Three further horses from the yard of Donald McCain have tested positive for equine flu, including Raise A Spark, who ran at Ayr on Wednesday, the British Horseracing Authority has announced.
A "separate suspicious case" - not yet confirmed as a positive sample - has been identified at another yard.
Raise A Spark finished last of four in an extended two-mile handicap chase won by Gordon Elliott's Veinard.
Three positives were already known from the McCain stable. Of the additional three cases, Raise A Spark is the only one to have run this week.
In a statement on Friday afternoon, the BHA said: "The test on this runner relates to a sample taken the following day, and the horse showed no clinical symptoms on raceday.
"It was in anticipation of this risk that the decision was taken by the BHA to restrict movement of horses on a precautionary basis at the yards of 120 other trainers who competed at these fixtures, and to suspend racing until Wednesday at the earliest.
"This action was taken to mitigate the impact of any of the runners which competed at these fixtures being found to be infected by the virus.
"Testing of horses at the 120 yards which have been put on hold is continuing on an ongoing basis. No further positive samples have been reported as yet."
But the statement added: "However, a separate suspicious case - which has not yet been confirmed as a positive sample - has been identified at another yard.
"No links have been identified between this yard and the original yard.
"This yard had runners at the fixtures at Newcastle on February 5 and Wolverhampton on February 6, and as such the BHA has taken the further precautionary steps of placing all 54 yards of trainers who also had runners at these fixtures on hold and initiating testing of horses from these yards."
The BHA said it was working closely with the Animal Health Trust "in order to manage the logistical challenge of providing sufficient swabs and handling the volume of tests being sent through the facility, considering that testing of a total of 174 yards is now taking place".
It added: "It will not be possible to test every horse from every yard before the end of the weekend, but we will work with trainers to identify any priority or risk horses and ensure that they are tested.
"This will all form part of the picture that is built in order to assist the decisions that will be made on Monday."