Racing has been suspended at Del Mar in California after 15 jockeys tested positive for coronavirus.
The San Diego track was due to hold fixtures on Friday, Saturday and Sunday.
All riders set to mount at the course in California were checked after leading jockeys Flavien Prat and Victor Espinoza tested positive.
Prat rode the 65-1 shot Country House to a dramatic victory in the 2019 Kentucky Derby. While Espinoza guided American Pharoah to the first US Triple Crown in 37 years.
"A common factor among all but one of the riders that tested positive is that they rode at the recently concluded Los Alamitos meet," the Del Mar Thoroughbred Club said.
Officials say the affected jockeys were asymptomatic and are now isolating at home.
They hope to be able to resume action at the course on July 24th. Dr Eric McDonald, the medical director of San Diego epidemiology and immunisation department said this is possible.
"Assuming these individuals continue to show no symptoms, they will be isolated for a total of 10 days and should be able to resume their usual activities, including riding after that time," Dr McDonald said.
The season at Del Mar began on July 10th without spectators for the first time in its 81-year history.
Safety measures for those taking part included temperature checks, face covering and social distancing requirements, additional hand washing and sanitiser stations, and regular disinfecting of all common areas.
Many of the contests that were due to be held this weekend were moved to the next race-card, including the $150,000 San Diego Handicap and the $200,000 Eddie Read Stakes.
“Canceling this weekend’s races will give us additional time to monitor the situation and give the individuals who tested positive additional time to recover," said Del Mar chief executive Joe Harper.
As of Thursday, California had over 355,000 confirmed cases of Covid-19, the second-most in the country behind New York.