PASSIVE TRANSFER, RATE OF DECAY, AND PROTEIN SPECIFICITY OF ANTIBODIES AGAINST EQUINE ARTERITIS VIRUS IN HORSES FROM A STANDARD-BRED HERD WITH HIGH SEROPREVALENCE

Objective-To determine rate of decay of passively acquired antibodies in Standardbred foals on a farm with a high seroprevalence to equine a rteritis virus (EAV) and to determine whether vertical or horizontal t ransmission of the virus was responsible for infection on the farm. De sign-Repeated-measures study. Animals-46 Standardbred horses (15 brood mares and their foals, 5 stallions, and 11 young horses). Procedure-S erum samples obtained from horses on the farm were evaluated by serum neutralization and western immunoblot analysis to detect EAV-specific antibodies. The half-life of passively acquired antibodies in foals wa s estimated by use of regression analysis. Results-Most (14/15) of the mares evaluated were seropositive to EAV. After suckling, their foals were also seropositive. Mean biological half-life for passively acqui red antibodies in serum samples obtained from foals was 32 days (r(2) = 0.61). The foal born to a seronegative dam and all 11 young horses f rom the farm were seronegative to EAV. At least 2 of 5 stallions on th e farm were persistently infected carriers that were shedding virus in their semen. Immunoblot analysis of seropositive serum samples most c onsistently recognized the M protein of EAV. Clinical Implications-Ana lysis of these data indicated that a modified-live EAV vaccine can be administered to foals after they are 8 months old without risk of inte rference from maternal antibodies, regardless of serologic status of t he foal's dam. Horizontal transmission of EAV via the respiratory trac t apparently was uncommon on the farm, indicating that mares primarily were infected by venereal transmission of virus from carrier stallion s.