Serological Evidence of Widespread Circulation of West Nile Virus and Other Flaviviruses in Equines of the Pantanal, Brazil

Abstract

A recent study reported neutralizing antibodies to West Nile virus (WNV) in horses from four ranches of southern Pantanal. To extend that study, a serosurvey for WNV and 11 Brazilian flaviviruses was conducted with 760 equines, 238 sheep and 61 caimans from 17 local cattle ranches. Among the tested equines, 32 were collected from a ranch where a neurologic disorder outbreak had been recently reported. The sera were initially screened by using a blocking ELISA and then titrated by 90% plaque-reduction neutralization test (PRNT₉₀) for 12 flaviviruses. Employing the criterion of 4-fold greater titer, 78 (10.3%) equines were seropositive for Ilheus virus, 59 (7.8%) for Saint Louis encephalitis virus, 24 (3.2%) for WNV, two (0.3%) for Cacipacore virus and one (0.1%) for Rocio virus. No serological evidence was found linking the neurological disease that affected local equines to WNV. All caimans and sheep were negative by blocking ELISA for flaviviruses. There were no seropositive equines for Bussuquara, Iguape, Yellow fever and all four Dengue virus serotypes. The detection of WNV-seropositive equines in ten ranches and ILHV and SLEV-seropositive equines in fourteen ranches of two different sub-regions of Pantanal is strong evidence of widespread circulation of these flaviviruses in the region. West Nile virus is maintained in cycles between birds and mosquitoes and recently reemerged as a worldwide major public health and veterinarian concern as the cause of human and equine encephalitis outbreaks. Recent studies have reported serological evidence of West Nile virus circulation in Pantanal, west-central region of Brazil. However, considering the co-circulation of various cross-reactive flaviviruses in Brazil and that most of the flaviviruses isolated in Brazil are unknown or understudied in the Pantanal, serological results should be interpreted with caution. Therefore, we conducted a serosurvey for West Nile virus and the 11 Brazilian flaviviruses of potential medical importance, utilizing equines, sheep and caimans as indicators, including 32 equines collected from a ranch where a neurologic disorder had been recently reported among the equines. We found serological evidence of Ilheus, Saint Louis encephalitis, West Nile, Cacipacore and Rocio viruses in Pantanal equines. West Nile virus infection was not associated with the neurological disease of equines. The detection of WNV-seropositive equines in ten ranches and ILHV- and SLEV-seropositive equines in fourteen ranches of two different sub-regions of Pantanal is strong evidence of widespread circulation of these flaviviruses in the region.