This Deadly Shellfish Poisoning Has No Antidote. A Protein Found in American Bullfrogs Could Change That

Each summer, warm weather and sunlight drive algae growth in ocean waters worldwide. But when the organisms grow out of control, they can transform into harmful “red tide” algal blooms that produce saxitoxin, a deadly substance that affects animals’ nervous systems. Saxitoxin can accumulate in shellfish and cause a life-threatening illness called paralytic shellfish poisoning in people who eat contaminated seafood.

It is such a potent poison that the United States stockpiled it during the Cold War, and it’s now considered a chemical weapon under international law. It also has no known antidote—though something found in American bullfrogs might eventually change that. A protein they make called saxiphilin can thwart the toxin in mice, researchers report in a study published in the journal Nature Communications on July 16.

Scientists already suspected that the protein allows bullfrogs and other ribbiting amphibians to resist the effects of the harmful agent by binding to it, but the new work offers a path forward for potential therapies. The protein acts like a “molecular sponge,” study co-author Daniel Minor, a biophysicist at the University of California, San Francisco, tells Lesley McClurg at KQED. Cooking or freezing seafood doesn’t kill saxitoxin, and affected shellfish look, smell and taste normal.

But in just 30 minutes after consumption of contaminated food, symptoms like nausea, vomiting, numbness, high blood pressure and paralysis can set in, and death can occur just hours later. It’s fatal in about 8.5 percent of cases. Researchers first reported that frogs are resistant to saxitoxin in the 1930s.

At the time, it was considered “muscle poison” because experts didn’t know it originated from algae. To find a potential antidote, Minor and his colleagues conducted a series of experiments to see whether the bullfrog protein could protect mice given lethal doses of saxitoxin. As a control, 13 animals received injections of the toxin, and nearly all developed poisoning symptoms, like limb paralysis, within three minutes and died soon after.

But providing the saxiliphilin before, during or after a toxic shot drastically changed the outcomes. Notably, giving just one jab with the protein one minute after poisoning allowed nine out of ten tested mice to avoid the toxin’s lethal effects. Protein analysis of the mice’s carcasses helped the team determine where the bullfrog protein traveled in the animals’ bodies.