Rift Valley fever in small ruminants

During the 2003 rainy season, the clinical and serologic incidence of Rift Valley fever was assessed in small ruminant herds living around temporary ponds located in the semi-arid region of the Ferlo, Senegal. No outbreak was detected by the surveillance system. Serologic incidence was estimated at 2.9% (95% confidence interval 1.0–8.7) and occurred in 5 of 7 ponds with large variations in the observed incidence rate (0%–20.3%). The location of ponds in the Ferlo Valley and small ponds were correlated with higher serologic incidence (p = 0.0005 and p = 0.005, respectively). Rift Valley fever surveillance should be improved to allow early detection of virus activity. Ruminant vaccination programs should be prepared to confront the foreseeable higher risks for future epidemics of this disease.

Rift Valley fever (RVF) is an arbovirosis caused by a phlebovirus (Bunyaviridae). In ruminants, RVF causes mass abortions and deaths in newborn kids and lambs. Human disease is often limited to a flulike syndrome, but severe forms have been reported. In West Africa, domestic ruminants are the main hosts of the virus, which is transmitted between animals by mosquitoes, particularly those belonging to the Culex and Aedes genera. Transmission is mostly horizontal, but a vertical mode was described for some Aedes species. Human cases are mainly caused by virus exposure after abortion or slaughtering of viremic animals.

A large RVF epidemic occurred in 1987 in southern Mauritania, with >200 reported human deaths. In the following years, several animal and human outbreaks occurred in Mauritania, Senegal, which emphasizes the need for understanding and modeling the risk for RVF in this region before implementing more efficient surveillance and control measures. For this purpose, a survey was conducted in the pastoral area of the Ferlo in northern Senegal. During the rainy season, this agro-ecosystem depends on the availability of surface water in temporary ponds that are flooded after the first rainfalls. These ponds also constitute a favorable habitat for RVF vectors. Previous studies showed that Barkedji, a village located in the central part of the Ferlo, was an area with active viral circulation. The purpose of this study was to assess RVF activity in the area of Barkedji during the 2003 rainy season and to identify risk factors for its transmission to livestock.

Emerging Infectious Diseases
November 22, 2005

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