Towards a better understanding of Rift Valley fever epidemiology in the south-west of the Indian Ocean

Abstract

Rift Valley fever virus (Phlebovirus, Bunyaviridae) is an arbovirus causing intermittent epizootics and sporadic epidemics primarily in East Africa. Infection causes severe and often fatal illness in young sheep, goats and cattle. Domestic animals and humans can be contaminated by close contact with infectious tissues or through mosquito infectious bites. Rift Valley fever virus was historically restricted to sub-Saharan countries. The probability of Rift Valley fever emerging in virgin areas is likely to be increasing. Its geographical range has extended over the past years. As a recent example, autochthonous cases of Rift Valley fever were recorded in 2007-2008 in Mayotte in the Indian Ocean. It has been proposed that a single infected animal that enters a naive country is sufficient to initiate a major outbreak before Rift Valley fever virus would ever be detected. Unless vaccines are available and widely used to limit its expansion, Rift Valley fever will continue to be a critical issue for human and animal health in the region of the Indian Ocean.

Figure 1

Geographical distribution of Rift Valley fever. Rift valley fever is historically endemic to many countries of sub-Saharan Africa. It spread from continental Africa to Madagascar in 1991 and in the Arabian Peninsula in 2000. In Mayotte in the Indian Ocean, RVF cases have been reported since 2004.

Figure 2

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.