In late 2374, the Federation Starship U.S.S. Destiny, commanded by Captain Raymer, was assigned to transport the ailing Dax symbiont from Deep Space Nine back to Trill after the death of its host Jadzia. En route, the symbiont's condition took a turn for the worse. Ensign Ezri Tigan, assistant ship's counselor, was the only Trill on board, so to ensure Dax's survival the symbiont was implanted in her by the ship's surgeon, in spite of Ezri's lack of preparation. The Destiny completed its journey to the Trill homeworld so that Ezri may visit with the Symbiosis Institute, but by then there was not much they could do. Greatly disoriented, Ezri requested a leave of absence from the ship, and Captain Raymer granted it.
Early the next year, on its way to join the Seventh Fleet at Kalandra during the Dominion War, the Destiny made another stop at DS9. Ezri Dax was to rejoin the Destiny's crew at that time in her prior position, but she opted instead to accept an offer to remain on DS9 as full counselor with a promotion.
References: Shadows and Symbols,