ANTANANARIVO — Researchers studying a coral disease that has devastated reefs across the Caribbean say they’ve come up with a promising treatment that avoids the long-term harm associated with antibiotics, currently the most effective remedy.

In a study published November 2024 in Frontiers in Marine Science, the researchers showed how the application of chlorine, typically used as a disinfectant to kill microorganisms, can reduce the impact of stony coral tissue loss disease (SCTLD). The disease attacks the soft tissue of a wide range of reef-building coral species, killing them quickly. Mortality rates in some cases can be as high as 100%.

Marine biologist Greta Aeby, an independent researcher in Hawai‘i, and colleagues mixed chlorine into cocoa butter so that they could get a paste to apply onto diseased corals at Horseshoe Reef, near the British Virgin Islands in the Caribbean.

After approximately 80 days, the rate of tissue loss among hard corals when treated this way was 17.6%. Over that same period, treatment with a paste of the antibiotic amoxicillin resulted in a tissue loss rate of around 1.7%. That makes the antibiotic, which is commonly used to treat SCTLD, significantly more efficient than chlorine.

But the use of antibiotics has long raised concerns among researchers, who warn that long-term use can contribute to marine pollution, which is harmful to living organisms due to the development of antimicrobial resistance. “Crabs, fish and even humans within the same environment face a high risk of contracting bacteria that are resistant to antibiotics,” Aeby said.

In this context, she said, the use of the chlorine and cocoa butter solution presents an efficient remedy against coral reef diseases.

Antimicrobial resistance is responsible for 1.27 million deaths worldwide every year, according to the World Health Organization, partly due to the excessive and indiscriminate use of antibiotics, including through self-medicating.

It was in an effort to find an alternative to antibiotics’ side effects, a crucial health challenge, that Aeby and her colleagues sought to innovate with the treatment of SCTLD. In addition, the increasingly popular concept of One Health demonstrates the need to unite efforts to treat complex interactions between human, animal and ecosystem health, in order to build a healthier and more sustainable future for all.

Advantages of chlorine-cocoa butter paste

The chlorine-cocoa butter mixture holds several advantages, according to the researchers. It disintegrates easily, and the chlorine naturally deactivates within a day, while antibiotics have a big impact on the environment.

The mixture is also easy to produce, with the required ingredients available from hardware shops and other authorized suppliers. The production of antibiotic paste, by contrast, is tedious and too expensive for conservationists who have limited financial resources, study co-author Argel Horton, a marine biologist with the British Virgin Islands’ environment ministry, said in a statement.

Scientists consider chlorine-based treatment for SCTLD a relatively effective method capable of stopping the spread of this disease. But they also acknowledge the limitations of their proposed solution.

They note that not all corals respond the same way to the treatment. Also, the study didn’t include coral species from different regions. They add they hope the effectiveness of different treatment methods will be tested elsewhere in the future.

According to Aeby, the treatment helps manage the disease but doesn’t eliminate it from coral populations. Direct treatments using the chlorine-cocoa butter paste can reduce the prevalence of pathogens in the environment. However, treating coral colonies in the wild through human intervention, as was done for the purpose of this study, isn’t feasible over the long term or on a large scale, she added.

Aeby noted that the best strategy would be to work toward improving environmental conditions such that corals have the opportunity to fight the disease on their own. This involves tackling water pollution as much as possible and maintaining the balance of ecosystems, which are currently highly disturbed in some areas of the world.

Mahery Randrianarivo, a marine biologist at WWF in Madagascar and a member of the country’s Coral Reef Network, shared this view. “Naturally occurring diseases … do exist, and they are worsened by human-induced water pollution,” he told Mongabay.

“Industrial wastewater discharges and everything that goes with them alters the composition of the sea. This in turn affects the physiology of corals, leading to the emergence of diseases as a result of disruptions to microorganisms such as bacteria, viruses, and microalgae within the [limestone structures] covering the body of the corals,” Randrianarivo said.

Enhancing the management of environmental issues

The presence of coral diseases is indicated by white, yellow and red bands on the coral tissue, according to Randrianarivo. But he added that such diseases are a rare occurrence in the seas surrounding Madagascar, in the Indian ocean.

José Randrianandrasana, a doctoral candidate in reef ecology at the Institute of Fisheries and Marine Science of the University of Toliara, in southwestern Madagascar told Mongabay: “In the last decades, more developed tropical countries have been investing in coral disease research, which has enabled the development of treatments like the ones mentioned in the study,” he told Mongabay.

Randrianandrasana said he prefers to focus on the overall and major issues in Madagascar, such as overfishing that impacts the balance of ecosystems and alters their entire food web, as well as destructive fishing practices that weaken and deteriorate the ecosystem.

He also points to sedimentation, exacerbated by massive soil erosion as a result of deforestation, leading to runoff from the land onto the reef flat during the flooding and cyclone seasons. “In addition to these threats are the persistent effects of climate change, which, through ocean acidification and warming, cause widespread coral bleaching,” Randrianandrasana said.

He called on countries like his to manage their environmental problems, which harm the health of the reefs. “Due to a lack of conservation interventions, the factors mentioned above increase the vulnerability of [Madagascar’s] reefs,” he said. “The accumulated delays in Madagascar — in terms of policies, research resources and expertise — highlight the urgent need to strengthen conservation initiatives and the restoration of the reefs.”

These shortcomings are not unique to the island; they’re also seen in other coastal countries. According to Randrianandrasana, countries facing similar limitations could consider partnerships to address the identified knowledge gaps and invest in an integrated reef management system, in order to accelerate innovation in coral disease prevention.

 
Banner image: Coral reefs of Komodo Island in Indonesia. Image by Mongabay.

This story was first published here in French on March 5, 2025.

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Citation:
Forrester, G. E, Arton, L, Horton, A. and Aeby, G. (2024). The relative effectiveness of chlorine and antibiotic treatments for stony coral tissue loss disease. Front. Mar. Sci. 11:1465173. doi: 10.3389/fmars.2024.1465173.