Serologic studies in 24 ponies indicated that prevaccination
antibodies to
Venezuelan equine encephalomyelitis (VEE) virus (strain TC-83) had no influence on hemagglutination-inhibition (HI) antibody stimulation by
western equine encephalomyelitis (WEE) or eastern equine encephalomyelits (EEE)-WEE
vaccines. However, studies of the effects of VEE
neutralizing antibodies on
neutralizing antibody stimulation by the heterologous alphavirus
vaccines were inconclusive. The VEE, WEE, and EEE antibody responses were studied in 18 VEE-vaccinated (strain TC-83) animals (13 ponies and 5 horses) at 9 to 14 months after challenge of immunity with virulent VEE virus. The VEE HI, and
neutralizing antibodies in combination were associated with greater suppression of VEE antibody stimulation than were VEE
neutralizing antibodies alone. The VEE geometric mean antibody titers (GMT) increased 26- and 64-fold for HI
antibodies and for
neutralizing antibodies, respectively, in 5 animals that had prechallenge exposure titers of VEE
neutralizing antibodies only. In contrast, the GMT increased only threefold and eightfold for VEE HI
antibodies and for
neutralizing antibodies, respectively, in 8 animals that had prechallenge exposure titers of both VEE HI and
neutralizing antibodies. Five animals which were seronegative for prechallenge VEE
antibodies and had survived challenge exposure gave primary VEE-immune response. Protection in these animals may have resulted from high-titer cross-reacting
antibodies to WEE or EEE viruses, or both. Changes in the GMT for WEE HI, neutralizing, and
complement-fixation
antibodies after challenge exposure were statistically insignificant. However, a statistically significant increase in the EEE HI-GMT occurred in ponies that had low (or no) prechallenge exposure titers of VEE
antibodies.