The Trill, or Trills, were a humanoid species native to the planet Trill. A small percentage of the Trill population co-existed with a sentient symbiotic organism known as a symbiont inside their bodies, a relationship that played a significant role in Trill culture and spirituality, going as far back as the 12th century. (DIS: "Forget Me Not")
Joined Trills had personalities which were a synthesis of the two beings including the memories, skills, and personalities of the previous hosts. As late as 2367, the fact that some Trill exist as a joined symbiotic species was not widely known, even to Federation scientists. (TNG: "The Host")
The Trill were a space-faring race by the early-23rd century, considered a generally peaceful people by the Federation. (DIS: "Forget Me Not") Trill could be found visiting Vulcan early-23rd century. (DS9: "Destiny") By the 2240s, they could be encountered on Earth, and by 2257, it wasn't unusual to see a Trill on Qo'noS. (DS9: "Trials and Tribble-ations", DIS: "Will You Take My Hand?")
Despite the relatively unusual nature of being a joined species, that aspect of their culture was not widely known until 2367, when the Odan symbiont had to be given a new host during the middle of tense negotiations on Peliar Zel. Such a public example of the dual nature of the Trill brought to the forefront what had previously been a very private matter to Trill for several millennia. (TNG: "The Host")
The Burn in the 31st century resulted in the decimation of the symbiont and joinable-host population. By the 32nd century, Trill could be found at the exchanges, but on the homeworld, reconnecting with a symbiont was considered a blessing. (DIS: "People of Earth", "Forget Me Not")
Most Trills were distinguished by two rows of dark spots going down each side of their bodies, from the top of their forehead down their neck to their toes. Their skin color could vary. One of the main neurotransmitters in the Trill brain was isoboramine. (DS9: "Equilibrium") Julian Bashir and Miles O'Brien were known to be curious about how far the spots went. (DS9: "What You Leave Behind") Joined Trills were also extremely allergic to insect bites; the biochemical connections between the host and symbiont could not tolerate the reaction caused by the insects' venom. (DS9: "The Siege") Trill were known for having cold hands. (DS9: "A Man Alone")
The few Trill that were given a symbiont were typically joined in their early- to mid-twenties. The physical process of being joined was irreversible. Once joined, the host and symbiont were dependent on each other after a ninety-three-hour period in which the symbiont bound itself to its host's nervous system. If the symbiont was removed from the host, symbiont and host would die within hours, even if they were otherwise healthy, unless they were implanted into another host or with another symbiont. (DS9: "Dax", "Invasive Procedures")
The brain of a joined Trill has two cerebral nuclei and two different brain wave patterns. In 2369, Julian Bashir compared them with two linked computers, which both work for the same task. (DS9: "Dax")
The Trill were a technologically advanced species and the Trill Science Ministry was a leading center of learning and experimentation. (DS9: "Rejoined", "In Purgatory's Shadow") The Guardians that cared for the symbionts were considered leaders in Trill spirituality. (DIS: "Forget Me Not")
Jadzia Dax joked that Trills didn't look for romance the way Humans do. Joined Trills considered it quite a nuisance and viewed it as a weakness of the young. While hosts may have romantic feelings as often as any other sentient species, symbionts tried to live on a higher, more spiritual plane and tried to rise above those sorts of temptations. (DS9: "A Man Alone")
In contrast to the greater revelation of their nature, the Trill were not a secretive species. To them, the joined nature of their culture was normal and not something they would think to comment on without prompting. Indeed, joined Trills could be seen as particularly genial and many served the Federation as distinguished ambassadors, including Odan and Daxs.
Trill law forbade reassociation between subsequent hosts of joined persons, whose symbionts were romantically involved in their previous hosts, and the people who the previous hosts were romantically involved with. This was because the main purpose of the transfer of symbionts was to experience new things in life. Trills who were found guilty of reassociation were expelled from Trill society, meaning that their symbionts die with their current host. (DS9: "Rejoined") Beyond the illegality of it, Trills almost universally viewed reassociation as a cultural taboo, but the degree to which they followed it was up to the individual. Some Trill took it so far as to not associate with any of their previous hosts' friends, family and colleagues, beyond the rare chance encounter. Others had a very narrow interpretation, and freely associated with the friends and acquaintances of their previous hosts and symbionts, avoiding only those with whom their previous hosts were intimately involved.
