EQUINE herpesvirus-1 (EHV-1) is an economically important pathogen affecting horses and exerts its major impact by inducing abortion storms or sporadic abortions in pregnant mares, early neonatal death in foals and respiratory disease in young horses (Ostlund 1993). Equine herpesvirus-1 myeloencephalopathy (EHM) is a relatively uncommon manifestation of EHV-1 infection but can cause devastating losses during outbreaks on individual farms or boarding stables (Lunn and others 2009). The factors determining whether horses develop EHM after EHV-1 infection are poorly understood. It has been proposed that the magnitude of cell-associated viremia is an important factor for the development of EHM because infection with the DNApol D752 neuropathogenic strain leads to a higher magnitude and duration of viremia (Goodman and others 2007, Allen 2008). Although all breeds of horses are susceptible to the neurologic form of EHV-1 infection, the authors are unaware of reports of neurologic EHV-1 clinically affecting donkeys and mules. However, donkeys and mules have shown seroconversion indicating infection with EHV-1 while in contact with affected horses during outbreaks (Pursell and others 1979, Franklin and others 1985). Furthermore, donkeys and mules returning from a show were thought to be responsible for dissemination of EHV-1 and propagation of multiple outbreaks of EHM in California in 1984 (P. E. Hughes, personal communication). The aim of this study was to investigate the role of mules as possible silent shedders during an outbreak of EHM occurring at a packing station in northern California. In early September of 2011, an EHV-1 outbreak with several neurologic horses occurred at a packing station located in the eastern Sierra of California. The station housed approximately 180 horses and mules at the time of the outbreak. The animals were kept in two large pens with shared feed and water troughs. All animals were moved to …