Identifying Host-Characteristics and Management Risk Factors in a California Equine Herpesvirus Myeloencephalopathy (EHM) Outbreak

Equine alphaherpesvirus-1 (EHV-1) is a highly contagious virus that can cause the neurological form, equine herpesvirus myeloencephalopathy (EHM). Understanding transmission-related risk factors is crucial for improving prevention strategies and guiding effective control measures. In this study, we collected data from 63 horses that had previously participated in the February 2022 winter horse show season at the Desert International Horse Park (DIHP) (26 cases and 37 controls) to identify host and management factors associated with EHV-1 infection and/or EHM development during the February 2022 outbreak at the DIHP in Thermal, California. Risk factors were evaluated using multivariable logistic regression and a random forest model with conditional permutation importance. Greater age was associated with higher odds of becoming a case (OR = 1.33; 95%CI: 1.04–1.69, p-value: 0.01). Compared with hunters, jumpers had greater odds of developing EHV-1 and/or EHM (OR = 7.37; 95%CI: 1.57–34.61, p-value: 0.01). Sharing a barn was also strongly associated with EHV-1 and/or EHM case status (OR = 7.37; 95%CI: 1.79–30.29, p-value: