Antibody studies in ponies vaccinated with Venezuelan equine encephalomyelitis (strain TC-83) and other alphavirus vaccines.

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.