Cycle of Rift Valley fever. The virus can be maintained in an enzootic cycle involving Aedes mosquitoes which are able to transmit the virus vertically to their offspring. Epizootic outbreaks are often linked with unusual rains or warm seasons, favoring the hatching of infected Aedes eggs that are then able to initiate the virus circulation. Subsequently, large numbers of secondary vectors belonging to the Culex genus could be infected and induce the emergence of epidemic/epizootic outbreaks. Transmission to humans occurs through direct contact with high virus loads when aborted fetuses are manipulated.