Rift Valley Fever
Discovery
Rift Valley Fever (RVF) virus was first isolated from a lamb in 1930. Throughout the 20th century, large epizootics (outbreaks in animals) of RVF were observed, starting as early as 1912. RVF causes abortion and high fever in livestock like sheep and cows. From the herd population the disease is spread to humans.
Outbreak
The origin of the outbreak can usually be traced to a period of excessive rain. All the rain creates many breeding places for Aedes genus mosquitoes. An excess of mosquitoes quickly turns a small outbreak into an area-wide epizootic. Once the mosquitoes are carrying the virus, it is spread to humans by mosquito bite.
Disease
Human infection with RVF is usually milder than in livestock. The incubation period lasts anywhere from two to six days. Infection lasts 2-4 days. Symptoms are usually mild and include fever, malaise, headache, photophobia, back pain, and joint pain. Usually, RVF patients have a rapid complete recovery after an influenza-like illness. Less than one in one hundred RVF patients develop more severe illness.
Some RVF patients contract hemorrhagic fever. Others can develop retinal vasculitis. Still others present with encephalitis up to four weeks after the initial infection. In these cases, patients suffer a recurrence of fever, confusion, hallucinations, and the disease can lead to coma and death. These patients’ brains show signs of focal necrosis, which is thought to be caused both by viruses destroying cells and overactive T-cells killing off healthy tissue.
Epidemiology
Cases of RVF in humans are typically seen in farmers and those living near livestock. All outbreaks have been in Africa, with the exception of a few epizootics in Yemen and Saudi Arabia. Scientists believe RVF can be spread by direct contact or aerosolized blood from infected animals. Workers who butcher or birth cattle are especially at risk for contracting RVF. Many cases of infection of lab workers by aerosol have also been observed.
Major Outbreaks
The first definite outbreak was a large epizootic among sheep in 1930 in Sub-Saharan Africa. There was a huge outbreak in 1977 in West Africa, the first recorded outbreak in the area. The outbreak was precipitated by damming a large river, which created a large breeding ground for mosquitoes. Another large outbreak in Egypt in 1977 caused many cases of human illness.
Prevention and Management
Since a vaccine exists for Rift Valley Fever, vaccination of livestock is the most effective means of prevention. Since RVF outbreaks are very sporadic and years may pass between epizootics, persuading farmers to vaccinate cattle regularly is a challenge. However, livestock vaccination is a very effective means of preventing cases of human RVF. Vaccinating high-risk workers is also effective to prevent localized outbreaks. Lab workers studying the virus should also be vaccinated as laboratory spread is relatively common. Lab workers should also use barriers like clothing, mask, and hood to protect themselves. Ribavirin may be helpful in treating RVF patients, but large-scale trials have not been performed.