- Stony coral tissue loss disease (SCTLD) is a novel coral disease that first emerged in Florida in 2014, and has now spread to 33 countries and territories in the Caribbean, including along the Mesoamerican Reef.
- SCTLD affects an unprecedented number of species (more than 30 species of reef-building corals), spreads quickly, and has a very high mortality rate.
- Researchers are still trying to figure out exactly what causes the disease.
- Researchers are also trying to understand how the coral microbiome is involved in or responds to SCTLD infection, and developing probiotics that they hope will offer an alternative treatment to antibiotics, with fieldwork in Belize, Colombia and elsewhere.
This May, divers found stony coral tissue loss disease on corals in Laughing Bird Caye National Park, Belize, for the first time.
The team from Fragments of Hope, a nonprofit, regularly monitors the site. A month previously there had been no sign at all of the disease. But on their trip in May, they found pillar (Dendrogyra cylindricus), symmetrical brain (Pseudodiploria strigosa) and mountainous star (Orbicella faveolata) corals covered with lesions, a telltale sign of this devastating disease that has slammed reefs in Florida and across the Caribbean over the past 10 years.
“That’s how fast it spreads,” says Lisa Carne, founder and director of Fragments of Hope.
Corals, like humans, sometimes get sick. But stony coral tissue loss disease (SCTLD) is different, researchers say. It afflicts an unprecedented number of species, has spread over a vast area, and kills incredibly quickly. Massive coral colonies, some hundreds of years old, can die within weeks or months, leaving reefs unrecognizable.
Miraculously, Laughing Bird Caye had, until now, been spared. The tiny island sits landward from Belize’s main barrier reef, separated by deep channels, and the team had hoped against hope that this might offer some protection.
“We’ve been in mourning for two days, because even though you know it’s coming, it’s really devastating,” Carne says.
As the disease continues to spread, and becomes endemic in parts of the Caribbean, researchers are mounting new efforts to figure out what is causing it — and what can be done to treat it.
What is stony coral tissue loss disease?
SCTLD is a novel disease that was first detected in the U.S. state of Florida in 2014. It affects a group called the stony corals, whose hard calcium carbonate skeletons build the reef structure. More than 30 species, about half of all Caribbean reef-building coral species, are affected. In the most susceptible species, mortality can be as high as 90%.
Researchers still don’t know exactly what causes SCTLD, but believe it’s a very complex disease, says Blake Ushijima, assistant professor at the University of North Carolina Wilmington. They know it’s transmitted via direct contact and through the water column, that it causes a breakdown in the symbiosis between coral and algae, and, because the disease responds to antibiotics, that pathogenic bacteria are involved, Ushijima says. But they don’t know if those bacteria are the cause of the initial infection, or just making things worse.
“The big part that’s really hard is, because we don’t know what the agent is that’s causing [SCTLD], we don’t know what to look for,” Ushijima says. “That’s kind of where we’re stuck. It’s this repeated thing.”
SCTLD hits the Mesoamerican Reef
The Mesoamerican Reef runs 1,000 kilometers (620 miles) from Mexico, through Belize and Guatemala, and into Honduras. It’s the second largest barrier reef in the world and of incalculable value for communities: as habitat for fish, for storm protection, tourism, cultural value, and more. “It’s one of the heartbeats of our country,” says Raphael Martinez, Belize coordinator for Healthy Reefs for Healthy People, a nonprofit that works throughout the Mesoamerican Reef.
SCTLD was first detected within the Mesoamerican Reef ecosystem in July 2018, in Puerto Morales, Mexico, one of the first sightings outside Florida. The scale of the outbreak was like nothing seen before, says Mélina Soto, Mexico coordinator at Healthy Reefs for Healthy People.
Researchers watched as the lesions crept over the corals, sloughing off the living tissue. Massive colonies of pillar, brain, boulder and maze corals were wiped out within days or weeks, Soto says.