On average, only three hundred symbionts were available for hosting each year and about a thousand Trills apply for joining. Because there were many more humanoid Trills than symbionts, prospective hosts were weeded out by a demanding selection procedure, overseen by the Symbiosis Commission. (DS9: "Equilibrium") The competition for the few symbionts was fierce and attracted the brightest and most highly motivated of Trill society. Often, the would-be hosts exceled in their chosen fields and it was not uncommon for them to hold several degrees or distinctions prior to their joining. Prospective hosts may eventually become initiates under the supervision of a field docent, a joined Trill who evaluates the prospective host's suitability for joining and makes a recommendation to the Commission. A negative recommendation usually meant the initiate was cut from the joining program. (DS9: "Playing God")
Common belief in Trill society holds that only one in a thousand Trills make acceptable hosts. In fact, this figure was vastly understated, and nearly half of the Trill population was capable of being joined. The myth was perpetuated very carefully, though, in order to avoid widespread chaos which would probably arise if the information were made public, since the symbionts would become, essentially, objects to be fought over, as people fought to gain the few prized symbionts. (DS9: "Equilibrium")
Unless the joined Trill objects, a prospective host may request a specific symbiont. If a host was weak, the personality of the symbiont would overwhelm it. Joined Trills had several tools at their disposal for dealing with various aspects of their previous hosts. The telepathic ceremony of Himantura allowed a current host to divest the symbiont of the personality and all the memories of a previous host, which were temporarily hosted in volunteers. The transferring process was performed and supervised by a symbiont Guardian, an unjoined Trill telepath. The ceremony created a chance for closure by having the new host address the previous hosts directly, as a means of distinguishing their voices and cementing the sense of finality of its latest transition. Similarly, the Rite of Emergence could focus the voice of a single previous host among the memories of the other, allowing for a more direct conversation in times of need for the current host.
The zhian'tara ritual was roughly similar to the Vulcan fal-tor-pan ritual, since both enable the respective species to perform a synaptic pattern displacement, the transfer of what could be considered as a soul (katra, pagh, etc.). (DS9: "Invasive Procedures", "Facets", "Field of Fire")
In the mirror universe, it was unclear which side (if any) the Trill government took in the conflict between the Klingon-Cardassian Alliance and the Terran Rebellion. A number of individual Trills were part of the Rebellion, while others would attempt to remain non-aligned. (DS9: "Through the Looking Glass")
Ships and technology
- Trill diagnostic tool
- Trill piano
- Trill transport
- Trill central database
- Trill weapon
- "The Host"
- Star Trek films:
- "Author, Author" (hologram)
At the time the concept of Trills was first conceived, the idea was (at least in the opinion of Brannon Braga) "very fresh." Ronald D. Moore suspected that many freelance writers would have centrally featured the Trill in the premise of TNG: "The Host", the first episode to depict a Trill, to a degree more than the installment actually does. (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 221)
For the make-up team tasked with designing the look of the Trills, the challenge was threefold: the humanoid host, the abdominal cavity, and the symbiont. (Star Trek: Aliens & Artifacts, p. 116) The facial appearance of Trills was originally designed by Michael Westmore for "The Host". "The original Trills had a forehead appliance and a little nosepiece," he said. (Cinefantastique, Vol. 23, No. 6, p. 31) The appliance used for the nose and forehead downplayed the alien nature of the Trills, causing them to seem just slightly extraterrestrial but still overtly human. (Star Trek: Aliens & Artifacts, pp. 116-117)
The requirement of creating a body cavity for the symbiont was much more challenging. The make-up department crafted an appliance for the stomach area, covered with enough hair to blend in with the actor's own body, with three air-bladders beneath the fake skin. Activating the bladders, by blowing air into them via tubes, could give the impression that the skin was being moved by the symbiont living inside the host. Since the stomach appliance was made with extremely soft rubber, the pulsating bladders were able to swell inside the whole area of the cavity, expanding it noticeably, which made for a very effective shot. (Star Trek: Aliens & Artifacts, p. 117)
At first, Trills were not to have featured as much on Star Trek: Deep Space Nine as they ended up being, because the creative staff initially considered a variety of alien species for the character of Jadzia Dax. "Of them, the Trill seemed the most interesting," noted Michael Piller. (Cinefantastique, Vol. 23, No. 6, p. 20) He also said, "Having a Trill seemed like a really, really good idea, at the time." ("New Frontiers: The Story of Deep Space Nine", DS9 Season 2 DVD special features) Once the Trill species was selected, explaining the concept of a Trill to those who auditioned for the role proved difficult, hampering the casting process. (Star Trek - Where No One Has Gone Before, paperback ed., p. 179) Afterwards, Piller remarked, "I could write a book about Trills now, but what does it mean in the day-to-day existence of these people? […] How do you make it something alien and yet accessible?" (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 333)
In the series bible of Star Trek: Deep Space Nine, the Trill symbionts and humanoids were said to have lived underground and on the surface of Trill respectively, "many centuries ago." The description continued, "Due to an environmental disaster, they were forced to 'join' to survive. As time went on, this mutual support evolved into a biological interdependency and thus two individuals became one – they speak with one voice (you can't ask to speak with the symbiont or the host, only the combined lifeform)." 
