In the mirror universe, the Trill were a humanoid species joined with symbionts.
In 2287, Saavik, part of a Terran Empire official state visit to Trill, learned of the existence of symbionts and journeyed into the Caves of Mak'ala to ascertain whether Trill might prove useful allies to the burgeoning Memory Omega project. Saavik learned that the symbionts had been infested with bluegill parasites and posed a dangerous threat not only to the Empire, but all life in the galaxy.
In response, Emperor Spock ordered the extermination of all symbionts and their hosts, although Curzon Dax's life was spared. In 2379, Curzon died, but not before Dax was transferred to Ezri Tigan. (ST - Mirror Universe novels: The Sorrows of Empire, Rise Like Lions)