All racing in the United Kingdom has been suspended until next Wednesday at the earliest.
The British Horseracing Authority (BHA) took the dramatic step on Wednesday night to cancel all four of yesterday's meetings after three vaccinated horses had tested positive.
Newbury's "Super Saturday" meeting - where last year's Cheltenham Gold Cup victor Native River was due to have a final race before his title defence - is among the casualties, although there are hopes some of the big races may be rescheduled for a later date.
The BHA is expected to make a further annoucnement on Monday announcing whether racing can continue after Wednesday, or whether the suspension will continue.
"The full extent of potential exposure is unknown and we are working quickly to understand as much as we can to assist our decision-making". "But at this stage, we can not know if the infection came from recent arrivals or from horses returning from racing".
"We felt clarity for the industry was the most important thing at this challenging time".
"I don't know if that will need a few more days or not".
"But by taking the action we have over the last couple of days - we're effectively locking down the movement of racehorses, instructing trainers to take extra special precautions - we're fairly confident we will manage to restrict it".
"We're just letting all the customers who bought tickets, hospitality boxes and restaurant places the bad news then we'll get round to thinking about the future after that".
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"It augurs well for Cheltenham to go ahead in a month's time".
Rust said: 'We hope we've got a hold of it quickly and that by Monday we'll be in a position to make another call.
Thurles staged the only meeting in Ireland as scheduled on Thursday, and runners from Britain will not be permitted to race in Ireland until further notice.
Equine flu is a highly contagious respiratory virus which affects horses, mules and donkeys.
France is the root of this equine influenza outbreak with several sport horses in northern France testing positive for the infection. Tens of thousands of horses can be affected by one outbreak.
They will also reveal whether the outbreak will have any impact on next month's showpiece Cheltenham Festival.
Since the incubation period for equine influenza can be up to 72 hours, samples will be taken on Friday from horses that raced at Ayr and Ludlow and sent to the Animal Health Trust in Newmarket for analysis. You will have to test all the other yards that had runners at Ayr and Ludlow. After every race meeting it is a requirement to to do deep cleaning of the equine areas.
The outbreak was discussed in parliament on Thursday.