Two Weld County properties connected with the horses, who were returned home to Colorado after the event, are under state-ordered quarantine. One of the horses has been euthanised because of severe neurologicial symptoms.
Organisers of the 2011 Breeder's Invitational decided on Saturday evening to cancel the 15-day event in Tulsa because of the cases.
Organisers were concerned that horses exposed to the virus in Ogden could potentially transport it to the Breeders International in Tulsa and, in doing so, further expose the horse population.
While no horses who arrived in Tulsa for the invitational - which was scheduled to start on Saturday - have exhibited any clinical symptoms of the disease, the decision to cancel was considered a precautionary measure.
"We did not feel comfortable with the potential of exposing horses arriving in Tulsa to EHV," said Breeder's Invitational board chairman Jeffrey Matthews.
"An outbreak at an event of this size could be devastating.
"The board of directors decided to take a conservative approach to the situation in the interest of protecting the health of the horses."
EHV has the potential to cause abortion and death in horses. Clinical signs include high fever, neurological symptoms and nasal discharge. The virus can spread through the air, contaminated equipment, clothing and hands.
Colorado State University Veterinarian, Dr Jerry Black, who consulted with event officials, said he was confident the organisation made a decision that took into consideration the best interest of the horses involved.
"I commend the Breeders Invitational on the decision they have made to make the welfare of the horse the priority.
"While I expect the risk in Tulsa may have been minimal, I do believe the decision was a prudent one," said Black, who urged horse-owners and trainers to quarantine any horses that may have been exposed to EHV.
"To be on the absolute safe side, any horses that have attended events where EHV is suspected to have been present should be quarantined separate and apart from un-exposed horses for a minimum of 10 to 14 days."
Breeder's Invitational executive director Bob O'Bannon said attempts will be made to refund entry fees to exhibitors entered in this year's event.
"We will be working the next several days to get this handled," he said.
"We are going to do our best to take care of our exhibitors and owners."
O'Bannon said the board will make a decision on whether to hold the event at a later date this year.
"This is an option that we would like to pursue, though no decision has been made at this time."