EHV-1 Outbreak Occurs Despite Rigorous Control Efforts at Colorado State

This fall, equine herpesvirus-1 (EHV-1) spread among patients in the veterinary hospital at Colorado State University (CSU), despite the fact the school runs one of the nation's top veterinary biosecurity programs. The finding forced 20 hospitalized horses into a quarantine, which is expected to be lifted Nov. 17.

The outbreak apparently began after hospital staff admitted a critically ill and partially paralyzed horse on Oct. 23. Because EHV-1 was suspected, the horse was isolated from the hospital population and managed with increased biosecurity precautions. Lab tests did not confirm the diagnosis for 10 days, but in the interim, two horses hospitalized for other reasons developed unexplained fevers.

EHV-1 Horse

This horse is displaying clinical signs of a neurologic disorder.

When those horses tested positive for EHV-1, the CSU large animal hospital officials immediately elected to quarantine all equine patients. In total, five hospitalized horses have been confirmed to be infected with EHV-1 after admission of the paralyzed horse. Four horses had mild fevers, and one developed mild neurologic signs that are improving. Since Oct. 31, the Veterinary Teaching Hospital has been accepting emergency patients only, and its staff has been treating them in isolation from the quarantined horses.

"We were surprised," admitted Paul S. Morley, DVM, PhD, director of biosecurity at CSU. "We have a strong biosecurity program to prevent incidents like this, and we did not expect it to occur."

Morley noted that it was possible the EHV-1 was not transmitted by the initial patient, but from another horse admitted to the hospital for a different reason. All horses carry the virus, and they can shed it when stressed by illness or other conditions, so all horses can be a potential source of the virus. Morley explained that the vast majority of horses never show signs of illness, especially after they are two years old. "Regardless of how the virus was introduced, we believed it was critical for us to act decisively to protect our patients," said Morley.

Equine herpesvirus can spread through direct contact or through the air, making it a concern where groups of horses are stabled. Race horses at Monmouth Park in New Jersey also were quarantined this October due to four horses testing positive for EHV-1, and patients at the Mid-Atlantic Veterinary Clinic in Ringoes, N.J. were quarantined after one horse tested positive in November.

Morley explained that only rarely does EHV-1 cause serious outcomes such as neurologic signs (including the inability to control the hind limbs or tail and urinary problems) or late-term abortions in mares. Most horses that show signs of illness have unexplained fever or nasal discharge--clinical signs that are indistinguishable from influenza. Special testing is required to definitively diagnose EHV infections through virus culture of blood or nasal secretions, PCR (a DNA test of blood or nasal secretions), or serology (antibody titers).

Morley cautioned horse owners against overreacting, because most infected horses recover fully with supportive care (antiviral medications might help). Infected and exposed horses should be isolated, and horses that have come in contact with them should be quarantined and monitored for fever. Horse owners should consult their veterinarians for expert advice on control and prevention of EHV.

"It is very likely that this is not an increase in the occurrence of EHV we are experiencing, but an increase of discovery," noted Morley. "We are more aware of the signs now, and we are watching more closely, so we are diagnosing EHV more often."

For more information on EHV-1, check out our free PDF library of EHV-related articles including images, or all our archived EHV-1 articles on

About the Author

Judith Lee

Judith Lee is a freelance health care writer who has written for a number of medical and health care journals and health care companies. As a long-time equestrian and horse owner, she has a particular interest in equine health care. She also operates an equestrian education program, Riding for Fun, geared toward adult beginners and returning riders.

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