And now a horse has died at the show, and others have tested positive, that previously tested negative. My condolences to the owner of the lost horse. 
The “isolated” horse at our small barn, who returned from Thermal during the initial outbreak, and was to be isolated for a week and temp taken twice a day (per the barn owner), didn’t stay isolated for the entire week. His teenage owner/rider came to ride him with her friends on their horses; the groom walked him down the barn aisle (not the barn where he was in isolation) from the community cross ties, past other horses in their stalls, and to the mounting block, so she could get on. During the week, when he was turned out, he’d drink from the same water bucket as the other horses that use the same turnout.
After watching all this, I understand now that it was the barn owner who insisted on the isolation, and the owner/rider didn’t have enough respect for barn management, and the other horse owners, to follow management’s policy.
If this can happen at our very small barn, it’s happening at larger ones too.