Dr. Ma was taking notes as the patient told his story, but when he mentioned this strange reversal of hot and cold, the doctor gasped. “I know what this is!” he shouted, interrupting the patient’s story. “I know what this is!” And with that he ran down the crowded hallway to where the attending physician and senior resident were sitting. “He doesn’t need a head CT! He has ciguatera poisoning.”

Image Contaminated seafood is a common cause of Hepatitis. Credit... Dan Winters

Ciguatera poisoning comes from eating fish that has been contaminated with a toxin produced by an organism that grows on reef algae in some infested tropical waters. Because the toxin is stored in fat, its concentration increases as it moves up the food chain from the little fish who eat the tainted algae to the larger, predatory fish, like shark, snapper, grouper and barracuda, and from there to the human consumer. Unlike most other causes of food poisoning, this toxin is colorless and odorless and isn’t destroyed by cooking.

The illness was first described in 1774 by a surgeon’s mate on the crew of Captain Cook’s South Pacific exploration aboard the HMS Resolution. The crewman, John Anderson, documented the symptoms described by several shipmates who had eaten a large fish caught in the tropical waters. There was “a flushing heat and violent pains in the face and head, with a giddiness and increase in weakness; also a pain, or as they expressed it, a burning heat in the mouth and throat.” Many since then have described the rapid onset of nausea, vomiting and diarrhea — similar to other types of food poisoning — but followed by the kind of strange neurological symptoms this patient had. Alterations in sensation — like the numbness, tingling and the bizarre hot-cold reversal — are most common and most characteristic. The toxin can sometimes affect the heart ­— causing it to beat too slowly or irregularly. It is rarely fatal, but there is no effective treatment, and the symptoms can persist for weeks, sometimes months, occasionally years.

It was a great diagnosis, the senior doctor told Dr. Ma. But how did he know? It was easy, Dr. Ma told his doctor-teachers. He took care of a family with ciguatera poisoning several months earlier. A whole family had eaten barracuda for their Christmas dinner. They came to the hospital a few hours later, after the nausea, vomiting and diarrhea had given way to these strange neurological problems. He’d never forget them.

Dr. Ma went back to the patient’s room. He apologized for his unexpected exit and began to explain the illness and where it had come from. Although the pathology of this poison is still not well understood, current thinking is that the toxin damages the protective sheath covering the nerves, causing the sheath to swell and compress the delicate tissue it’s supposed to protect.

“Even before they told me, I knew it had to be the fish — the barracuda,” the patient told me sadly. Barracuda has recently been identified as a common source of the toxin, and the C.D.C. now advises against eating the fish, especially when it’s been caught in the Caribbean. These days ciguatera is not just a tropical threat. In the U.S., it has become one of the most common fish-related illnesses as the waters off the coasts of Florida, Texas, South Carolina and most recently North Carolina become warm enough to host these once tropical organisms.

It has been more than six months since the patient returned from the Caribbean, and he still has not fully recovered. He can eat again — he lost 20 pounds in the first few weeks of the illness. He still has occasional numbness and weakness. The patient sighed: “And it was such good fish too. I ate a lot of it.”