Rift Valley fever virus. Coloured transmission electron micrograph (TEM) of a section through tissue infected with Rift Valley fever virus particles (virions, blue). These virions are sequestering themselves in vacuoles (spaces, white) in the cell, where they will replicate. Rift Valley fever occurs in Africa, infecting humans, cattle, sheep, rodents and other species. It is transmitted 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 visual impairment.