Ataxia and Paresis with Equine Herpesvirus Type 1 Infection in a Herd of Riding School Horses

An outbreak of neurologic disease associated with serologic evidence of equine herpesvirus type 1 (EHV-1) infection occurred in a herd of 46 riding school horses. Ataxia and paresis were observed in 14 geldings and 5 barren mares. Eight affected horses had distal limb edema, 1 horse had a head tilt, and 3 others had urinary incontinence. Other clinical signs included fever, depression, and inappetance in 30 horses. Seven horses with neurologic signs were treated with acyclovir. Serum neutralizing antibody titers against EHV-1 increased 4-fold between acute and convalescent samples or exceeded 1:256 in 19 of 44 horses, confirming recent infection. A significantly greater proportion of horses that seroconverted were mares (P =.014). Of the 19 horses exhibiting ataxia and paresis, 17 made a complete recovery, 1 made a partial recovery, and 1 was euthanized.