VECTOR-BORNE AND ZOONOTIC DISEASES Volume 7, Number 3, 2007 © Mary Ann Liebert, Inc. DOI: 10.1089/vbz.2006.0593 West Nile Virus Outbreak Detection Using Syndromic Monitoring in Horses A. LEBLOND, 1 P. HENDRIKX, 2 and P. SABATIER 1 ABSTRACT Recent outbreaks of West Nile virus–associated (WNV) diseases, both in the old World and Americas, underline the importance for early warning systems that rapidly identify emerging and re-emerging diseases and thus help in their control. Traditional approaches of disease monitoring become less reliable and increasingly costly when used for rare health-related events, such as WNV outbreaks in southern France. The objective of this work was to discuss methodological issues related to syndromic monitoring of WNV-associated disease in Camargue horses by veterinary practitioners. Tracking cases of equine encephalitis by veterinarians is an example of such syndromic monitoring of an emerging disease. Signs of illness, observed prior diagnostic confirmation, can be of interest be- cause they may provide an early warning for WNV circulation in a given area and allow authorities to take ap- propriate preventive measures for public health. Key Words: Monitoring—Epizootic—Syndrome—West Nile virus—Veterinary practitioners—Horses. Vector-Borne Zoonotic Dis. 7, 000–000. 403 INTRODUCTION W EST NILE VIRUS (WNV)–associated disease is a mosquito-borne flaviviral infection transmitted in natural cycles between birds and mosquitoes. Infection can cause febrile, some- times fatal, illnesses in horses, birds and hu- mans (Komar 2000). The frequency and sever- ity of WNV outbreaks have increased during recent decades. In 1999, the virus reached the North American continent, with subsequent rapid geographic spread resulting in 3000 hu- man cases with 119 deaths in 2005 (for United States Geological Survey 2006, see http://west- nilemaps.usgs.gov/). In Europe, even if WNV outbreaks are of little concern for public health, they can have an important economic impact. Tourist activity tends to fall when the virus cir- culates along the Mediterranean coast during the end of summer. In the south of France, out- breaks of WNV were observed in horses in 2000, 2003 and 2004, and human cases were di- agnosed east of Marseille in 2003. Serological studies conducted in 2000 among blood donors living in the South of France showed a signifi- cant level of WNV-IgG and IgM prevalence (Gallian et al. 2005). At the present, even though circulation of the virus among horses or in the avian population does not always pre- dict an outbreak in the surrounding human population, the French blood banks exclude blood donors living in areas at risk from July to October. For these reasons, public authori- ties are interested in implementing reliable monitoring systems that would be capable of identifying rapidly WNV outbreaks, and thereby helping in their control. The WNV outbreak in New York State (NYS), in the summer and fall of 1999, resulted in an increasing number of dead birds. So, the 1 UMR CNRS 5525 TIMC, Unit Environnement et Prévisions de la Santé des Populations, Ecole Nationale Vétéri- naire de Lyon, Marcy l’Etoile, France. 2 Direction des Services Vétérinaires du Gard, Nîmes, France.