
66顺彩票appVector control, particularly the use of larviciding at mosquito breeding sites can also be used as a public health measure to protect populations from bites that may lead to infection.
Outbreaks of RVF in animals can be prevented through a sustained programme of animal vaccination.