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Prevalence of antibody against West Nile virus in volunteer blood donors living in southeastern France

1320 TRANSFUSION Volume 41, October 2001 www.transfusion.org Prevalence of antibody against West Nile virus in volunteer blood donors living in southeastern France Since 1996, West Nile virus (WNV; family Flaviviridae, genus Flavivirus) has been repeatedly responsible for epidemics in Romania, the United States, Israel, and Russia.1 The Camargue region of southeastern France includes the cities of Arles and Avignon and a large protected wildlife area (national park) mainly composed of marshes. It is recognized that its ecosystem favors the circulation of multiple arthropod-borne viruses.2 In particular, WNV has been circulating for at least 40 years, vectored by Culex mosquitoes during the warm season and potentially reintroduced seasonally by birds, including flamingoes, migrating from Africa.3 In the literature, there are data concerning equine epizootics in the Camargue in 1942, 1962, and 1964-1965.2 From April to November 2000, an important new epizootic occurred, affecting a large number of horses. While we are unaware that WNV has been transmitted by the transfusion of blood components, the possibility of such transmission during a donor’s viremic phase is at least theoretically possible. We present the results of a serologic study of WNV in blood donors living in southeastern France that may be useful in establishing the laboratory and epidemiologic framework for similar studies in other geographic areas. At the end of the equine epizootic, we investigated the seroepidemiology of WNV infection in two groups of blood donors living in contiguous regions of southeastern France. Cohort A included 1053 blood donors (mean age, 41.2; range, 1880) living in the Camargue, in the areas of the 2000 equine epizootic (Region A). Cohort B included 1104 blood donors (mean age, 43.4; range, 18-74) living outside the region of the WNV epizootic, in areas located to the east of Region A and characterized by the absence of marshy zones where mosquitoes could breed (Region B). Sera of both cohorts were collected between September and November 2000 and tested by IgG and IgM ELISA immunocapture assays as described by Saluzzo et al.,4 using a WNV antigen and an anti-WNV mouse polyclonal, hyperimmune, ascitis fluid prepared from the prototype strain Eg-101. As shown in Table 1, none of the blood donors’ sera were positive for IgM, whereas the prevalence of IgG antibody to WNV was significantly higher in Cohort A than in Cohort B (1.42% vs. 0.18%; p<0.005). Four dilutions (1-in-10, 1-in-40, 1in-160, 1-in-640) of each IgG-positive serum were tested in quadruplicate for their capability to neutralize the multiplication in Vero cells of a WNV strain, titrated Eg-101 solution (seroneutralization test), by reference to a control serum. Under the specific conditions of the assay, the control serum (drawn from a sick horse and positive for IgG and IgM antibody against WNV) exhibited a neutralizing activity at the 1in-160 dilution. Therefore, a neutralizing activity observed at a dilution greater than or equal to 1-in-160 was considered positive for human sera, and it was shown that eight blood donors in Cohort A and none in Cohort B (p<0.005) had an indisputable history of infection by WNV. Together, these results indicate that populations living within Region A (including vast areas that are covered by marshes and known for an extremely intense mosquito activity during the warm season) present a risk of being infected by WNV that is significantly higher than that of people living in Region B (Table 1). Within both cohorts, no relationship could be established between the serologic status and age or sex. In conclusion, our data demonstrate that WNV is responsible for human infections in the Camargue and that the population of volunteer blood donors is exposed to the risk of infection. The fact that none of the blood donors with antibody to WNV had detectable anti-WNV IgM suggests that they were infected before the epizootic episode of the summer of 2000; this confirms that WNV activity is recurrent in the Camargue. During the summer of 2000, a study conducted with patients hospitalized for meningitis and meningoencephalitis at the University Hospitals of Marseilles did not indicate that WNV was the etiologic agent of these clinical manifestations.5 However, it is known that a substantial proportion (>95%) of patients (in particular, young adults) infected by WNV may remain asymptomatic or develop a mild disease characterized by flu-like manifestations.3 Therefore, an efficient epidemiologic survey of WNV infection in humans should not be limited to severe central nervous system manifestations. Including blood donors in a sentinel system for the detection of WNV activity would provide additional and, perhaps, more pertinent information on the potential risk of transmitting WNV infection via blood transfusion. L E T T E R S T O T H E E D I T O R