During the course of investigation to determine the vector of sylvan yellow fever in Honduras, an unidentified neurotropic virus was isolated from mosquitoes of the genus Psorophora (Rodaniche, 1956). On the basis of cross-immunity and neutralization tests it was determined that this virus was not that of yellow fever. Unfortunately, other appropriate virus strains or immune serums were not available here for making a final classification. However, Dr. Jordi Casals, of the Rockefeller Institute, generously offered to carry out systematic immunological studies with a pooled mouse hyper-immune serum forwarded to his laboratory, and found the virus to be “identical with or closely related to Ilhéus.” Animal pathogenicity and other properties of the Honduran strain as observed in this laboratory also conform to those of Ilhéus.
The virus of Ilhéus encephalitis was isolated for the first time in 1944 by Laemmert and Hughes (1947) from a pool of mosquitoes of the genera Aedes and Psorophora captured in the vicinity of Ilhéus in the State of Baia, Brazil.