In a letter issued by the California Department of Food & Agriculture on Wednesday, the state recommends all equine events - including jumping events and other gatherings - be postponed through March 28.
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All "non-essential horse movements" should be postponed through March 31, the state added.
The virus known as equine herpesvirus myeloencephalopathy, or EHM, can cause "significant consequences" to horses, the state said.
It can cause respiratory disease and neonatal death. Officials say it spreads in aerosolized secretions, by direct contact, and by contact with surfaces.
The CDFA reports 10 horses who were infected with the virus had to be euthanized. Another nine that have been infected are showing significant symptoms of neurologic disease. Another 137 horses have tested positive for the virus but are not yet showing signs of neurologic disorder.
"It's a neurological virus, they get symptoms of a snotty nose and the symptoms come on very quickly," said Donna Hyde, who runs NDR Therapeutic Riding, a nonprofit horse ranch in Norco.
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In Orange County, hundreds of horses have been exposed and more than a dozen have tested positive for the virus.
"It's also a virus that will survive in the environment for seven days and can be spread with buckets and shoes, garments, people touching one horse, going to the next horse," said Dr. J Ward Thompson, a veterinarian based in Yucaipa.
Even if horses aren't showing neurological symptoms, the state encourages owners not to move them.
"We are going to not take them out of the ranch, we are not going to allow any horses into the ranch," said Hyde.
The Swallows Day Parade in San Juan Capistrano, which typically features 400 to 500 horses, went on without them this past weekend. There's hope with the state's precautions, the virus will be contained.
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"I think most people will be pretty cautious and we'll get this taken care of," said Thompson.
Sanctioned horse racing tracks are exempt from the state recommendation.
For more information, read the full notice from the state here.