Epidemiology of Saint Louis encephalitis virus in the Brazilian Amazon region and in the State of Mato Grosso do Sul, Brazil: elevated prevalence of antibodies in horses

The immunity of horses (n = 1401) against Saint Louis encephalitis virus (SLEV) was investigated in the Brazilian Amazon region (Braganca/Para, Salvaterra/Para, Macapa/Amapa and Rio Branco/Acre) and Maracaju, State of Mato Grosso do Sul, by the hemagglutination inhibition (HI) and plaque reduction neutralization (PRNT) tests. HI and neutralizing antibodies specific (monotypic reactivity, MR) for SLEV and other flaviviruses included in the tests were detected, as was cross-reactivity (CR) against flaviviruses. In the HI test, MR was observed in 248 (17.7%) serum samples, 137 of which were (55.2%) against SLEV; CR was detected in 380 (27.1%). The frequency of MR against SLEV was significantly higher in Macapa and CR was significantly higher in Salvaterra. In the PRNT, neutralization of SLEV was observed in 713 (50.9%) samples, and the prevalence of neutralizing antibodies was significantly higher in Macapa than in Salvaterra (p = 0.0083). This study adds new data regarding the immunity of horses against SLEV in Brazil, and it confirms the wide distribution of SLEV and the diversity of flaviviruses in the country, as well as the apparent absence of disease in SLEV-infected horses. Encephalitis, St. Louis; Horses; Serologic Tests; Encephalitis, Arbovirus.