Cologne racecourse general manager Benedikt Fassbender said: "It's a nightmare. We are the biggest training centre in Germany and one horse has tested positive for equine infectious anaemia (EIA).
"For three months we are now in quarantine – no horses can come in to the track, and no horses can come out. It is a disaster, especially as Danedream cannot take part in the Arc at the orders of the special vets.
Danedream has not been infected by the disease, but every horse will be tested for the infection in the next few days."
Patrick Barbe, racing manager for Dane-dream's part-owner, Teruya Yoshida, said: "At the moment, Danedream is very well and has not been infected. The only horse who has been infected is on the other side of the racecourse.
"Because the Arc is only now in a few days' time, I'm very pessimistic she will be allowed to run."