Amoxicillin-Based Compound Successfully Treats Stony Coral Tissue Loss Disease

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Stony coral tissue loss disease treated with antibiotic paste (L), and shown recovered 11 months later (R) (Photo: E.N. Shilling et al/Scientific Reports)

A common antibiotic may help prevent the spread of stony coral tissue loss disease, according to a study published in Nature.com's Scientific reports. Amoxicillin – a semi-synthetic derivative of penicillin – is one of the most commonly prescribed drugs in the world; generically manufactured and available at low cost.

Stony coral tissue loss disease (SCTLD) was first observed off the coast of Florida in 2014 and has since spread throughout the Caribbean, causing infected reef sections to lose as much as 60 per cent of their living corals. A large part of Cozumel's reefs were closed to divers in 2019 to help prevent the spread of the disease.  

SCTLD is one of a number of known coral diseases, affecting at least 20 different hard coral species. Its appearance as white bands of diseased tissue makes it difficult to distinguish from other pathogens. However, unlike other diseases, SCTLD does not appear to be affected by seasonal variations, causing it to progress rapidly throughout the year. 

Little is known about the cause of the disease and how it is spread, but the new study builds on previous research, during which experimental laboratory treatments with a range of antibiotics had proven successful, with amoxicillin demonstrating the highest survival rate among the coral fragments to which it was applied. Other studies have involved the application of chlorinated agents, and 'trenching', whereby the coral is cut to separate healthy polyps from diseased tissues.

To apply the antibiotic to the coral and prevent it leaching into the water supply, the team, led by Erin Shilling of the Harbor Branch Oceanographic Institute at Florida Atlantic University, developed a specially formulated silicone-based product which they called 'Base 2B'. The compound was combined with amoxicillin and applied to diseased portions of hard coral reefs located approximately 2km off the coast of Florida.

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The white paste shown covering the white-coloured diseased coral with a trench cut through the middle of the colony (Photo: E.N. Shilling et al/Scientific Reports)

The antibiotic paste was applied to 41 colonies of Montastraea cavernosa – the great star coral – spread across three widely separated locations. After regular monitoring during the subsequent 11 months, the corals that had been treated with the amoxicillin compound were found to have survived with a 95 per cent success rate.

The experiment also gave some indication as to how the disease spreads and what sort of pathogen may be causing it. Trenching proved unsuccessful, reinforcing the theory that SCTLD is waterborne, as well as spread through contact. Although successful treatment with an antibiotic does not guarantee the form of pathogen behind a disease, it does strongly imply that it is bacterial in nature, rather than a virus. 

The results of the research are good news in the fight against coral disease, particularly for scuba divers. There is growing speculation that dive tourism may be helping to spread the disease-causing pathogens, and this new study mentions that the antibiotic paste is both effective and 'relatively easy to use by scuba divers'. Perhaps diseased coral treatment may be – at a later date – added to the growing repertoire of conservation-minded dive initiatives.

Before the widespread use of amoxicillin for the treatment of coral disease is put into practice, however, the study calls for further research into the possible secondary effects of antibiotics on coral reef systems which are, after all, home to a vast host of beneficial micro-organisms. 

Nevertheless, while caution must be urged until the effects of the treatment on the wider reef can be assessed, research is becoming increasingly confident that coral diseases can be treated. At a time when coral reefs are globally under threat, that's a win worth taking.

The complete study 'Assessing the effectiveness of two intervention methods for stony coral tissue loss disease on Montastraea cavernosa' by E.N. Shilling, I.R. Combs and J.D. Voss is published in Scientific Reports. 10.1038/s41598-021-86926-4.

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