It was initially planned for the Trills of DS9 to look basically identical to Odan, who had established the Trill appearance in "The Host". The make-up was consequently to still consist of several distinguishing marks on the forehead. (Cinefantastique, Vol. 23, No. 6, p. 21) Actress Terry Farrell recalled how the Trill makeup was being planned when she was cast as Jadzia Dax; "[Makeup Supervisor Michael] Westmore [...] was about to put this whole thing on me [....] I didn't know that the [makeup] testing would take months to figure out that I didn't need a forehead." ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) Indeed, a new prosthetic forehead, much like the original, was designed by Westmore. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) The appliance was filmed in test footage involving Farrell. (Star Trek: Deep Space Nine Companion (p. 7); "Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) At first, the forehead matching Odan's was used for only two days. "Then they kept reducing it with each test," related Farrell, "until it really looked like someone had just hit me in the forehead. But Paramount didn't want to make me look strange." (Star Trek: Deep Space Nine Companion (p. 7)) The forehead appliance was used over the course of a few weeks. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) "We had only shot one of her scenes, which [the producers] wanted to redo for some other reason, so they took a new look at the makeup," explained Westmore. (The Official Star Trek: Deep Space Nine Magazine issue 1, p. 28)
After Paramount executives watched the dailies of Terry Farrell in DS9 pilot "Emissary", they issued a rare ultimatum about the Trill make-up scheme. The executives insisted the make-up be changed via removal of the forehead markings; having spent a long time seeking a beautiful actress for the role of Dax, Paramount wanted to avoid Farrell being defaced by prosthetics. (Cinefantastique, Vol. 23, No. 6, p. 21) The actress offered, "I just remember that somebody said, 'What did you do to her? She used to be beautiful and you gave her this weird head!'" ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) The alarmed reaction to the Trill forehead finally resulted in the more common spots seen on Deep Space Nine. Hence, the reasons for the change were purely cosmetic. (Star Trek: Deep Space Nine Companion (p. 7); "Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) Like the unused forehead, the Trill spots were designed by Michael Westmore. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) He took inspiration for the spots from Famke Janssen's Kriosian make-up in TNG: "The Perfect Mate". (TV Zone, Special #34, p. 12) The DS9 makeup team thought the Trill spots would look sexier than the earlier ones, though. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) "I didn't want to do the exact same thing," recollected Westmore, "so we experimented around the front of Terry's face and the powers-that-be finally accepted that design." (TV Zone, Special #34, p. 12) The resultant distinguishing marks were tested over a couple of weeks. One of the tests, labeled "Take 3", was filmed with a TNG clapperboard. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) Observing the entire make-up layout usually worn by Farrell, Westmore said, "Aside from her regular beauty makeup, she has a pattern around her hairline and ears, and down her neck." (The Official Star Trek: Deep Space Nine Magazine issue 1, p. 28)
Although the writers of DS9 were obviously aware of the physical differences between Odan and the Trill in DS9, no canon explanation has ever been offered. In an attempt to explain the variations, Terry Farrell once suggested that the spotted members of the species were from the north of Trill whereas Odan was from the planet's south. (Cinefantastique, Vol. 23, No. 6, p. 21) She also regarded the spots as natural-looking. "It's not much different than freckles or leopard spots or zebra stripes," Farrell observed. (Hidden File 05, DS9 Season 1 DVD special features)
