An epizootic of equine viral arteritis at a Thoroughbred racetrack in Illinois: 223 cases

An epizootic of equine viral arteritis (EVA) occurred over an 8-wk period in the summer of 1993 at a midwestern Thoroughbred (TB) racetrack that ceased operations in 2021. Based on clinical signs, viral isolation, and serology, 223 horses (213 TBs, 10 lead ponies) were affected. Consistent clinical signs included fever (38.3–40.6°C; 101–105°F), depression, inappetence, and diffuse urticaria, including distal limb and preputial edema. Equine arteritis virus (EAV) was cultured from buffy coat samples from 24 of 28 acutely ill horses. Seventeen of 18 convalescent horses and 1 of 92 non-affected horses were EAV-positive on virus neutralization testing. Geldings (OR 2.08; p < 0.001) and 2-y-olds (OR 2.33; p < 0.001) were at greater risk than intact males and older horses, respectively. The source of infection was traced to 2 horses that earlier had been hospitalized for unrelated elective procedures at a nearby veterinary referral clinic. Initially, 2 of 33 barns were affected. Subsequent relocation of horses resulted in secondary spread to 7 additional barns of similar design. All EVA-affected barns were of one design; no EVA cases occurred in horses in barns with an older, better-ventilated design despite the transfer of horses from EVA-affected barns into those barns. Isolation of acute cases, restriction of movement of horses, and vaccination were combined effectively to halt the epizootic while permitting racing to continue. Thirty-nine cases developed despite EAV vaccination. Of 213 affected TBs, 58% returned to racing within 60 d of the last clinical case (1993 Aug 21).