Serological evidence of co-circulation of West Nile and Usutu viruses in equids from western Spain

Abstract

West Nile virus (WNV) is a mosquito-borne emerging virus in Europe with capacity to cause neurological complications such as encephalitis or meningoencephalitis in humans, birds or equids. In Spain, WNV is actively circulating in mosquitoes, birds and horses in different regions, but never has been deeply studied in Extremadura. Therefore, the aim of this study was to evaluate the seroprevalence of WNV in equids of those areas and to analyse the risk factors associated with exposure to the virus. A total of 199 out of 725 equids presented antibodies against WNV by competition ELISA (27.45%), while 22 were doubtful (3.03%). Anti-WNV IgM antibodies were detected in 16 equids (2.21%), and 3 animals were doubtful (0.41%). All ELISA-reactive positive/doubtful sera (N = 226) were further tested by micro-virus neutralization test (VNT), and a total of 143 horses were confirmed as positive for WNV, obtaining a seroprevalence of 19.72% in equids of western Spain. In addition, specific antibodies against USUV were confirmed in 11 equids. In 24 equids, a specific flavivirus species (detected by ELISA test) could not be determined. The generalized linear mixed-effects models showed that the significant risk factors associated with individual WNV infection in equids were the age (adults) and hair coat colour (light), whereas in USUV infections, it was the breed (pure). Data demonstrated that WNV and USUV are circulating in regions of western Spain. Given the high WNV seroprevalence found in equids from the studied areas, it is important to improve the surveillance programmes of public health to detect undiagnosed human cases and to establish a vaccination programme in equid herds in these regions.

El virus del Nilo Occidental (VNO) es un virus emergente en Europa transmitido por mosquitos con capacidad para causar complicaciones neurológicas como encefalitis o meningoencefalitis en humanos, aves o équidos. En España, el VNO circula activamente en mosquitos, aves y caballos en diferentes regiones, pero nunca ha sido estudiado en profundidad en Extremadura. Por ello, el objetivo de este estudio fue evaluar la seroprevalencia del VNO en équidos de estas zonas y analizar los factores de riesgo asociados a la exposición al virus. Un total de 199 de 725 équidos presentaron anticuerpos frente al VNO por ELISA de competición (27,45%), mientras que 22 fueron dudosos (3,03%). Se detectaron anticuerpos IgM anti-VNO en 16 équidos (2,21%), y 3 animales eran dudosos (0,41%). Todos los sueros positivos/dudosos reactivos a ELISA (N = 226) se analizaron posteriormente mediante la prueba de neutralización de microvirus (VNT), y un total de 143 caballos se confirmaron como positivos a VNO, obteniéndose una seroprevalencia del 19,72% en équidos del oeste de España. Además, se confirmaron anticuerpos específicos contra el VNU en 11 équidos. En 24 équidos no se pudo determinar una especie específica de flavivirus (detectada mediante la prueba ELISA). Los modelos lineales generalizados de efectos mixtos mostraron que los factores de riesgo significativos asociados a la infección individual por VNO en équidos eran la edad (adultos) y el color de la capa de pelo (claro), mientras que en las infecciones por USUV lo era la raza (pura).