Diane Jones got the news Friday morning: The annual Casanova Hunt Point-to-Point, a steeplechase race expected to draw 5,000 people to a farm near Warrenton on Saturday, was being canceled.

State agriculture officials told her that after the outbreak of a horse virus at an equine medical center in Leesburg, they didn't want to take chances.

"It's a big deal to cancel the day before," said Jones, the race chairman, who estimated that organizers would lose $30,000 to $40,000 in revenue.

At the Charles Town Races and Slots track in West Virginia, officials barred all horses from Virginia and Maryland from racing until further notice. Morven Park Equestrian Center in Leesburg canceled all its events through March 9, including a dressage show and a competition among area high schools.

Many owners of horses and stables across the region were imposing their own restrictions: no movement of horses on or off their property for at least two weeks.

Four confirmed cases of a strain of equine herpes virus called EHV-1 have put the horse community on high alert, because the virus is highly contagious and there is no cure. Infected horses undergo therapy for respiratory problems and such neurological symptoms as stumbling and weak legs.

In severe cases, horses are unable to stand and must be euthanized. In December, several horses in Florida were put down after a particularly bad outbreak of EHV-1.

One of the new cases is a horse on a Loudoun County farm that since last week has been under state quarantine, which minimizes the risk that other animals were exposed, said Elaine J. Lidholm, a spokeswoman for the Virginia Department of Agriculture and Consumer Services.

The other case is a horse that was admitted to the Marion duPont Scott Equine Medical Center in Leesburg for surgery and went to a convalescent center in Maryland to recuperate. The horse, which came from a farm in Fauquier County, was euthanized because of complications from surgery, but test results revealed she also was infected with the virus known as EHV-1.

In Virginia and Maryland, officials are trying hard to prevent such an outcome. The duPont Center, the Virginia Tech facility where the cases of infection were discovered, is under a state-imposed quarantine that is expected to last two to four weeks.

The quarantine began Wednesday, when officials announced that a horse admitted to the hospital Feb. 7 for an unrelated reason had tested positive for the virus. On Friday, officials said that a second horse had tested positive and that they were awaiting lab results on two other horses with neurological symptoms of the disease.

One of the infected horses went to the medical center from Virginia and the other from Maryland, officials said.

By Friday afternoon, Virginia officials had quarantined 10 farms with a total of about 175 horses: six farms in Loudoun, two in Fauquier, one in Culpeper and one at the Quantico Marine Corps Base. Each farm has horses that could have come into contact with the infected animals, said Lidholm.

Maryland health officials imposed a quarantine on at least eight horse farms.

Officials in both states said the list could grow as they continue to trace the movement of horses over the past two to three weeks. They declined to identify the quarantined farms, citing state confidentiality laws, but said clients at the properties had been notified.

As news of the outbreak spread through the local horse industry last week, owners tried to understand what it could mean for their animals.

Veterinarians reported a high volume of calls from horse owners, and equine Web sites were buzzing.

"Of course all of us are very concerned, and we're watching it very carefully," said Penny Denegre, joint master of the Middleburg Hunt, whose season runs through March. "This is the kind of thing you have to take very seriously. You have to keep watch on it."

Tom Waters, a horse trainer and the owner of Faircroft Stables in Leesburg, admitted a client's racehorse to the equine medical center last week for colic. Now the 5-year-old thoroughbred, Darla, is undergoing blood tests to make sure she wasn't infected. She has two more tests to go before she is cleared.

"You always worry a little bit. [But] better her being there, knowing exactly where she was, than at a show where you are unsure who could have had it," said Waters, who has 11 other horses on his farm.

Several horse farm operators said they were restricting access even though their properties are not on the Virginia or Maryland embargo list.

"You avoid contact if you can," Waters said. "You just make sure you wash everything down with Clorox."

Leoni Corbett, manager of Windsor Stables in Lovettsville, said she always monitors new horses that enter her herd.

"If I get a new boarder, I keep them in confinement for a month to make sure they're okay," she said. "I will be extra careful until they say everything is clear."

James Joyce, a veterinarian at Blue Ridge Veterinary Associates in Purcellville, recommended that horse and stable owners institute a voluntary quarantine on their properties "and then weather the storm and let it run its course."

"The simplest thing is to know what horses are coming to and from your premises and where they're coming from," he said.