In two areas of Senegal where previous evidence of Rift Valley fever (RVF) virus circulation was detected, Barkedji in the Sahelian bioclimatic zone and Kedougou in the Sudano-Guinean zone, a longitudinal study of the enzootic maintenance of RVF virus was undertaken from 1991 to 1993. Mosquitoes, sand flies, and ticks were collected and domestic ungulates were monitored with serologic surveys. Rift Valley fever virus was not isolated in Kedougou. In Barkedji, RVF virus was isolated from Aedes vexam and Ae. ochraceus mosquitoes collected in traps near ground pools and cattle droves and from one healthy sheep. Sand flies were not involved in the maintenance cycle. Seroconversions were recorded in three (1.9%) of 160 monitored sheep and goats. The interepizootic vectors appeared to belong to the Aedes subgenus Neomelaniconion in East Africa, and to the subgenus Aedimorphus in West Africa. Epizootics in East Africa are associated with an increase in rainfall. However, factors associated with epizootics remain unknown for West Africa.

During September–October 2010, an unprecedented outbreak of Rift Valley fever was reported in the northern Sahelian region of Mauritania after exceptionally heavy rainfall. Camels probably played a central role in the local amplification of the virus. We describe the main clinical signs (hemorrhagic fever, icterus, and nervous symptoms) observed during the outbreak.

Serologic incidence was estimated at 2.9%.

No abstract

BackgroundRift Valley fever (RVF) is a vector-borne viral zoonosis of increasing global importance. RVF virus (RVFV) is transmitted either through exposure to infected animals or through bites from different species of infected mosquitoes, mainly of Aedes and Culex genera. These mosquitoes are very sensitive to environmental conditions, which may determine their presence, biology, and abundance. In East Africa, RVF outbreaks are known to be closely associated with heavy rainfall events, unlike in the semi-arid regions of West Africa where the drivers of RVF emergence remain poorly understood. The assumed importance of temporary ponds and rainfall temporal distribution therefore needs to be investigated.Methodology/Principal FindingsA hydrological model is combined with a mosquito population model to predict the abundance of the two main mosquito species (Aedes vexans and Culex poicilipes) involved in RVFV transmission in Senegal. The study area is an agropastoral zone located in the Ferlo Valley, characterized by a dense network of temporary water ponds which constitute mosquito breeding sites.The hydrological model uses daily rainfall as input to simulate variations of pond surface areas. The mosquito population model is mechanistic, considers both aquatic and adult stages and is driven by pond dynamics. Once validated using hydrological and entomological field data, the model was used to simulate the abundance dynamics of the two mosquito species over a 43-year period (1961–2003). We analysed the predicted dynamics of mosquito populations with regards to the years of main outbreaks. The results showed that the main RVF outbreaks occurred during years with simultaneous high abundances of both species.Conclusion/SignificanceOur study provides for the first time a mechanistic insight on RVFV transmission in West Africa. It highlights the complementary roles of Aedes vexans and Culex poicilipes mosquitoes in virus transmission, and recommends the identification of rainfall patterns favourable for RVFV amplification.

Eighteen strains of Rift Valley fever (RVF) virus collected over a period of 38 years and isolated from diverse localities in Africa and from various hosts (human, animal and arthropod) were investigated by RT-PCR followed by sequencing of the NS S protein coding region. This region was chosen to analyse variability because, in contrast to the N protein, the NS S protein differs in various phleboviruses and there exists an RVF virus (clone 13) in which 70 % of the NS S ORF is deleted, suggesting that this sequence is under a weak selective pressure. Sequence data indicated that percentage di-

Rift Valley fever virus (RVFV), a phlebovirus of theBunyaviridae family, is an arthropod-borne virus which emerges periodically throughout Africa, emphasizing that it poses a major threat for animal and human populations. To assess the genetic variability of RVFV, several isolates from diverse localities of Africa were investigated by means of reverse transcription-PCR followed by direct sequencing of a region of the small (S), medium (M), and large (L) genomic segments. Phylogenetic analysis showed the existence of three major lineages corresponding to geographic variants from West Africa, Egypt, and Central-East Africa. However, incongruences detected between the L, M, and S phylogenies suggested that genetic exchange via reassortment occurred between strains from different lineages. This hypothesis, depicted by parallel phylogenies, was further confirmed by statistical tests. Our findings, which strongly suggest exchanges between strains from areas of endemicity in West and East Africa, strengthen the potential existence of a sylvatic cycle in the tropical rain forest. This also emphasizes the risk of generating uncontrolled chimeric viruses by using live attenuated vaccines in areas of endemicity.

To investigate p d t infection in and transmission of Rift Valley fever (RVF) virus to humans within an endemic focus, we undertook a retrospective cohort study of the seminomadic Peul people living in sub-Saharan northcentral Senegal. Residents of the rural settlement of Yonofere five years of age or older were studied during February-May 1989. Anti-RVF virus IgG was found in blood samples of 22.3% of 273 persons who responded to a standard questionnaire; none had IgM antibodies. Seropositivity was similar for males (25.4%) and females (21.1%), increased markedly with age for both sexes, and varied considerably among compounds (groups of huts) (O-37.5%). F s k factors for past RVF virus infection were nursing sick people, assisting animals dunig abortionshirths, and treating sick animals. In all age groups, odds ratios (ORs) for RVF viral antibody among females who reported treating sick animals were three to six times greater than for those who did not. The ORs for males who reported assisting with animal birthdabortiom and nursing sick people were approximately five times those for males who did not. Se-rologic prevalence of RVF viral antibody among sheep averaged 30.1% overall (0.8% IgM), but varied among compounds (O-66.7%) in a manner different from that of humans. The seasonal abundance and relative density of potential mosquito vectors were estimated by monthly samples captured in Centers for Disease Control and Prevention-type traps. Mosquito abundance varied seasonally with rainfall (> 90% captures during four months). Species diversity was large (28 spp.), dominated by Aedes and Culex. Rift Valley fever virus was bot isolated from 142 pools of 2,956 unengorged mosquitoes tested, although three ot&rr'-grboviruses were found. Results indicate that RVF is endemic in this region, people are at considerable risk of infection, and that a heretofore unrecognized mode of human infection under nonepizootic conditions may be transmission via contact with infected animals or humans.