Cross-protective immunity between equine encephalomyelitis viruses in equids.

Eighteen equids were inoculated with eastern equine encephalomyelitis (EEE) and 18 equids with western equine encephalomyelitis (WEE) viruses to produce EEE virus- and WEE virus-immunized equids. Twelve surviving EEE virus-seropositive equids, 15 surviving WEE virus-seropositive equids, and 10 nonimmunized, seronegative equids (controls) were subsequently inoculated with an equine pathogenic (epizootic) strain of Venezuelan equine encephalomyelitis (VEE) virus to determine cross-protective immunity. Challenge infection produced 90% mortality in control (nonimmunized) equids, and 40% mortality in WEE virus-seropositive equids; all EEE virus-seropositive equids survived. Postchallenge exposure VEE viremia levels in EEE virus- or WEE virus-seropositive equids were lower than those in the 10 nonimmunized VEE virus-inoculated control equids. Plaque-neutralizing antibody responses to VEE virus in the EEE virus- and WEE virus-seropositive equids were similar in time of onset and titer to the antibody responses of nonimmunized equids. Neutralizing antibody to the third equine encephalomyelitis virus (either EEE virus or WEE virus) was detectable in 19 of 27 equids after inoculation with the challenge virus, VEE. Demonstration of cross-protective immunity between EEE or WEE virus and VEE virus in equids confirmed field observations made during the VEE epizootic in Texas in 1971.