On DS9, Dax's Trill makeup was regularly applied by Michael Westmore; faced with having to select only one makeup to ordinarily apply, he chose the Trill spots, which pleased Terry Farrell and meant they spent a huge amount of time together. "And it was a really exciting makeup," enthused Farrell, "because makeup artists would always say, 'My god. How do they do the spots?' [....] And so, when he started doing it, I said, 'You know, it's really neat and I get a lot of compliments from you doing my makeup.'" ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features) Westmore used brown make-up for the Trill spots, and subsequently accented them with burnt orange coloration. He applied the spots by hand, employing a technique whereby he deliberately made his hands quiver while painting the markings on. Westmore never used a stencil, so the spots were always unique.  "It's a laborious job," he remarked. "There seems to be no easy way to make up a stencil for that. It just has to be done every day." (Cinefantastique, Vol. 23, No. 6, p. 31) Farrell stated, "Michael Westmore [...] [used] two different colors of watercolor. The first season we experimented with art pens, but they would take me two or three days to get off of my skin – not pleasant!" Applying the spots generally took slightly longer than an hour each day, though Farrell thought the process would likely have taken less time if she and Westmore hadn't enjoyed conversing with each other quite so much as they did. (Star Trek: Deep Space Nine Companion (p. 7)) In hot shooting conditions, the sweat of the Trill-playing actors might cause the watercolor to run down the side of his or her face, so the actor had to be kept cool. ("Crew Dossier: Jadzia Dax", DS9 Season 2 DVD special features)
The disparity between Trills seen on TNG and the later Trills goes beyond makeup. In contrast to DS9's depictions of the species, "The Host" seems to make it clear that the symbiont was dominant (in fact, the symbiont is referred to as a "parasite" on more than one occasion), and that the host was merely a body with no influence over the joined entity. Odan was apparently unable to safely use the transporter, while Dax used it often with no trouble at all. Ronald D. Moore noted, "The Trill were not supposed to go through transporters and we slid by it (like the original Trill makeup)." (AOL chat, 1997) Furthermore, whereas Odan was ready to pick up where they and Crusher had left off, a Trill taboo against reassociation was, early in the series run of DS9, suggested by Michael Piller.
Trill society was foremost on Michael Piller's mind when he devised the ban on reassociation. "He felt they'd have to have a very strict taboo in order to avoid an aristocracy of the joined," reflected René Echevarria. "Otherwise, they'd all want to hang out with each other, their dear old friends from five hundred years ago, and it would become a really screwed-up society." (Star Trek: Deep Space Nine Companion (p. 279))
Though Star Trek: Deep Space Nine Companion (p. 86) pretends the DS9 writers occasionally used a handbook called "Everything You Always Wanted to Know About Trills", details about the species were actually established as and when required, such as for the DS9 episode "Invasive Procedures". "In this case, it was important to establish how difficult it was to get a symbiont," recalled Robert Hewitt Wolfe, who was credited with co-writing the installment. "It said something about [...] the Trill culture that was interesting." The ad hoc method of inventing additional aspects of the species also resulted in the writers later revealing, in the DS9 outing "Equilibrium", that it wasn't as hard to qualify for a symbiont as the Trill populace had been led to believe. Regarding the improvisational system the writers used to define the species, Ira Steven Behr commented, "It's dangerous but fun. The whole Trill concept is very difficult, but 'Invasive Procedures' dealt very successfully with it." (Star Trek: Deep Space Nine Companion (pp. 86-87))
