CROSS-PROTECTION IN ANIMALS INFECTED WITH GROUP A ARTHROPOD-BORNE VIRUSES

The phenomenon of cross-protection among Venezuelan equine encephalomyelitis VEE, Eastern equine encephalomyelitis EEE and Semliki Forest SF viruses in a variety of laboratory animals was investigated after immunization by the intraperitoneal, subcutaneous, or respiratory route the last was effected by exposing the animals to aerosols of virus. One injection of an attenuated strain of VEE 9t protected guinea pigs against a lethal challenge dose of EEE or SF virus in guinea pigs and mice, respectively. Two injections of live SF virus protected guinea pigs against small doses of VEE or EEE virus. Mice vaccinated with 9t responded by demonstrating resistance mechanisms that appeared to operate in series. This consisted of, first, an early nonspecific interference phase, followed by a second, specific phase. The second phase also included a partially specific mechanisms of resistance of unknown origin and of relatively long duration, manifested as cross-protection in the group A viruses.