Risk factors for development of neurologic disease after experimental exposure to equine herpesvirus-1 in horses.

OBJECTIVE: To identify risk factors associated with development of clinical neurologic signs in horses exposed to equine herpesvirus-1 (EHV-1). ANIMALS: 36 adult horses. PROCEDURES: Blood samples collected before and after challenge inoculation with nonneuropathogenic or neuropathogenic EHV-1 were analyzed for leukocyte-associated viremia, serum neutralizing antibody, and EHV-1-specific cytotoxic T-lymphocyte precursors (CTLPs). Associations between variables and neurologic disease and correlations between age category or breed and development of neurologic disease were examined.
RESULTS: 9 horses developed CNS signs (ataxia, hind limb paresis or paralysis, bladder atony, or recumbency). Neurologic deficits were correlated with infection by a neuropathogenic strain of EHV-1, age>20 years, high postexposure viremic load, and low preexposure concentration of CTLPs. No significant correlations were observed between preinfection titers or horse breed and postinfection development of neurologic signs. CONCLUSIONS AND CLINICAL RELEVANCE: Horses with high concentrations of preexisting CTLPs, regardless of age, strain of virus, or titer, were more likely to control the magnitude of postinfection leukocyte-associated viremia and subsequent development of neurologic disease; therefore, CTLPs appear to be a critical requirement for protective immunity against EHV-1-induced myeloencephalopathy. The importance of achieving immunity related to high concentrations of vaccine-induced CTLPs in horses at high risk for exposure to neuropathogenic strains of EHV-1 is indicated.