The notion of structuring the DS9 episode "Playing God" around a storyline about Trills, featuring Jadzia Dax and a Trill initiate, was suggested by Michael Piller. (Star Trek: Deep Space Nine Companion (p. 126)) Jim Trombetta, who wrote the story, recalled, "I had a little trouble writing that because I didn't really believe there was anybody that wanted to have a worm inside their body [....] [The DS9 staff writers] said, 'No, we've gotta have it. It's an alien race; it means something different to them than it does to us.'" (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 70) Ira Steven Behr commented, "I thought it [...] was an interesting show about Trills." (Cinefantastique, Vol. 25/26, No. 6/1, p. 105)
Another DS9 installment that features the Trill species is "Equilibrium". Terry Farrell remarked, "It revealed a lot about Trill society that we didn't know anything about before. The really big revelation was that about half of all Trills could actually be joined." (The Official Star Trek: Deep Space Nine Magazine issue 14, p. 7) For Robert Hewitt Wolfe, the opportunity to explore Trill society in that episode intrigued him. Wolfe especially enjoyed how the installment demonstrates that the Trill were meant to be considerably unpredictable and different from Humans. "I think the culture is more complicated than that," he explained, "and I think this show demonstrates that complication, which I like." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 84) Ira Steven Behr also approved of how the Trill were developed in "Equilibrium". "It's a fascinating subject. So fascinating it's deep waters to get into. I just love the scene with the guardian and the Trills – weird kind of stuff we don't quite understand. I also like a society that's basically medical. Its a bunch of people really involved in their bodies." (Cinefantastique, Vol. 27, No. 4/5, p. 95)
By DS9 Season 4, the duration needed to apply the Trill makeup for Jadzia Dax had been reduced to forty minutes daily, despite the Trill spots still being done entirely by hand. (Star Trek Monthly issue 9, p. 46)
In the fourth season DS9 installment "Rejoined", a romance between two female Trill, namely Lenara Kahn and Jadzia Dax, was highly controversial both for the Trill in general, due to it being in breach of their taboo against reassociation (the prohibition was first established in "Rejoined"), and for the viewing audience, as some viewers rejected the same-sex relationship as well as a kiss between the Trill lovers. "Trills have no reason to be against same sex couples," Robert Hewitt Wolfe remarked, "but what a better way to show that they were a totally different, alien society than to see them reacting in self-righteous indignation just because they were lovers in a previous life?" (Cinefantastique, Vol. 28, No. 4/5, p. 44) Never having played a Trill before, Lenara Khan actress Susanna Thompson consulted Terry Farrell with lots of questions about the species. "She was very helpful to me, to give me the Trill background, to answer all my picky questions [....] She was quite open with all of that, as much as she knew," Thompson recollected. (Star Trek: Deep Space Nine - The Official Poster Magazine, No. 5) Farrell herself noted that, upon directing the kiss, Avery Brooks tried "to pretend we're not aliens in outer space," instead concentrating on the emotional honesty of the scene. (Cinefantastique, Vol. 28, No. 4/5, pp. 44 & 46)
There is relatively a lot of exposition about Trills in "Rejoined", for the benefit of viewers not intimately familiar with the species or the characters. Commented Ronald D. Moore, "Not everybody knows Dax's family history, or what Trill society is about." Ira Steven Behr remembered, "We just wanted to make it clear that 'This is why this is going to happen, because we're not doing a show about lesbians, we're doing a show about Trills, and to make sure you know that, we're going to tell you what Trills were.'" (Star Trek: Deep Space Nine Companion (p. 280))
The emblem of the Trill was designed by Anthony Fredrickson. (Star Trek Sticker Book, p. 22) A white and silver gradient version was first used in the wardroom in "Rejoined", though the transparency was displayed backwards. This version was used in the Star Trek Encyclopedia, with the intended orientation. Both a gray and a gold version appear on monitors in "Prodigal Daughter."
