Rift valley fever (RVF) is a zoonotic disease of the Phlebovirus genus (family Phenuiviridae, order Bunyavirales) endemic to areas of eastern and southern Africa, the Middle East, and Madagascar. The disease is naturally spread to humans by mosquitoes, contact with or inhalation of aerosolized body fluids of infected animals (eg, cattle, sheep, goats, buffalo, camels), or by drinking raw milk from infected animals.
 
Aedes mosquitoes are born infected, as the virus transmits from the mother to the eggs. Animals are infected by mosquito bites, and then other types of mosquitoes and biting insects (eg, midges) subsequently become infected by feeding on these animals. Disease rates are higher during the rainy season or flooding, as the eggs are reconstituted from their desiccated state.
 
Although the mortality rate of typical RVF is less than 1%, the mortality rate, if it progresses to a hemorrhagic fever, is approximately 50%. In addition, the economic repercussions of an outbreak can be severe due to the loss of infected farm animals.

RVF has an incubation period of 2-6 days. Many patients may be asymptomatic or have mild, nonspecific symptoms that do not prompt medical evaluation. Those with symptoms present with the sudden onset of fever, headache, generalized weakness, myalgia, backache, photophobia, and vomiting. Less frequently, the disease progresses into eye disease (see below), meningoencephalitis, or hemorrhagic fever.
 
Eye disease, which includes lesions in the macula and permanent vision loss, vasculitis, hepatitis, or meningoencephalitis, occurs in about 10% of all patients within 1-3 weeks. Hemorrhagic fever develops within an average of 3 days and manifests with liver disease, jaundice, hematemesis, hematochezia, purpura, and gingival bleeding.
 
Person-to-person transmission is not known to occur. A vaccine has been developed for veterinary use as well as human use, but the human vaccine is not commercially available.

RVF is uncommon in young children and infants because the virus is most effectively spread via direct contact with infected animals and/or animal products.