Spatial Risk Assessment of Rift Valley Fever in Senegal

Abstract

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.

Interepidemic Rift Valley Fever Virus Seropositivity, Northeastern Kenya

“…Our current study shows that large seroprevalence discrepancies can also occur over short distances. Spatial risk assessments of RVF in animals in Senegal have been predicted by using measurements of seasonal rainfall, land surface temperature, distance to perennial water bodies, and time of year (18). Designing such risk maps with human risk factor data may enable improved surveillance systems and better prediction of the spatial distribution of RVFV.…”

Rift Valley Fever Epidemiology, Surveillance, and Control: What Have Models Contributed?

“…Not detecting these cases by monitoring NDVI anomalies could be explained by the fact that either local NDVI anomalies were below the EWS threshold (i.e., lack of sensitivity), or that emergence of these cases was associated with factors other than heavy rainfall. In Senegal, the existence of potential local factors involved in RVF virus circulation has been suggested (Clements et al 2007b). In the Ferlo region of Senegal, these local factors could be specific rainfall patterns producing temporary ponds, alternating conditions of humidity and drought favoring Aedes egg hatching .…”

“…Although the data used were not primarily collected for this purpose, the final risk maps allowed the identification of areas for potentially targeted sentinel surveillance. Finally, the presence of residual small-scale spatial variation suggested that additional spatial local risk factors might influence disease distribution (Clements et al 2007b).…”

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“…The methods of serological data collection are published in detail elsewhere [21]. Validation of the epidemic suitability map was undertaken by visually comparing the suitability estimates to a map of the locations of known RVF epidemics that occurred during the 20 th century, in addition to the observed serological data in Senegal.…”