For DS9 Season 5 outing "Let He Who Is Without Sin...", portraying the Trill spots called for extensive work by Michael Westmore. Explained Terry Farrell, "Mike had to come in and paint them all the way down to my toes, 'cause I had a bathing suit on in that [episode]." ("Crew Dossier: Jadzia Dax", DS9 Season 3 DVD special features)
The Star Trek: Deep Space Nine Companion proclaims, "Trills don't like heat," and observes that this can be seen when Jadzia Dax takes off parts of her uniform while on the surface of jungle planet Soukara in DS9 Season 6 entry "Change of Heart". (Star Trek: Deep Space Nine Companion (p. 545))
At the time Terry Farrell left DS9's cast, Michael Westmore decided to no longer hand-illustrate Dax's unique Trill spots himself, though he ended up "detailing" Ezri Dax actress Nicole de Boer throughout the show's seventh and final season. (Star Trek: Deep Space Nine Companion (p. 594))
When the character of Ezri Dax was created at the start of DS9 Season 7, Ira Steven Behr also hit upon the idea that a newly joined Trill could be driven crazy by the previous lifetimes experienced by his or her symbiont. "I just loved this idea of joined Trills being basically schizophrenic," Behr recollected. "We'd never played up the idea in the past, but it made sense. What must it be like to have all those voices and opinions? 'Do I like raktajino or do I only think I like it?' It would give us all this wonderful stuff to play." (Star Trek: Deep Space Nine Companion (p. 593))
While preparing to do a Los Angeles audition for the part of Ezri Dax, Trill society was one subject which Nicole de Boer questioned the production personnel about. "I asked, 'What kind of people were Trills? Was it an honor to be joined?'" she recalled. (Cinefantastique, Vol. 32, Nos. 4/5, p. 44)
As early as 1993, the Trill were seen by some fans as having parallels with transgender people. A reviewer of "Invasive Procedures" saw a transsexual metaphor and commented: "Watching this episode I was struck by the parallel between the Trills and transsexuals here on Earth. In either case, the goal of surgery is long sought for and desperately wanted. There are " gatekeepers " who decide who may and who decide who may and who may not proceed. And those who fail to get the precious approval can feel they've lost their reason for living". (Where Few Have Gone Before, Volume 1, Issue 1, page 6) In 1997, Jadzia appeared on the front cover of a transgender magazine. 
In Mark A. Altman's opinion, the Trill had to (at least when DS9 started) be depicted in "a sort of humorous way." Altman commented, "If you play it straight, it could become a kind of hokey science fiction concept." (Trek: Deepspace Nine, p. 68)
Even after both TNG and DS9 ended, Michael Westmore applied the Trill makeup one last time in 2014, applying the Trill spots to Terry Farrell's face once again, for a Star Trek convention event called Dax 539.  
Dating First Contact
It is not known when Trills and Humans made First Contact, but, according to DS9: "Trials and Tribble-ations", Emony Dax was known to have visited Earth in the mid-23rd century, where she was apparently close to Leonard McCoy.
According to the reference book Star Trek: Star Charts (p. 33), the Trill civilization had warp capability since antiquity and joined the Federation in 2285. Living on the planet Trill in 2370 were 650 million Trill and 11 million symbionts.
A link between the Trill symbionts and the alien parasites seen in TNG: "Conspiracy" is suggested in the Pocket DS9 short story "Sins of the Mother" (in the anthology The Lives of Dax) and the novel Unity. This was expanded upon in Worlds of Star Trek: Deep Space Nine, Volume Two. The neural parasite was first seen as it traveled in a comet passing close to the Trill homeworld. Audrid Dax and her husband, Jayvin Vod, went to the comet with a Federation team to examine it when Trill scanners showed a being similar to a symbiont in the comet. However, as the Federation was not yet aware of the Trill's symbiotic relationships, it was kept in secrecy. Upon coming into contact with the chamber of the parasite, it bonded with Jayvin, as well as the Vod symbiont, ultimately causing both of their deaths.
The inconsistencies between TNG and DS9 Trill were dealt with in the novels by suggesting that ridged Trills and Trills who could not be transported were subgroups of Trill society; both were mentioned or shown during the scenes on Trill, and the inability of some Trill to be transported was a plot point.
The comics took a far different course, such as the Star Trek: Divided We Fall mini-series, where both Odan hosts were shown without facial ridges but with spots. All Trills in the series were shown with the spots instead, no reasoning given.
In another novel, Forged in Fire, the ridged Trills were said to result from a strain of the Augment virus that managed to infect a Trill colony through visiting Klingon traders. However, those Trills did not seem to be interested in working on methods of restoring the original Trill look, with this "sub-group" having recently (at the time the novel was set) being re-accepted back into Trill society. This division among the groups possibly explains why Odan was so different from other Trills encountered in the series. The novel also notes that the Trill avoidance of transporters was actually a means of avoiding the nature of the symbiote being disclosed rather than the process being dangerous to the symbiote.