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A popular antibiotic helps to slow the progression of a mysterious coral disease

On stony coral colonies, amoxicillin is 95 percent effective at curing contaminated tissues.

New research indicates that slathering corals in a common antibiotic temporarily relieves a mysterious tissue-eating disease.
Researchers note in Scientific Reports on April 21 that a form of coral infected with stony coral tissue loss disease, or SCTLD, showed widespread improvement several months after being treated with amoxicillin. Although the deadly disease finally reappeared, the findings are a ray of hope as scientists pursue their investigation into what causes it.

“The antibiotic treatments give the corals a break,” says Erin Shilling, a coral researcher at Florida Atlantic University’s Harbor Branch Oceanographic Institute in Fort Pierce. “It’s very good at halting the lesions it’s applied to.”

SCTLD was discovered on reefs near Miami in 2014 by divers. The disease, which is characterized by white lesions that rapidly eat away at coral tissue, affects nearly the entire Great Florida Reef, which stretches 580 kilometers from Marin County’s St. Lucie Inlet to Dry Tortugas National Park outside the Florida Keys. SCTLD has spread to Caribbean reefs in recent years.
If scientists try to figure out what’s causing the lesions, they’re left to treat them by trial and error.
Divers applying a chlorinated epoxy or an amoxicillin paste to contaminated patches are two promising therapies.

“We wanted to experimentally assess these techniques to see if they’re as effective as people have been reporting anecdotally,” Shilling says.

Shilling and colleagues discovered 95 lesions on 32 colonies of great star coral (Montastraea cavernosa) off the coast of Florida in April 2019. The researchers dug trenches into the corals around the lesions to isolate diseased from healthy tissue, then filled the moats with antibiotic paste or chlorinated epoxy and tracked the corals for 11 months.
95% of contaminated coral tissues treated with amoxicillin had recovered within three months of treatment. Meanwhile, only about 20% of infected tissue treated with chlorinated epoxy had healed in that period, and untreated lesions had healed no better.

However, the researchers discovered that a one-and-done procedure would not prevent new lesions from forming over time. The scientists also point out that some important questions remain unanswered, such as how the drug functions on larger scales and what, if any, long-term side effects the antibiotic can have on corals and their surroundings.

“Erin’s work is fabulous,” says Karen Neely, a marine biologist at Nova Southeastern University in Fort Lauderdale, Fla. 

In their two-year project at the Florida National Marine Sanctuary, Neely and her colleagues saw identical effects. More than 2,300 lesions on upwards of 1,600 coral colonies representing eight species, including great star coral, were treated with the same amoxicillin paste and chlorinated epoxy treatments by the researchers.
According to Neely, the antibiotic treatments were more than 95% successful for all habitats. After the initial treatment, spot-treating new lesions that emerged after the initial treatment appeared to prevent corals from being reinfected over time. Frontiers in Marine Science is currently peer-reviewing the report.

“Overall, putting these corals in this treatment program saves them,” Neely says. “We don’t get happy endings very often, so that’s a nice one.”

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