...I do detest the novel (ha!) idea of tying the symbionts with other Trek parasites, biologically and historically. We could just as well invent a relationship between Theodore Roosevelt and the sehlat species, or IKEA and the Borg.
But it's quite possible the Trill symbionts are cosmopolitan. If I lived in a milk bath, never seeing anything but fellow slugs and those lactose-tolerant animals that came there to drink and fell prey to my kin, and suddenly managed to invade the body of a humanoid astronaut, surely I wouldn't settle with examining just planet Trill. I'd be out at warp nine, and then back with more humanoids for my family so that they could see the wonders, too. Depending on how long ago the first Trills got starborne, there could well be related cultures and even new, related species all across the galaxy.
However, if the Trill have been at it for more than a couple of centuries, it becomes all the more difficult to believe that they could have pulled off their galaxy-wide conspiracy of non-exposure. And in "a couple of centuries", nobody gets to spread through the entire galaxy - if the absolutely manic humans didn't manage it, nobody will!
Timo Saloniemi