• Open Access

Case–Control Study of a Multistate Equine Herpesvirus Myeloencephalopathy Outbreak


  • Data collection for this study was performed in states with primary exposed horses. Study design, coordination, data analysis, and manuscript preparation were performed at USDA–APHIS–VS in Fort Collins, CO. This work was conducted as part of the regular duties of State Animal Health Officials and USDA–APHIS–VS personnel. No external funding supported the work. At the time of submission, this work has not been presented at a previous meeting.

Corresponding author: Dr. Josie Traub-Dargatz, Colorado State University College of Veterinary Medicine and Biomedical Sciences, Animal Population Health Institute (APHI), Department of Clinical Sciences, 300 West Drake, 1678 Campus Delivery, Fort Collins, CO 80523; e-mail: jtraub1@lamar.colostate.edu.



A large multistate outbreak of equine herpesvirus myeloencephalopathy (EHM) occurred in May 2011 among horses that participated in a competitive event.


To identify EHM risk factors among horses with a common exposure venue.


A total of 123 horses: 19 horses with EHM, 14 equine herpesvirus-1 cases with no reported neurologic signs, and 90 control horses.


EHM case survey data were compared with data from EHV-1 cases with no neurologic signs and healthy controls using univariable and multivariable methods.


Significant factors associated with higher risk for EHM compared with EHV-1 cases with no neurologic signs were (1) greater number of biosecurity risks at the event, (2) female sex, (3) increasing number of classes competed in at the event, and (4) an interaction between sex and number of classes competed in. In the EHM versus controls comparison, in addition to sex and biosecurity risks, factors associated with higher EHM risk included EHV-1 vaccination in the 5 weeks before the event and increasing number of events attended in April 2011; zinc dietary supplementation was associated with decreased risk. An interaction between sex and the number of events attended in April 2011 also was significant.

Conclusions and Clinical Importance

Findings from this study suggest that dietary zinc supplementation may be associated with decreased risk of EHM. Several factors were associated with increased risk of EHM. Additional investigations of factors associated with risk of EHM are warranted to evaluate the importance of these factors in this complex disease of horses.