Officials with the Maryland Department of Agriculture have determined that the illness that killed three horses at the Columbia Horse Center in Laurel, Md., was the neurological form of equine herpesvirus-1 or “rhino.”

Two additional horses that veterinarians were treating for symptoms of the virus are doing well, said Nanci Steveson, general manager for the horse center. She said the horses are not showing any additional symptoms and have completed their medication.

The investigational animal-hold order will remain in effect for another 10 days, until April 16. Dr. James Fearer, acting state veterinarian, said they are still waiting for the results of a final test but did not expect it to show anything new.

“We just want to make sure everything is going to stay quiet,” he said.

The neurological form of rhino is a genetic mutation of the virus, and the vaccine for respiratory rhinopnuemonitis does not prevent it. All of the horses at the Columbia Horse Center were up to date on their vaccinations. Fearer said it is important that horses be vaccinated for the respiratory form of the virus so that it is less prevalent and, therefore, less likely to mutate into the more serious neurological form.

Steveson said that the Columbia Horse Center is continuing to take precautions in the quarantined area of the barn. The virus can be transmitted through the air when a horse coughs or sneezes up to a distance of 35 feet, as well as be transmitted from direct contact with infected horses, feed and equipment. Steveson said that the horse center staff is doing everything it can to ensure there are no new cases.

Boarders will also not be permitted to take their horses off the farm for another 30 days, even after the hold order is lifted. Horse center officials will allow boarders to begin riding again on April 16, and a veterinarian will be meeting with boarders to discuss how to recondition their horses. The center will reopen on April 18.