Within five months, SCTLD had spread south along the entire Mexican portion of the reef, showing up in Belize in 2019 and in Honduras the following year.
“We had one [pillar coral] that was super iconic … 4 meters wide and 6 meters long [13 by 20 feet], a massive colony that took 100 years to grow, and in less than a month, it was already all dead,” Soto says. “Sometimes we were getting back on the boat, and just crying because of the helplessness.”
The impact on the reef has been dramatic, causing a shift to more weedy coral species, a loss of functional diversity, and threatening the ability of reef building to keep up with erosion and sea level rise due to climate change, according to a 2022 Communications Biology paper that looked at changes on the Mexican section of the Mesoamerican Reef.
“We lose individuals. But it’s also worrisome because we are also losing what is creating the reef, which gives all the other ecosystem services,” Soto says.
The only effective treatment for SCTLD, so far, is an amoxicillin paste, a type of antibiotic, developed by researchers in the U.S. Karen Neely, a research scientist at Nova Southwest University, helped develop the treatment, and it has been used by multiple U.S. agencies to treat some 30,000 corals in Florida. Her research has found it’s effective in slowing or stopping lesion progression, with no ill effects on things like gamete production or physiology. And, she adds, the amounts being applied are a fraction of the antibiotics entering the ocean from other sources.
Across the Mesoamerican Reef, various agencies are using the treatment as well, albeit in more limited trials.
Those trials have shown that antibiotics can be used to save some iconic corals or reefs, those with high tourism or community value, Soto says. But the treatment is time-consuming, needs to be reapplied, and can get expensive, she adds. There are also concerns about unintended consequences of applying antibiotics to reefs.
Ushijima says he’s glad that biologists have found something that works, but adds that antibiotics aren’t a long-term solution.
“The real danger is we don’t know exactly what’s going to happen when we generate antibiotic resistance. Is a pathogen involved with this coral disease involved with infecting other things?” he says. “We just don’t know. To me, it’s a little spooky.”
Microbiomes and probiotics
As an alternative to antibiotics, Ushijima and other researchers are trying to develop a probiotic treatment for SCTLD.
Some coral colonies are more resilient to the disease than others — and Ushijima says part of the reason could lie in their microbiome, the associated assemblage of bacteria, algae, fungi and other microbes found on and within corals.
Ushijima is working with collaborators in Florida and around the Caribbean to understand what happens to a coral’s microbiome when exposed to SCTLD. The researchers are also identifying specific beneficial microbes within the microbiome that seem to offer protection. The idea is that the researchers can then develop a probiotic full of those beneficial bacteria, which can outcompete the harmful bacteria or otherwise promote resilience.
“I always like to say, probiotics aren’t the silver bullet … but it’s another tool,” Ushijima says.
Some laboratory and field trials of probiotics in Florida have been promising. But Ushijima and other researchers caution that probiotics will likely need to be developed for specific regions, to minimize any risk of introducing novel microbes.
Valeria Pizarro, a senior researcher with the Perry Marine Institute, is working with Ushijima and other partners to develop a probiotic for the Colombian island of San Andreas.
Pizarro did her Ph.D. research on San Andreas, and knows the reefs so well she used to be able to navigate underwater without a dive light, by the shadows alone. But now many of the familiar landmarks are gone, she says. That includes some of the most iconic pillar corals.
“I have never [before] seen a species disappear from a reef in a couple of years,” Pizarro says.
She’s now collecting microbiome samples from the survivors by suctioning up the slimy mucus on the outside of the coral, and sending it to Ushijima’s lab, where bacteria are isolated, evaluated for effectiveness against SCTLD, and then tested for toxicity. The team has already identified some promising candidates, and if all goes well, hopes to start field trials by the end of the year. They say they hope these probiotics will have a range of applications, including as a prophylactic, to treat diseased corals and to help with restoration.
