Rift Valley fever: A viral disease that is acute, causes fever in domestic animals (such as cattle, buffalo, sheep, goats, and camels) and humans, and is associated with mosquito-borne epidemics during years of heavy rainfall. Rift Valley fever is more deadly than West Nile virus.
Rift Valley fever has not occurred in the United States (through 2003). However, there has been concern that it could become permanently established in the US if it does enter the country. People with Rift Valley fever usually have flu-like symptoms but may develop complications such as kidney or liver disease. In outbreaks, the death rate has been about 15% for those seriously ill with Rift Valley fever.
Rift Valley fever is caused by the RVF virus. It is found mainly in regions of eastern and southern Africa where sheep and cattle are raised. RVF primarily affects livestock and can cause disease in a large number of domestic animals (a situation referred to as an "epizootic"). An RVF epizootic can lead to an epidemic among people exposed to animals with RFV. The most notable epizootic of RVF to date occurred in Kenya in 1950-1951 and resulted in the death of an estimated 100,000 sheep. The first epidemic of RVF in West Africa was reported in 1987.
Epizootics of RVF tend to occur during years in which there is heavy rainfall and localized flooding. The excessive rain allows mosquito eggs, usually of the genus Aedes, to hatch. The mosquito eggs are infected with the RVF virus, and the resulting mosquitoes transfer the virus to the livestock on which they feed. Once the livestock are infected, other species of mosquitoes can become infected from the animals and spread the disease.
People get RVF from the bites of mosquitoes. (In addition, it is possible that the virus can be transmitted by other biting insects.) People can also get the RVF by exposure to the blood or other body fluids of infected animals. This exposure may be in slaughtering or handling of infected animals or by touching contaminated meat during the preparation of food. Infection through aerosol transmission of RVF virus has resulted from contact with laboratory specimens containing the virus.
Most people with RVF have no symptoms or only a mild illness associated with fever and liver abnormalities. People with more serious symptoms usually experience fever, weakness, back pain, dizziness, and extreme weight loss at the onset of the illness. Typically, they recover within two days to one week after the onset of illness. But RFV can progress to hemorrhagic fever (bleeding which can lead to shock), encephalitis (inflammation of the brain which can cause headaches, seizures, or coma), and eye disease. About 1% of people infected with RVF die of the disease.
A common long-term complication of RVF involves the eye. It is due to inflammation of the retina (at the back of the eye), About 1-10% of affected patients suffer some permanent loss of vision.
There is no established treatment for RVF. Ribavirin, an antiviral drug, has shown promise in animal trials. Interferon, immune modulators, and convalescent-phase plasma may possibly help in the treatment of RVF.
Sleeping outdoors at night in regions where outbreaks occur risks exposure to mosquito vectors. Animal herdsmen, abattoir workers, and other individuals who work with animals in RVF-endemic areas (areas where the virus is present) have an increased risk of infection. Persons in high-risk professions, such as veterinarians and slaughterhouse workers, have an increased chance of contracting the virus from an infected animal.
International travelers increase their chances of getting the disease when they visit RVF-endemic locations during periods when sporadic cases or epidemics are occurring. To avoid RFV, the use of mosquito repellents and bednets is strongly recommended. Exposure to blood and tissues of animals that may be infected is taboo.