Published May, 2011

The recent spread of a neurologic form of equine herpesvirus type 1 (EHV-1), believed to have originated at a cutting horse show in Ogden, Utah, held April 29 to May 8, and subsequent outbreaks of the disease across the western United States and Canada highlight the importance of equine infectious disease research.

The University of Kentucky Department of Veterinary Science has a long and illustrious history of equine herpesvirus research. The late William Dimock, DVM, and Philip Edwards, PhD, first investigated the etiology of abortion storms on many Central Kentucky breeding farms in the 1920s (before the disease was known as EHV-1). Today the Gluck Equine Research Center continues to conduct research on the virus.

A significant number of EHV-1 outbreaks also have been reported from Europe this year with some affected horses exhibiting respiratory and neurologic signs. In addition, there have been reports of sporadic cases of EHV-1 abortions and occasional cases of infection in neonatal foals, according to Peter Timoney, MVB, FRCVS, PhD, Frederick Van Lennep Chair in Equine Veterinary Science, and Udeni Balasuriya, BVSc, PhD, virology professor at the Gluck Center.

"The development of neurological disease in particular horses may be influenced by a variety of virus, host, and environmental factors," Balasuriya said. "More basic research is needed to understand the molecular basis of the neurologic form of equine herpesvirus-1 infection. There should be more collaborative research between scientists who are working on EHV-1 around the world."

In 1993 the Office International des Epizooties (the animal equivalent of the World Health Organization) designated the Gluck Center as a World Reference Center for three of the important equine viral diseases, including equine rhinopneumonitis (equine herpesvirus 1 and 4). Former Veterinary Science faculty member Roger Doll, DVM, PhD, and others were responsible for determining a great deal of what is known today about EHV-1 and the virus' clinical signs. They devised laboratory procedures to study EHV-1 infections, including vaccine development and evaluation. The late George Allen, PhD, a former Gluck Center professor, was one of the world's leading authorities on equine herpesvirus diseases. In collaboration with researchers at the Animal Health Trust in Newmarket, United Kingdom, he identified virus strains of a particular genotype (genetic code) associated with neurologic disease outbreaks.

Several equine herpesvirus research projects are currently under way at the Gluck Center. One research project on EHV-1 is "Molecular characterization of neurovirulent EHV-1 strains," a two-year study funded by the Grayson-Jockey Club Research Foundation that began last year. Balasuriya is the principle investigator and is collaborating with Frank Cook, PhD, associate research professor at the Gluck Center, and Timoney.

The study will attempt to identify possible additional neurovirulence (capacity to cause disease of the nervous system) determinants of EHV-1 by sequencing genes essential for replication from a panel of archived virus isolates. It will also attempt to increase understanding of how neuropathogenic strains emerge in nature. The goal is to develop improved diagnostics, a vaccine that is effective in protecting against this disease, and more accurate predictors of the clinical outcomes associated with orses infected with particular viral genotypes.

Over the past decade there has been an unexpected increase in equine herpesvirus neurologic disease (equine herpesvirus myeloencephalopathy [EHM]) incidence. Previous research by other scientists suggests a significant percentage of EHM or paralytic herpes outbreaks are caused by a mutant strain. A single mutation has been identified in the gene encoding of the viral replication enzyme, which seems to confer the power of enhanced pathogenicity (a pathogen's ability to cause disease in an organism) or neurovirulence to such strains.

"It is uncertain whether there are other genotypes of the virus with similar pathogentic potential," Timoney said.

"In my opinion there may be other mutations in the viral genome that may be associated with neurologic disease and this warrants further investigation," Balasuriya said. "EHV-1 is a very complex virus and like all known mammalian herpesvirus, can give rise to lifelong latent carrier state in horses (no evidence of virus replication or shedding, and the horse appears normal). Laboratory diagnosis of latent infection in horses is a considerable challenge."

Balasuriya said some of the major questions to be addressed through his research include:

  • Does the single mutation in viral gene involved in replication alone precipitate neurologic disease? Are there additional nucleotide changes in the viral gene involved in replication or perhaps other viral genes that may contribute to the development of neurologic disease?
  • What are the major genetic determinants of EHV-1 neurovirulence?
  • Is there a correlation between other equine herpesvirus infections (for example, EHV-2 and EHV-5) and EHV-1 disease outbreaks?
  • How do we improve the sensitivity and specificity and overall reliability of current molecular diagnostic assays for EHV-1?

To answer these questions, other current equine herpesvirus research at the Gluck Center includes:

  • Molecular epidemiology and evolution of EHV-1.
  • Identification of virulence determinants of EHV-1 and characterization of the molecular mechanisms of EHV-1 pathogenesis and host immune response to the virus.
  • Development of improved diagnostic assays and vaccines against EHV-1.
  • Comparison of in vitro (in the laboratory) antiviral activity of herpesvirus DNA polymerase inhibitors against neuropathogenic and non-neuropathogenic strains of EHV-1.
  • Investigation into equine herpesviruses' role in respiratory disease outbreaks on horse farms.

"In my opinion it is important to isolate viruses from EHV-1 outbreaks for further molecular characterization," Balasuriya said. "We are one of the few laboratories that continue to attempt isolation of the virus from clinical specimens. It is important to isolate EHV-1 strains for evaluation of their respective biological and molecular properties. We have also developed an attenuated strain(s) of EHV-1 as a possible modified live virus vaccine(s) candidate, but have not tested these cell culture adapted strain(s) in horses."

The Gluck Center has not yet received any clinical samples from the most recent EHM outbreaks. However, Balasuriya said he expects to receive samples from the University of California, Davis, by the end of May.

Jenny Blandford is the Gluck Equine Research Foundation assistant at the Gluck Center.