State Veterinarian: EHV-1 Outbreak Winding Down, Normal Equine Activities Can Resume

According to a June 6, 2011 press release from State Veterinarian Dr. Don Hansen of the Oregon Department of Agriculture, sufficient time has passed for most horses that may have been exposed to the virus traced to a horse show in Utah last month. The vast majority of horse owners in Oregon and in the Pacific Northwest should feel free to participate in horse shows, rodeos, and other equine events as a recent outbreak of the neurological form of Equine Herpesvirus appears to be well contained.

Per Dr. Hansen, the few horses that have shown symptoms of the disease will remain quarantined in their barns or stalls and monitored closely until it is clear the virus is no longer present. Horses that participated in the National Cutting Horse Association’s Western National Championship in Ogden, Utah from April 30 through May 8 may have been exposed to the virus.

In total, 20 horses from Oregon attended the show, five tested positive for the virus, and one died. The virus also affected horses in other states including Washington, Idaho, and California.

“Any horse that hasn’t tested positive, shown any symptoms, or not exposed to a confirmed positive horse should be okay for travel and participation in equine events,” says Hansen. “We’ve been in close contact with owners of the affected horses and their stable mates. Those animals have been kept isolated and under close watch for the past few weeks. If these horses have gone 28 days without any signs of illness, including fever, they are most likely no longer contagious and can be considered for quarantine release.”

Hansen continues to emphasize the need for horse owners to practice good biosecurity and
hygiene at all times.

“While we appear to be out of the current episode, herpes viruses in general are common
in horse populations as they are in human populations,” says Hansen. “It’s always a good idea to
take steps that minimize the threat of disease. That was the case before the recent outbreak and
will continue to be the case in the future. There is no reason at this time to avoid shows, exhibitions, competitions, and other equine activities,” says Hansen.

Case History

The State Veterinarian reports that all Oregon horses that have tested positive (4 EHV-1, 1 EHM) are from Clackamas, Deschutes and Umatilla counties, including this most recent case.

A horse from Clackamas County developed neurologic signs of EHM and was euthanized last weekend. The horse had attended the Ogden show. Test results from the horse were positive for EHM.

Horses from several states that attended attended the National Cutting Horse Show in Ogden, Utah, held in late April/early May 2011 were exposed to EHV-1, the virus that can cause EHM.

The first confirmed Oregon case of EHV-1 was in a horse from Clackamas County that attended the show.

The second confirmed Oregon case of EHV-1 was in a horse from Umatilla County that did not attend the show, but traveled in the same trailer with two horses that were returning from the show. Those two horses tested negative.

The third confirmed case of EHV-1 was a horse from Deschutes County that attended the show.

The fourth Oregon horse that tested positive for Equine Herpesvirus (EHV-1) was also from Clackamas County horse and was stabled with others that attended the National Cutting Horse Show in Ogden, Utah, where the current outbreak stems from. The horse remains in quarantine in Clackamas County with its stablemates.

A total of 20 Oregon horses attended the Ogden show. As described above, one horse was positive for EHM and was euthanized. The other 19 Oregon horses that attended the show continue to be monitored and all remain under quarantine with their stablemates. 

Report Suspect & Confirmed EHV-1/EHM Cases to State Veterinarian

All suspect and confirmed cases of EHV-1/EHM are REPORTABLE to the State Veterinarian.  A suspect case would be any horse showing neurological signs without another obvious cause or any horse that was exposed to a case of EHM and spikes a temperature over 101.5. Please notify the State Veterinarian by phone at (503) 986-4680.

Veterinarians should work closely with their equine clients to develop plans that address the clients’ ability to prevent EHV-1 in their hoses. And, if a horse is suspected of having the viral disease, it should be isolated.  Equipment should not be shared.

The State Veterinarian also recommends that it would be prudent to continue taking temperatures of horses exposed at the Ogden show or exposed to horses from the Ogden show for 14 days beginning with the day they were exposed.

Enforcement of strict biosecurity measures and hygiene practices are likely to be more effective than widespread vaccination in reducing the risk of acquiring infection. Nevertheless, recent research demonstrates that viral shedding is much reduced in horses with high circulating titers of virus-neutralizing (VN) antibody, as well as in horses that have been vaccinated recently with the Rhinomune MLV vaccine.

Western Cases by State

From USDA report (6.2.11)

*Information not reported

Potential Clinical Signs of EHV-1

1. Respiratory signs (Signs may be minimal and of short duration.)

• Increased rectal temperature may be the only clinical sign
• Horses can have two fever spikes
• The initial rise in rectal temperature is usually mild – 101.5 to 102.5 F
• After the initial temperature rise, which may be missed, the horse can either be clinical normal, develop respirator signs of nasal discharge, increased temperature (> 102.5), minimal coughing, can abort if pregnant, or, in a small number of cases develop neurological signs.

2. Neurologic signs

• Horses become ataxic (in coordination), have an inability to empty the bladder, and weakness of the tail. Some horses will become completely paralyzed; the prognosis for these horses is poor. In a small number of cases, horses can show abnormal mentation and develop cranial nerve signs. Most horses become mildly to moderately neurologic and stabilize rapidly. The neurologic signs can persist but most horses are normal by 3 to 6 months after onset of clinical signs.

3. Abortion

• Pregnant horses can experience spontaneous abortion between 7 days and several months after exposure. The mare will exhibit limited initial signs.

Other Facts

• Many times when horses are incubating the virus, fever in excess of 102 F may be the only observable sign of infection. However, fever may not be present in all neurogenic clinical cases.
• Stress and lack of previous exposure to the virus may make the horse more susceptible to becoming clinically ill.
• Currently, there is no equine vaccine that has a label claim for protection against the neurological strain of the virus.
• It is virtually impossible at this time to know if horses in our native population have ever been naturally exposed to recently diagnosed strains of EHV-1 and whether these individuals develop any subsequent immunity to the virus.
• EHV-1 infected horses, whether clinically ill or not, may periodically shed both active and nonreplicating (dead) virus in their nasal secretions. If the virus being shed is active, these horses have the potential to spread the virus to other horses. Horse-to-horse contact, contaminated hands, equipment, tack, feed, and aerosol transmission all play a role in its spread.
• It is felt that horses with severe clinical signs of neurological EHV-1 illness often have very large viral loads in both their blood and nasal secretions. These high viral loads are thought to be a significant aspect in the transmission of the disease to exposed horses. Therefore rapid separation and isolation of identified suspect cases and biosecurity are key elements for disease control.
• As with all contagious diseases, if horses are commingled with strange horses, an unknown degree of inherent risk exists for exposure to EHV-1. Many factors may enhance or reduce the amount of risk. If the choice is made to commingle with a population that has the potential to harbor EHV-1 infected individuals, there is no foolproof way to completely eliminate the risk of exposure.
• If horses are exposed to new horses, especially in stressful competitive environments or following long distance travel, it is helpful to establish a disease-monitoring plan under the advice of a veterinary practitioner. Temperature monitoring (2x / day) is a tool to be used for a differential diagnosis that could include EHV-1.

For more information, contact Don Hansen DVM, MPVM - State Veterinarian, Animal Health & Identification, Oregon Department of Agriculture at (503) 986-4680.

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