GRAND ISLAND, Nebraska (AP) — Hundreds of racehorses have been quarantined at Fonner Park in Grand Island because three of them tested positive for equine herpes.

One of the horses has been euthanised, but the remaining two are getting better with treatment, the Nebraska Agriculture Department said.

Fonner CEO Bruce Swihart said officials became aware of the sickness Sunday evening and that he doesn’t know how the horses contracted the virus. They’d been at Fonner for a couple of months. The US Department of Agriculture’s National Veterinary Services Laboratory in Ames, Iowa, evaluated nasal swabs and blood samples from the three horses to confirm the disease.

Swihart said 750 to 850 horses at Fonner Park can’t leave until the minimum 21-day quarantine is lifted. Fonner will continue racing through May 7, but the 150 or so horses in the same barn as the three that tested positive won’t be allowed to race.

Prairie Meadows Race Track in Altoona, Iowa, has quarantined and is monitoring horses that arrived from Fonner Park within the past week.

The virus cannot be spread to humans or other animals, but it can be transferred to other horses through boots, buckets, clothes, feed, tack or alternative means. Symptoms of the disease include fever, decreased coordination, hind limb weakness, leaning against a wall to maintain balance, lethargy and the inability to rise.

“It is of utmost importance that horse owners and facility managers take appropriate precautions to prevent the spread of this disease,” said Nebraska State veterinarian Dennis Hughes in a news release.

Several tracks have encountered the equine herpes virus during the past year. Four horses died at Parx Racing in Pennsylvania and an outbreak in January forced Sunland Park in New Mexico to stop racing for more than a month. Horses were also quarantined at training facilities in Texas and at Turf Paradise in Arizona.

Nebraska Horse Racing Commission Director Tom Sage declined to comment.