“I'm apalled,” said Breido Graf zu Rantzau, president of the German equestrian society (FN) from Warendorf, near Dortmund in North Rhine-Westphalia. “This is a catastrophy for equestrian sports.”
Werth's horse Whisper failed a drugs test the FN revealed on Wednesday.
The German Olympic equestrian team was dramatically disbanded just weeks ago, after four-time jumping champion Ludger Beerbaum was found to have doped his horse.
Christian Ahlmann's horse Cöster also tested positive for drugs during last summer's Beijing Olympics.
A hearing will be held on Thursday for Werth, who has demanded that the B-sample be tested. In the meantime, the two-time Olympic gold winner is suspended from all forthcoming events.
Horses are typically doped with Capsaician, an extract from the chilli plant, which stops horses feeling pain from old injuries but hyper-sensitises them to new stimuli such as a rider's instructions.
Four horses tested positive for the product during the the Beijing Olympics and their riders were subsequently disqualified, including Ahlmann.
In Germany, a three-member panel has been set up by the Federal Olympic Committee to investigate the slew of positive tests that have emerged since the Beijing Games.