EHV-1 Outbreak: Weekend Cases Boost Total Numbers

Several new cases of equine herpesvirus-1 (EHV-1) were diagnosed over the long holiday weekend. Confirmed cases are currently confined to nine states in the Western U.S. and two Canadian provinces. The outbreak is believed to have stemmed from a cutting horse championship competition held recently in Utah.

Although it's not transmissible to humans, EHV-1 is highly contagious among horses and camelids, and it is generally passed from horse to horse via aerosol transmission (when affected animals sneeze/cough) and contact with nasal secretions. The disease can cause a variety of ailments in equines, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and myeloencephalopathy (EHM, the neurologic form). Myeloencephalopathy is characterized by fever, ataxia (incoordination), weakness or paralysis of the hind limbs, and incontinence.

Additionally, the USDA Animal and Plant Health Inspection Service released an updated situation report on May 26, which can be viewed online.

New Cases

Idaho--A statement released by the Idaho Department of Agriculture late Friday (May 27) indicated that one additional horse tested positive for EHV-1. The horse is a stablemate of one of the horses that attended the cutting competition in Utah.

"The horse has only mild neurologic signs and is expected to recover," the statement read.

The total number of confirmed EHV-1 cases in Idaho now stands at seven. Four horses (including the most recently confirmed positive animal) have displayed neurologic signs, and three have only exhibited a fever.

California--One additional EHV-1 case was confirmed over the weekend in California, according to a statement from the state's Department of Agriculture. The recently confirmed case brings the total number of EHV-1 positive horses to 19. One horse with severe neurologic signs was euthanized, seven horses displayed neurologic signs, and the remainder were febrile or showed no clinical signs. The horses are located in the following counties: Glenn, Plumas, and Shasta in Northern California; Amador, Colusa, Marin, Napa, Placer, and Sacramento in North Central California; Stanislaus in Central California; Kern in South Central California; and Los Angeles and Ventura in Southern California.

Additionally, the department is reporting a horse suspected of having EHV-1 as displaying a fever and neurologic signs, however a positive diagnosis has not yet been achieved.

Alberta--The Office of the Chief Provincial Veterinarian is reporting eight confirmed cases of EHV-1, and three suspected cases for which officials are awaiting laboratory test results. Of the confirmed cases, one horse has displayed neurologic signs, three have shown respiratory signs and/or a fever, and four have been asymptomatic (lacking any clinical signs). The office noted that there have been no equine fatalities reported relative to the EHV-1 outbreak.

Washington--A press release issued today by Washington State University (WSU) indicated that a university-owned horse tested positive for EHV-1 during a postmortem examination, despite showing no signs of disease prior to euthanasia.

The release explained that the horse was euthanized after a "long-standing lameness" had gotten progressively worse. Although the horse had not shown signs of disease or produced a positive nasal swab through PCR testing, the team at the Veterinary Teaching Hospital decided to test the horse's lung tissue as a "part of the college's monitoring efforts." The tissue sample confirmed the horse was positive for the neurotrophic form of the virus.

"Five horses have been confirmed as positive for EHV-1at the university," the press release read. "The first horse was released to the care and control of the State Veterinarian's Office. Three horses remain at WSU that are confirmed positive for EHV-1.

"The new confirmation at WSU restarts the clock for the veterinary college's isolation procedures," the release continued. "The restrictions for new, non-emergency horse or camelid patients coming to WSU's Veterinary Teaching Hospital remain in place for a minimum of 21 more days from today if no more new cases develop."

The newest confirmed case brings Washington's total to eight, four of which (including the most recent case) did not display any clinical signs. The signs associated with the other horses were not reported.

Arizona--The USDA Situation Report indicated a total of 11 confirmed cases of EHV-1 in the state of Arizona, one of which was euthanized after developing neurologic signs. There is no word on the clinical signs the EHV-1 positive horses are showing or where in the state the horses are located.

Texas--The Texas Animal Health Commission released a statement Friday (May 27) indicating that one new case of EHV-1 had been confirmed; however, animal health officials do not believe this case is related to the outbreak. The facility where the EHV-1 positive horse (a Quarter Horse racehorse) is housed has been quarantined and other horses are being monitored for exposure. The USDA Situation Report indicates that there are currently no other confirmed cases in Texas.

No Change

  • British Columbia--0 positives
  • Colorado--9 positives (Six horses have shown neurologic signs; the remaining three have displayed respiratory signs and/or a fever)
  • Montana--0 positives
  • Nebraska--0 positives
  • Nevada--3 positives (two have displayed neurologic signs)
  • New Mexico--4 positives (USDA Situation Report indicates two have been euthanized; clinical signs not reported for other horses)
  • North Dakota--0 positives
  • South Dakota--0 positives
  • Oregon--5 positives (one euthanized with neurologic signs; four showing no clinical signs)
  • Utah--7 positives (one euthanized after becoming recumbent, or unable to rise; clinical signs of surviving horses were not reported)
  • Wyoming--0 positives

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.

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