Three horses in an active racing yard tested positive for the highly contagious disease, and the British Horseracing Authority took the extreme measure to call off meetings at four United Kingdom tracks in a bid to prevent it spreading further.
As a result all meetings on Thursday have been abandoned.
All four of Thursday's race meetings have been cancelled because of an outbreak of equine flu.
It takes about a week for the equine flu virus to live and die, so the BHA are clearly hoping that a week with no movement of horses will contain the outbreak.
The decision comes after three vaccinated horses in an active yard tested positive for the disease.
A statement from the British Horseracing Authority (BHA) read, "Horses from the infected yard have raced at Ayr and Ludlow, potentially exposing a significant number of horses from yards across the country and in Ireland".
The British Horseracing Authority (BHA) said in a statement there was significant concern over the disease spreading.More news: Apple starts selling refurbished iPhone X from $769
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Equine influenza is endemic in Britain's horse population, but vaccination against the disease is mandatory for thoroughbreds and most other breeds that are used competitively, such as eventers.
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.
In a statement, the BHA said: "This precautionary approach is meant to ensure we put the health of the horse population and control of the virus first, and avoid any unnecessary risk that might come from returning to racing too quickly". It is communicating with yards potentially exposed to ensure appropriate quarantine and biosecurity measures are put in place and horse movements restricted.
"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".
"We are scrupulous about observing the health status of horses in our care and taking the necessary steps to treat any condition that may affect them".
McCain reported the cases of flu to the BHA after his vets relayed the news on Wednesday evening, and the trainer was keen to emphasise he would never knowingly have run a potentially infected horse.
Symptoms to be aware of range from an increased temperature, coughing and nasal discharge and the horse being off feed to more severe respiratory signs - your veterinary surgeon should be contacted for advice under current circumstances if these signs are present.