McCain also sent a runner to Wolverhampton's meeting on Monday evening and all horses which competed on the card will be tested for equine influenza, along with all runners at the meetings at Ludlow and Ayr on Wednesday.
Racing is still set to continue in Ireland, where Thursday's meeting at Thurles took place, but no entries from British stables will be accepted until further notice.
The British Horseracing Authority has confirmed three cases of equine influenza, triggering the cancellation of all racing in Britain for the next several days and the shutdown of training yards across the country.
Leading Welsh horse trainer, Tim Vaughan, says the equine flu outbreak, which has forced the cancellation of all race meetings across the United Kingdom, could last up to 10 weeks, and could affect the Cheltenham Festival.
In the wake of last night's announcement of the flu outbreak, all of Thursday's meetings in the United Kingdom, at Ffos Las, Doncaster, Huntingdon and Chelmsford, were cancelled.
Horse racing in Britain will not resume until next Wednesday at the earliest after three vaccinated horses tested positive for the disease.
The governing body's statement added: "The BHA is presently communicating with yards potentially exposed to ensure appropriate quarantine and biosecurity measures are put in place and horse movements restricted to avoid possible further spread of the disease". Symptoms in non-immune animals include high fever, coughing and nasal discharge.
A further update on the possible continued extent of disruption is expected from the BHA - with a packed weekend of Cheltenham trials and other big races scheduled at Newbury, Warwick, Musselburgh and in Ireland.
"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". We recommend that any trainer who has concerns about the health status of any of their horses should contact their veterinarian'.
It can be airborne over reasonable distances as well as be transmitted indirectly, including via people.
It's not usually fatal if the infected horse is healthy and fully grown, however foals, pregnant mares, and horses who are already sickly are particularly vulnerable to equine flu, and could die as a result of exposure to the disease.