Circulation of Zoonotic Arboviruses in Equine Populations of Mallorca Island (Spain)

The presence of major arbovirus vector species, climate change that promotes the expansion and increase of their populations, and potential animal reservoirs mean that vector-borne diseases represent a significant health risk for Mallorca's inhabitants. Microbiological monitoring of circulating arboviruses, particularly flaviviruses causing encephalitis, was initiated using domestic horses from localities near wetlands as sentinel hosts. A total of 291 blood samples were taken from 172 horses between 2011 and 2012, using paired samples to highlight seroconversion events. A multiplex immunoassay and confirmatory reference serological assays were used to screen sera for immunoglobulin G antibodies against West Nile (WNV), Usutu (USUV), and tick-borne encephalitis (TBEV) viruses. The seroprevalence was 6.4% (confidence interval [95% CI] 3.2%-11.0%) for WNV, 1.2% (95% CI 0.1%-4.1%) for USUV, and 0.6% (95% CI 0.0%-3.2%) for TBEV. In addition, eight horses (4.6%; 95% CI 2.0%-8.9%) were found positive for unidentified flaviviruses. Seroconversion events were detected for WNV and USUV, reflecting recent arboviral infections. These results highlight the active transmission of zoonotic arboviruses in Mallorca wetlands.