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Introduction: In the period of 1996-1999 some virus associated with encephalitis have been reported in horses from different regions of Paraná State, Brazil. To identify the etiologic agent associated with this illness, mosquitoes and serum samples were collected in the endemic area., Methods: The study area corresponded to four municipalities of Paraná State, Brazil. Mosquitoes were captured in Shannon trap and human bait. After identification, they were processed for virus isolation. Blood of equines were collected in the municipalities of Querência do Norte and Colorado. Antibodies to different Alphavirus and Flavivirus were analyzed by hemagglutination inhibition test. Specific seroneutralization reactions were performed in those sera with a positive reaction in the hemagglutination test., Results: The mosquitoes genus collected were: Culex, Aedes, Mansonia, Coquillettidia, Psorophora, Sabethes, Wyeomyia, and Limatus. Even thought no virus was isolated, serologic analyses showed hemagglutinazing antibodies to Eastern equine encephalitis, Mucambo, Pixuna, Maguari, and St Luis encephalitis viruses. The neutralization test showed specific reaction to Eastern equine encephalitis virus in 12 tested sera., Conclusions: Species of mosquitoes that could be potential vectors of encephalitis, buniavirus, and other arboviruses of epidemiological importance were collected. It is believed that Eastern equine encephalitis virus affected the equines populations in the study regions because of the symptoms and antibodies for the virus in the sera detected in these equines.