Sarah Gignoux-Wolfsohn, an assistant professor at the University of Massachusetts Lowell, is another one of Ushijima’s collaborators. She’s been monitoring corals at Carrie Bow Caye, in Belize, since 2019, and says SCTLD profoundly changed the reef.
“The impact [of SCTLD] was devastating,” Gignoux-Wolfsohn says. “There are some species that previously we saw a decent amount of, and now since 2021 we have not seen any individual of that species, and then other species, you know, it might be 50% mortality,”
As well as looking at changes on the reef, Gignoux-Wolfsohn and her team are sampling the coral microbiomes, by collecting the outer mucus layer and taking biopsy punches. Right now there aren’t plans to develop a Belize-specific probiotic, but the research will help shed light on how the coral microbiome responds to SCTLD, and how it varies across the Caribbean. The biopsy samples are also being used to look at the corals’ immune response to SCTLD infection, in collaboration with researchers at Texas State University.
Researchers are also trying to figure out what exactly causes SCTLD, which remains a major stumbling block. To do so, Ushijima’s lab and eight other universities and institutions have embarked on a mammoth collaboration to scrutinize SCTLD-infected corals from multiple angles. They’ve taken corals raised under biosecurity, and exposed fragments to SCTLD. The various institutions are now running different analyses. That’s hopefully going to yield some clues as to what’s causing the disease, Ushijima says, and they hope to have the data analysis finished by 2026.
SCTLD is not the only serious threat to Caribbean reefs. There are ongoing challenges with water quality and pollution. And, over the past two years, the reefs have been slammed by severe bleaching events due to elevated ocean temperatures.
Right now, it appears that SCTLD is not made worse by bleaching; in fact, infections seem to wane at higher water temperatures, Gignoux-Wolfsohn says.
“What’s really devastating is that the coral species that are resistant to SCTLD are often ones that are more susceptible to high temperatures and bleaching, and so we kind of see these alternating pressures on the reef that are leading to a more dramatic decline,” Gignoux-Wolfsohn says.
For researchers across the Caribbean, the combination is heartbreaking.
“We are at a point that we have to do everything we know and we can to be able to save coral reefs,” Pizarro says. “Because what we are doing right now, I don’t think that is enough.”
Banner image: A diver applies an amoxicillin paste to stony coral tissue loss disease-affected corals in Roatan, Honduras. The antibiotic paste can slow or halt lesion progression. Image by Antonio Busiello.
Citations:
Lee Hing, C., Guifarro, Z., Dueñas, D., Ochoa, G., Nunez, A., Forman, K., … McField, M. (2022). Management responses in Belize and Honduras, as stony coral tissue loss disease expands its prevalence in the Mesoamerican reef. Frontiers in Marine Science, 9, 883062. doi:10.3389/fmars.2022.883062
Mazurek, H. E., Neely, K. L., & Renegar, D. A. (2025). The impact of stony coral tissue loss disease and amoxicillin treatments on coral gametogenesis. Frontiers in Marine Science, 11, 1445735. doi:10.3389/fmars.2024.1445735
Neely, K. L., Macaulay, K. A., Hower, E. K., & Dobler, M. A. (2020). Effectiveness of topical antibiotics in treating corals affected by Stony Coral Tissue Loss Disease. PeerJ, 8, e9289. doi:10.7717/peerj.9289
Neely, K. L., Whitehead, R. F., & Dobler, M. A. (2024). The effects of disease lesions and amoxicillin treatment on the physiology of SCTLD-affected corals. Frontiers in Marine Science, 11, 1460163. doi:10.3389/fmars.2024.1460163
Ushijima, B., Gunasekera, S. P., Meyer, J. L., Tittl, J., Pitts, K. A., Thompson, S., … Paul, V. J. (2023). Chemical and genomic characterization of a potential probiotic treatment for stony coral tissue loss disease. Communications Biology, 6(1), 248. doi:10.1038/s42003-023-04590-y
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