EHV-1 Outbreak: Number of Confirmed Cases Rising

It's been nearly a week since the first indications of a neurologic equine herpesvirus-1 (EHV-1) outbreak began to surface, and the outbreak shows no signs of slowing down. Veterinarians have been confirming new cases of neurologic EHV-1 daily in the western United States and Canada. The National Cutting Horse Association's (NCHA) Western National Championship competition, held April 28-May 8 in Ogden, Utah, has been identified as a common denominator in the outbreak, as many of the horses confirmed or suspected of having the disease were involved in the competition or exposed to horses coming home from the event.

Equine herpesvirus-1 is highly contagious and can cause a variety of ailments in horses, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and myeloencephalopathy (evident in the neurologic form). The virus is not transmissible to humans. Clinical signs of EHV-1 myeloencephalopathy include fever, ataxia (incoordination), weakness or paralysis of the hind limbs, and incontinence. The virus is generally passed from horse to horse via aerosol transmission (when affected animals sneeze/cough) and contact with nasal secretions.

As veterinarians and horse owners work to control the outbreak, updated case numbers from Western states continue to emerge. At press time, the most up-to-date numbers from individual states are as follows:

Arizona - In a press release issued Wednesday (May 18), the Arizona Department of Agriculture indicated that one horse had tested positive for neurologic EHV-1.

"The owners of the affected horse, under the direction of an attending veterinarian, have restricted animal movement to prevent the spread of disease to other horses," the department noted in its release.

California - The University of California, Davis is reporting that two more California horses have tested positive for neurologic EHV-1, bringing the total number of confirmed cases to 12. These horses are located in Amador, Kern, Napa, Stanislaus, Tehama, and Placer counties (all in the Central and Northern California regions). A statement posted on the state's Department of Food and Agriculture website indicated that all of the EHV-1 positive horses participated in the NCHA championship competition.

However, only one California horse has been euthanized as a result of the EHV-1 outbreak, the statement said. The others are quarantined and undergoing veterinary treatment.

Colorado - Eight horses have tested positive for EHV-1 in Colorado, according to a release from the state Department of Agriculture. Two of the EHV-1 positive horses were euthanized as a result of the virus. There are currently 22 suspected cases--animals that owners confirmed were exposed to the virus--but officials have not yet received results from laboratory diagnostic testing.

The Department of Agriculture has placed 10 facilities in six counties (Boulder, Larimer, Morgan, and Weld in north central Colorado; Mesa in the western part of the state; and Bent in the southeast corner) under quarantine.

New Mexico - One confirmed case of EHV-1 was diagnosed at a Texas veterinary hospital, according to the Texas Animal Health Commission (TAHC). That horse was treated and returned home, where it is under quarantine and undergoing veterinary treatment. The New Mexico Livestock Board issued a statement indicating that another horse in Central New Mexico died after the onset of acute neurologic signs. The report indicated that the euthanized horse's stablemate is displaying a fever and remains under quarantine.

The statement also indicated that all known New Mexico participants of the cutting horse show are implementing voluntary quarantine of their facilities.

Oregon - There is currently just one confirmed case of neurologic EHV-1 in Oregon. A horse in Clackamas County (located in Northwest Oregon) was in close contact with other horses that attended the NCHA show before testing positive for EHV-1. All of the exposed horses--including the EHV-1-positive animal, the show attendees, and other in-contact horses--are under quarantine. None of the others are currently showing signs of disease, a release from the Oregon Department of Agriculture said.

Texas - Late Wednesday a horse residing near Weatherford, Texas, that had attended the Utah show tested positive for neurologic EHV-1. The horse's home farm is under quarantine, according to a report from the NCHA.

Utah - Officials in Utah confirmed late Wednesday that five horses are now confirmed positive for EHV-1. A press release issued by the Department of Agriculture and Food indicate that all five horses are being quarantined at a single facility.

Washington - As of Wednesday The Washington Department of Agriculture reported three confirmed cases of EHV-1 and three suspected cases with results pending. Debra C. Sellon, DVM, PhD, Dipl. ACVIM, a professor of equine medicine at Washington State University, confirmed that all of the horses at the Veterinary Teaching Hospital (which has been under restriction since the index Washington case was discharged May 13) remain negative for EHV-1.

Alberta - Chief Provincial Veterinarian Gerald Hauer, DVM, confirmed today that two new cases were confirmed in Alberta, bringing the total to three, however little further information was available at press time: "We're going to gather some more information and try to put the picture together and try to see what's going on here," he said.

There are suspected but unconfirmed cases of neurologic EHV-1 in:

  • Idaho ("several" horses have shown signs, two of which have died or were euthanized, per the Idaho Department of Agriculture);
  • British Columbia (according to a report from Paton Martin Veterinary Services, there are currently believed to be three suspect horses under quarantine);

There are still no known cases of neurologic EHV-1 in the other home states of horses that attended the show: Montana, Nebraska, Nevada, North Dakota, South Dakota, and Wyoming are currently free of EHV-1 reports.

Travel Restrictions

Wyoming has implemented a new protocol for horses entering the state, according to a press release issued by the Wyoming Livestock Board.

"Due to concern over the recent equine herpesvirus-1 neurologic situation in several Western states, the Wyoming Livestock Board will immediately begin requiring an official certificate of veterinary inspection within 72 hours of entry into Wyoming for imported equine(s)," the press release read. "The certificate must include the temperature recorded for each horse listed. The statements, 'No horses listed have been exposed to any horse infected with or exposed to EHV-1,' and 'No horses listed are showing any clinical signs of EHV-1,' must be written on the certificate."

The new protocol is effective immediately.

Origin of Virus Still Unknown

New information surfaced yesterday about a horse attending the Ogden show that was considered by many as a possible source for the outbreak. "Uncle Kunkel" began displaying neurologic signs while he was stabled on the show grounds, according to a press release from the CSR Equine Sports Medicine Center in Bozeman, Mont. The horse was transported and admitted to CSR on April 30, where veterinarians suggested he might be affected with cervical stenotic myelopathy (or Wobblers syndrome).

After not responding to treatment, the horse was euthanized May 2 and his body was transported to the Utah Veterinary Diagnostic Center in Salt Lake City for examination. Extensive postmortem diagnostic testing was negative for EHV-1, thus eliminating him as the source of the outbreak. Veterinarians confirmed at necropsy that Wobblers was the cause of Uncle Kunkel's neurologic signs.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More