A report of equine infectious anemia at the training center in Cologne prompted German authorities to impose a quarantine preventing movement of any horses, including Danedream. There reportedly has been only one confirmed case of the disease, also known as swamp fever, but that was sufficient to trigger measures to prevent its spread.
"We regret extremely that Danedream cannot go and defend her title," the board of the Cologne race track said in a statement. "It is catastrophic for all involved."
Danedream was among the favorites for Sunday's Group 1 Arc at Longchamp in Paris. She easily won her prep race, the Grosser Pries von Baden, last month, after taking down the King George VI and Queen Elizabeth Stakes at Royal Ascot.
Her defection leaves Japanese Triple Crown winner Orfevre among the favorites for Sunday's race. He also won his prep race, two weeks ago over the Longchamp course that will host the Arc. British-based Nathaniel also is among the well-backed prospective starters with the field to be declared later in the week.
Danedream's absence also could play into trainer Aidan O'Brien's decision whether to send his superstar 3-year-old, Camelot, to contest the Arc. As of Monday, he was unsure about Camelot's plans.