Rift Valley fever virus. Coloured transmission electron micrograph (TEM) of Rift Valley fever virus particles (virions). One of the virions has split, allowing its genetic material (green strips, upper right) to spill out. This virus occurs in Africa, infecting humans, cattle, sheep, rodents and many other species. It is transferred between hosts by insects, mainly mosquitoes. In humans its symptoms include chills, fever, a headache and joint and muscle pain. It is rarely fatal, but can cause retinal haemorrhages that may lead to temporary or permanent visual impairment. Magnification unknown.