Objective: To determine rate of decay of passively acquired antibodies in Standardbred foals on a farm with a high seroprevalence to equine arteritis virus (EAV) and to determine whether vertical or horizontal transmission of the virus was responsible for infection on the farm.
Design: Repeated-measures study.
Animals: 46 Standardbred horses (15 brood mares and their foals, 5 stallions, and 11 young horses).
Procedure: Serum 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 was 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 acquired antibodies in serum samples obtained from foals was 32 days (r2 = 0.61). The foal born to a seronegative dam and all 11 young horses from the farm were seronegative to EAV. At least 2 of 5 stallions on the farm were persistently infected carriers that were shedding virus in their semen. Immunoblot analysis of seropositive serum samples most consistently recognized the M protein of EAV.
Clinical implications: Analysis of these data indicated that a modified-live EAV vaccine can be administered to foals after they are 8 months old without risk of interference from maternal antibodies, regardless of serologic status of the foal's dam. Horizontal transmission of EAV via the respiratory tract apparently was uncommon on the farm, indicating that mares primarily were infected by venereal transmission of virus from carrier stallions.