Induction of immune response and protection from equine viral arteritis (EVA) by formalin inactivated-virus vaccine for EVA in horses.

Thirty-nine horses included 3 pregnant mares were examined by inoculating with formalin inactivated-virus vaccine for EVA. Antibody response of horses after one dose vaccination was somewhat poor and 50% effective inoculum dose of the vaccine should be included 10(8.4) pfu of virus before inactivation. After 2 doses given at an interval of 4 weeks, the horses developed such high titer of SN antibody as up to 1:5,120. The SN titer declined rather rapidly, but supplemental administration of the vaccine at an interval of more than 2 months elicited a prompt antibody response and SN titers persisted as 1:80 to 1:320 at 6 months after the administration. Therefore, supplemental administration of the vaccine as booster every 6 months or 1 year would be capable of maintaining high titer of SN antibody. The inactivated-virus vaccine prevented horses from clinical disease of EVA and protected pregnant mares from abortion by challenge exposure with virulent virus. Fifty percent protective dose in SN titer of 1:43 was confirmed by clinical signs and viremia.