Rift Valley Fever Virus
by: Jenny Dorth
Rift Valley Fever virus is a member of the Phlebovirus genera of the Bunyaviridae family. Members of the Phlebovirus genus (Rift Valley Fever and Sandfly Fever viruses) are so named because they can be transmitted by mosquitoes or phlebotomine flies. RVF virus was placed in this family because it has a tripartite genome and is antigenically similar to other phleboviruses.
History and Transmission
RVF virus was first isolated in 1930 during an investigation of an epizootic among sheep in the Rift Valley in East Africa, hence its name derivation. After this first isolation, other epizootics were seen in many sub-Saharan countries as well as Egypt. The epizootic/epidemic of RVF was intense in several areas clustered around the Nile in Egypt. The disease first occurred in animals (as it usually does) and then spread to humans, resulting in infection rates between 25 and 50% in sheep and cattle and 200,000 human cases with at least 600 deaths. Once an epizootic is underway, it usually leads to an epidemic that can't be controlled because humans become infected both through exposure to the tissues or blood of animals. What increases this transmission rate is the fact that mosquitoes, usually of the genus Aedes, are also able to transmit this virus between animals and humans. Rates of infection increase exponentially during rainy seasons, when there are more mosquitoes around to act as vectors for RVF. Infection can also occur in the laboratory through aerosolization of a virus-infected laboratory specimen.
The clinical manifestations of RVF virus are relatively severe in both humans and animals. Sheep are the most susceptible of the domesticated animals and lambs experience a 90% mortality rate during epizootic periods, adult sheep a 25% mortality rate and pregnant ewes usually abort. Patients that manifest clinical symptoms usually have fever, generalized weakness, back pain, dizziness, and extreme weight loss at illness onset.
RVF in humans is fatal 1% of the time and associated long-term complications are permanent vision loss, occurring in 1-10% of patients. However, most individuals recover within 2 days to 1 week after infection.
In humans the incubation period is 2 to 6 days, so the virus replicates swiftly, in some cases producing no symptoms, in others causing a mild illness associated with fever and liver abnormalities. In some patients, this virus causes a hemorrhagic fever, encephalitis or ocular disease.
RVF can be controlled and prevented by taking precautions to avoid international transport of infected animals. Important prevention strategies are through mosquito vector control and the practice of safe methods for killing and disposing infected animals.
Live attenuated and formalin-inactivated vaccines are available for use in livestock, but they can cause birth defects and abortions in sheep and only induce low-level protection in cattle. Another limitation of the veterinary vaccine is that it needs to be administered on a large scale to prevent outbreaks in epizootic areas. There is also a human live-attenuated vaccine, MP-12, that has yielded promising effects in the laboratory, but further research must be done before it can be administered in the field.
No antiviral therapy is available.
Game Action: When RVF card is drawn, all players will lose a turn because RVF is transmitted at a rapid rate. It is spread by mosquito vectors and can lead to epizootics that increase the transmission to humans. Ultimately, everyone loses.
Next time, think before you engage in " intimate" practices with your livestock or you might become infected with RVF and die!