Recent outbreaks of rift valley Fever in East Africa and the middle East

Abstract

Rift Valley fever (RVF) is an important neglected, emerging, mosquito-borne disease with severe negative impact on human and animal health. Mosquitoes in the Aedes genus have been considered as the reservoir, as well as vectors, since their transovarially infected eggs withstand desiccation and larvae hatch when in contact with water. However, different mosquito species serve as epizootic/epidemic vectors of RVF, creating a complex epidemiologic pattern in East Africa. The recent RVF outbreaks in Somalia (2006-2007), Kenya (2006-2007), Tanzania (2007), and Sudan (2007-2008) showed extension to districts, which were not involved before. These outbreaks also demonstrated the changing epidemiology of the disease from being originally associated with livestock, to a seemingly highly virulent form infecting humans and causing considerably high-fatality rates. The amount of rainfall is considered to be the main factor initiating RVF outbreaks. The interaction between rainfall and local environment, i.e., type of soil, livestock, and human determine the space-time clustering of RVF outbreaks. Contact with animals or their products was the most dominant risk factor to transfer the infection to humans. Uncontrolled movement of livestock during an outbreak is responsible for introducing RVF to new areas. For example, the virus that caused the Saudi Arabia outbreak in 2000 was found to be the same strain that caused the 1997-98 outbreaks in East Africa. A strategy that involves active surveillance with effective case management and diagnosis for humans and identifying target areas for animal vaccination, restriction on animal movements outside the affected areas, identifying breeding sites, and targeted intensive mosquito control programs has been shown to succeed in limiting the effect of RVF outbreak and curb the spread of the disease from the onset.

Keywords: Aedes mosquitoes; East Africa; RVFV outbreaks; rainfall.

Figure 1

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. Source: Balenghien et al. (34), with permission from Thomas Balenghien, CIRAD, UMR Contrôle des maladies, F-34398 Montpellier, France.

Figure 2

Sudan map shows states with confirmed Rift Valley fever cases are in boldface during 2007 and 2010 outbreaks. Source: Aradaib et al. (8), with permission from Stuart T. Nichol, Centers for Disease Control and Prevention, Atlanta, GA, USA.

Figure 3

Distribution of village-level space-time clusters of RVF cases from 1947 to 1978. The authors set model parameters for maximum spatial and temporal window sizes and that such cluster could include a maximum of 50% of all cases. They indicated there were no clusters detected in 1930, from 1947 to 1978 three primary clusters were persistently detected in Ngorongoro district, each involving one village. An asterisk represents the center of cluster that involved more than one village; relative risk for each cluster is displayed (RR) along with the buffer (circle) size in kilometers (km). Source: Sindato et al. (68), with permission from Calvin Sindato, National Institute for Medical Research, Tabora, Tanzania.

Figure 4

Distribution of village-level space-time clusters of RVF cases in humans and domestic ruminants. The authors set model parameters for maximum spatial and temporal window sizes and that such cluster could include a maximum of 50% of all cases. They conducted the analysis of clustering of cases separately for humans and domestic ruminants during the 2006/2007 outbreak wave. Between January and February 2007, there was an overlap of livestock and human primary clusters in the same location. Asterisks correspond to villages that were included within human space-time clusters; relative risk for each cluster is displayed (RR) along with the buffer (circle) size in kilometers (km). Source: Sindato et al. (68), with permission from Calvin Sindato, National Institute for Medical Research, Tabora, Tanzania.

Figure 5

Cases (No. received the treatment) and deaths from RVF over the period of the outbreak in Gezira State, Sudan, September–December 2007. Source: Epidemiology Unit, Ministry of Health, Gezira State.