Veterinarians are still working to determine the extent of a multi-state outbreak of neurologic equine herpesvirus (EHV-1). The number of horses confirmed or suspected to have EHV-1 remains on the rise, with several states reporting cases. A common point of origin for all the positive horses is the National Cutting Horse Association's (NCHA) Western National Championship competition, held April 28-May 8 in Ogden, Utah.

In California the number of EHV-1 positive horses has grown to six. According to Gary Magdesian, DVM, Dipl. ACVIM, ACVECC, associate professor in the department of medicine and epidemiology at the University of California, Davis (UC Davis), School of Veterinary Medicine, four horses at referral practices in Northern California tested positive for the virus May 16, and two horses currently in quarantine at UC Davis' isolation facility tested positive the same day. Magdesian reported that veterinarians were treating the horses at UC Davis and that the animals were in stable condition as of Tuesday morning (May 17).

Magdesian emphasized that while the isolation facility at UC Davis is currently under quarantine, the general hospital at the William R. Pritchard Veterinary Teaching Hospital is not under quarantine and remains open: "The two horses with EHV-1 (at UC Davis) are under maximal isolation with no contact with the general hospital. All horses currently at the general hospital have tested negative for EHV-1. However, because of the external cases in the horse community, we have high level precautions in place to do everything possible to prevent a positive horse from entering our hospital.

"Due to reports of a number of additional febrile and neurological horses in the state of California, including Northern California, we are taking every precaution to prevent any positive horses from being admitted to the regular hospital and to protect all of our patients. We are testing all horses admitted to the hospital and using every infection control precaution to protect the hospital from being affected."

On May 16 the Colorado Department of Agriculture released new information on the number of affected horses within the state. There still are only two confirmed cases (one of which was euthanized), as previously reported, however six additional horses exposed to the virus are showing clinical signs consistent with neurologic EHV-1. A second horse was euthanized after displaying severe signs, however a positive diagnosis has not been confirmed. The remaining affected horses in Colorado are being kept under quarantine held in Boulder, Larimer, Mesa, and Weld counties.

Additionally, the Colorado State University Veterinary Teaching Hospital has closed its doors to new equine and camelid cases that aren't considered emergencies.

While no positive cases have been reported in Nebraska, State Veterinarian Dennis Hughes, DVM, has placed five farms in Lancaster, Dawson, and Scotts Bluffin counties under quarantine as a precautionary measure. The farms are home to horses that recently competed at the NCHA Western National Championships.

"The horse premises that are quarantined in Nebraska may have come into contact with infected horses during the National Cutting Horse Association's Western National Championships in Ogden, Utah," Hughes said in a press release. "We are acting quickly to take appropriate measures to protect our horse industry. Our staff is working with the quarantined horse premise owners to monitor for signs associated with the disease."

No additional cases have been diagnosed at Washington State University's (WSU) Veterinary Teaching Hospital, according to Debra C. Sellon, DVM, PhD, Dipl. ACVIM, a professor of equine medicine at WSU. The hospital remains under voluntary quarantine after a horse tested positive for the virus May 13.

A press release issued May 17 by the Montana Department of Livestock stated that while 30-35 horses from the state attended the NCHA competition, there have been no reports of positive diagnostic tests.

Equine herpesvirus is highly contagious and can cause a variety of ailments in horses, including rhinopneumonitis (a respiratory disease mostly of young horses), abortion in broodmares, and myeloencephalopathy (evident in the neurologic form). The virus is not transmissible to humans. Clinical signs of the neurologic EHV-1 form 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.

TheHorse.com will continue to provide updates on the EHV-1 outbreak as more information becomes available.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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