In humans the virus can cause several different syndromes. Usually sufferers have either no symptoms or only a mild illness with fever, headache, myalgia and liver abnormalities. In a small percentage of cases (< 2%) the illness can progress to hemorrhagic fever syndrome, meningoencephalitis (inflammation of the brain), or affecting the eye. Patients who become ill usually experience fever, generalized weakness, back pain, dizziness, and weight loss at the onset of the illness. Typically, patients recover within 2-7 days after onset.
Approximately 1% of human sufferers die of the disease. Amongst livestock the fatality level is significantly higher. In pregnant livestock infected with RVF there is the abortion of virtually 100% of fetuses. An epizootic (animal disease epidemic) of RVF is usually first indicated by a wave of unexplained abortions.
In November 2006, a Rift Valley fever outbreak occurred in Kenya. The victims are from the North Eastern Province and Coast Province of Kenya, which have received heavy rain in recent months, causing floods and creating breeding grounds for mosquitoes which spread the virus of the fever from infected livestock to humans.
As of 23rd January 2007 cases had started to crop at the Kenyan capital Nairobi. An estimated large number of businesses were supposedly suffering large losses as customers were shunning the common meat joints for the popular Nyama Choma (Roast meat), as it was believed to be spreading the fever.
In December 2006 and again in January 2007, Taiwan International Health Action (TaiwanIHA) began operating missions in Kenya consisting of medical experts assisting laboratory training and health facility personnel, and included donations of supplies such as mosquito sprays. The United States Centers for Disease Control has also set up an assistance mission and laboratory in Kenya.
By the end of January, 2007, some 148 people have died since the outbreak began in December.
As at 14 March 2007, the Kenyan government declared RVF as having diminished drastically after spending an estimated 2.5 Million in Vaccine and deployment costs, It also lifted the ban on cattle movement in the affected areas.
As of 2 November 2007, 125 cases including 60 deaths have been reported from more than 10 localities of White Nile, Sinnar, and Gezira states in Sudan. Young adult males are predominantly affected. More than 25 human samples have been found positive for RVF by PCR or ELISA.