Rift Valley Fever (RVF), at least in the African country of Kenya, has been well known for over 60 years. As early as 1913, a disease fitting the description of RVF was blamed for the loss of sheep in the Rift Valley in Kenya. However, it was not until scientists studied an outbreak of the disease in 1931 that a virus was isolated and shown to cause the disease. Since that time, major outbreaks of RVF have been noted throughout sub-Saharan Africa, with occasional outbreaks noted in other parts of the Africa continent. For example, in Egypt, several hundred people were infected and thousands died from RVF during a violent epidemic of this disease in 1977-78.
RVF-El Niño Link
Using data from Earth-orbiting environmental satellites that simultaneously measure ocean temperature and vegetation conditions, scientists have discovered that the combination of warmer-than-normal equatorial Pacific Ocean temperatures associated with El Niño and rising sea-surface temperatures in the western equatorial Indian Ocean increase rainfall over a large area of eastern Africa, leading to a dramatic green-up of vegetation. This February 1998 image illustrates the close relationship between ocean temperature (warmer-than-normal ocean temperatures are shown in red, cooler-than-normal temperatures in blue), rainfall, and their impacts on land vegetation (greener-than-normal vegetation shown in light green). By closely monitoring the vegetation in the region affected by the increased rainfall, scientists can identify likely habitats for the mosquitoes that carry the RVF virus, and provide advance warning of large-scale outbreaks of the disease.
Between October 1997 and January 1998, heavy rains drenched eastern Kenya and southern Somalia, which led to an outbreak of RVF that killed many cattle and eventually spread to the local human population. The World Health Organization estimated a total of 200-250 deaths and 89,000 cases in southern Somalia and northeastern Kenya, one of the largest outbreaks in history.