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

## TL;DR

This study identifies risk factors for equine herpesvirus myeloencephalopathy outbreaks in California, focusing on age, horse activity, and barn-sharing practices.

## Contribution

The study provides new insights into specific host and management risk factors for EHM outbreaks using both statistical and machine learning approaches.

## Key findings

- Older horses had higher odds of developing EHM (OR = 1.33).
- Jumpers had significantly higher odds of EHM compared to hunters (OR = 7.37).
- Sharing a barn with horses from different home barns increased EHM risk (OR = 7.37).

## Abstract

Despite extensive efforts to mitigate the spread of equine alphaherpesvirus-1 and equine herpesvirus myeloencephalopathy, including new vaccination strategies and treatment alternatives, the frequency of outbreaks reported worldwide has been increasing. The objectives of this study were to identify the main risk factors associated with the 2021–2022 outbreak of equine herpesvirus myeloencephalopathy in Thermal, California, and provide recommendations to mitigate the risk of future outbreaks. Sixty-three questionnaires regarding host demographics, biosecurity, and diet management were obtained. We identified that increasing age, main activity (jumpers vs. hunters), and sharing a barn with horses from different home barns were associated with higher odds of becoming a case. These findings provide useful insights for refining biosecurity protocols against equine alphaherpesvirus-1 and equine herpesvirus myeloencephalopathy, particularly for horses that routinely participate in equestrian events.

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: <0.01). The machine-learning-based rankings were concordant with the regression estimates. Age, main activity, and sharing a barn were the most influential risk factors associated with elevated odds of developing EHV-1 and/or EHM. These results highlight specific demographic and management-related risk factors that could inform targeted prevention strategies.