Several factors were identified to be significantly different when comparing EHM cases to horses that had EHV-1 with no neurologic signs, and when comparing EHM cases with healthy horses (controls).

The study showed that the odds of a horse developing neurologic signs (EHM) when compared with developing EHV-1 with no neurologic signs and when compared to healthy controls “were greater among horses that were exposed to more biosecurity-related risks (2+) than those that experienced fewer risks (0 or 1).”

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Biosecurity-related risks from the horse show included: 

  • Tied in a barn
  • Used shared water
  • Grazed on grounds
  • Used wash rack
  • Had a veterinary examination or treatment
  • Worked on by farrier


Researchers found that mares were more likely to develop the neurologic form of herpesvirus (EHM) than male horses when compared to EHV-1 cases without neurologic signs and when compared to male horses that were healthy (controls).

When looking at individual risk factors for EHM cases versus EHV-1 cases, mares were 4.3 times more likely than male horses to develop EHM, noted the study. Comparing individual risk factors for EHM versus controls, mares were 2.8 times more likely to develop EHM. However, this “gender effect” was influenced by the number of classes that horses competed in during April and May of 2011.

The study also showed that the risk of being an EHM case compared with EHV-1 cases without neurologic signs increased as the number of classes competed in at the NCHA event increased. The research found that “as the number of classes increased, the likelihood of a male horse becoming an EHM case increased more rapidly.”

Horses that were shown in more classes from May 3 onward (the show ran from April 29 to May 8) were more likely to have neurologic signs (EHM) no matter their sex.

The research concluded that the “main effects and one interaction term remained significant when comparing EHM cases to controls.” Those were:

1.     Greater number of biosecurity risks

2.     Vaccination against EVH-1 in the five weeks preceding the event

3.     Dietary zinc supplementation

4.     Increasing number of events attended in April 2011

5.     Sex modified by the number of events attended in April 2011


One management practice was significantly associated with a decreased risk of a horse becoming an EHM case: dietary supplementation with a product containing zinc.



The neurologic form of equine herpesvirus type 1 (EHV-1) is one of the most significant diseases concerning the horse industry today. The show horse industry was caught off guard in 2011 when more than 400 horses were potentially exposed to EHV-1 at the National Cutting Horse Association Western National Championship in Utah. Those horses then dispersed to 19 states and 242 premises.

Ultimately, 90 laboratory-confirmed and 72 suspect cases of EHV-1 or neurologic herpesvirus (EHM) were identified among the primary and secondary exposed horses from this multistate outbreak, according to the study conducted by USDA-APHIS-VS The study was published in the Journal of Veterinary Internal Medicine.

A total of 123 horses that attended the Utah NCHA event were included in the case-control study: 19 neurologic (EHM) cases, 14 EHV-1 cases with no reported neurologic signs, and 90 controls (healthy horses).

A study of a previous EHM outbreak at a Findlay, Ohio, equine facility had implicated recent or repeated vaccination against EHV-1 as a risk factor for neurologic signs (EHM). In the study comparing neurologic horses with control (healthy) horses exposed at the NCHA event, “there was a 3.3 times greater odds of EHM among horses reported to be vaccinated against EHV-1 in the five weeks preceding the event, and a 1.9 greater odds with eachincrease in number of EHV-1 vaccinations in the year before the event.”

In comparing horses with EHM to control (healthy) horses, the study found, “Horses greater than five years of age had a lower risk of being an EHM case, but the number of horses in the youngest age category was small, and most cases were 5–9 years of age.”


Zinc Shows Promise

In the case control study using the 2011 Utah NCHA show horses, one positive factor researchers found was that when comparing neurologic (EHM) horses with controls (healthy horses), “feeding a nutritional supplement containing zinc was associated with decreased risk” of a horse becoming a neurologic (EHM) case.

“In light of the important role that zinc may play in the control of herpesvirus infection in humans, our surveys included questions regarding nutritional supplementation of this trace mineral,” noted the study. “We found a significant association between reported dietary zinc supplementation and a decreased risk of EHM when EHM cases were compared with healthy exposed control horses.

“However, this association must be viewed cautiously as only one reported EHM case was receiving zinc supplementation,” the study continued. “No zinc measurements were made from tissues or sera taken from these horses, and zinc variables were calculated based on assumptions that zinc was included in supplements according to manufacturers’ packaging and that horses consumed and efficiently utilized the estimated amount of daily supplemental zinc.”

The study explained that T-lymphocytes (CTL) are recognized as important for controlling the magnitude of post-infection leukocyte-associated viremia and subsequent development of neurologic disease. “One study has implicated low pre-exposure concentrations of CTL precursor cells in horses as a predisposing factor of horses to the neuropathogenic form of EHV-1.”

Researchers noted that bodily zinc status has been reported to influence the quantity of circulating CTLs. “Zinc ions themselves also may have a direct antiviral effect on herpesviruses,” the study noted.

One part of the study stated that in a recent report, “serum concentrations of zinc, along with other micronutrients, were measured in horses with EHV-1 infection. Results showed concentrations of zinc and copper to be lower in the nine naturally EHV-1-infected horses when compared with nine healthy horses.”

Researchers involved in the NCHA show outbreak of neurologic herpesvirus (EHM) suggested that the associations found in that study should be further investigated either through challenge studies or through the collection and analysis of epidemiologic data from future outbreaks.

The research reported on here was published as “Case–Control Study of a Multistate Equine Herpesvirus Myeloencephalopathy Outbreak” in the Journal of Veterinary Internal Medicine 2013. Authors were: J.L. Traub-Dargatz, A.M. Pelzel-McCluskey, L.H. Creekmore, S. Geiser-Novotny, T.R. Kasari, A.M. Wiedenheft, E.J. Bush, and K.E. Bjork. More information can be obtained from the February 2013 USDA document online entitled Descriptive Epidemiologic Characteristics of Cases from the 2011 Multistate EHV-1 Outbreak and Summary of Epidemiologic Investigation.

About the Author: Kimberly S. Brown has more than 30 years of experience writing and editing in the equine medical industry. She is president of The Homestead Information Network, Inc., a company focusing on marketing, business development, and editorial services.


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