As previously described, Equine Herpes Virus (Myeloencephalopathy) (EHM) can be devastating to horses exposed, especially when an outbreak occurs in stables or equestrian events where large numbers of horses are present.

State and Federal Animal Health Officials have been responding to these events, imposing quarantines when warranted, and requiring daily examinations and testing to segregate out horses with clinical signs of disease and those testing positive for the virus.

Now, the United States Animal Health Association has published “Equine Herpesvirus Myeloencephalopathy Incident Guidelines for State Animal Health Officials,” which memorializes the best practices that have been developed over the years.  “During Equine Herpesvirus Myeloencephalopathy (EHM) incidents, the state animal health officials’ (SAHO) goal is to prevent the spread of the disease agent, specifically Equine Herpesvirus- 1. Science-based disease control protocols, adapted to the specific incident control disease spread while ensuring compliance and minimizing the impact on equine movement.”

The document, described as “an evolving document,” provides animal health officials “with science and field experience based control guidance to be considered during an EHM incident.”

Notably, the guidance document defines important terms that are necessary to mount an organized and effective response, including:

EHM Incident: Detection of one or more confirmed cases of EHM, and where there is confirmation of disease agent spread or evidence of potential for disease transmission from an EHM confirmed case to additional horses in a population.

Index Definitions:

  1. Confirmed Index EHM Case: A horse displaying signs of central nervous system (CNS) dysfunction, including but not limited to hindlimb incoordination, weakness, recumbency or urinary bladder atony, with evidence of Equine Herpesvirus -1 infection based on virus isolation and /or PCR testing of nasopharyngeal/nasal swab or blood (buffy coat) specimens. If the horse dies or is euthanized, the case can be confirmed based on histological evidence and detection and/or demonstration of EHV-1 in CNS tissues collected at necropsy.

  2. Suspect Index EHM Case: Highly suggestive EHM case, defined as a horse displaying signs of central nervous system (CNS) dysfunction, including but not limited to hindlimb incoordination, weakness, recumbency or urinary bladder atony which may or may not have been preceded by fever, respiratory signs or abortion in other horses on the premises.

The detection of infected horses amongst those exposed is particularly challenging because of the numbers of horses typically involved in these outbreaks, and the logistical challenges that arise because of the difficulty of housing potentially infected horses in isolated barns.  With each newly identified infected horse, the clock is restarted to eventually release the quarantines that are inevitably imposed.

With an incubation period that can span 10-21 days, the length of the quarantine can significantly impact the equine competitive season or barn affected.  While I was the Director, Division of Animal Health, New Jersey Department of Agriculture, a quarantine was imposed at Monmouth Race Track for months before horses could move back to their home stables or out of state.  Fortunately, expect for the index case, no horses died in that outbreak.