Fishing for Trouble : A Parasitic Worm Underscores the Risks of Eating Raw or Lightly Cooked Delicacies
Not long after he had to undergo an emergency operation on his small intestine, the 71-year-old retired surgeon--let’s call him Dr. Maxwell--remembered the first time he ate the tainted gravlax.
It was a Tuesday night in July when his neighbor in a coastal town north of San Francisco brought over the brine-cured, fresh-caught raw salmon, which he had proudly prepared. Over cocktails, the two men and their wives tore into the pink-fleshed delicacy. Maxwell happily ate it again Thursday and on Saturday.
Then, that night, red-hot pain pierced his abdomen, which began to swell. “By 3 a.m.,” he said, “I was convinced I had something serious.” His wife drove him to the nearest emergency room.
Maxwell does not want his real name used in this fishy tale. But it is clear now that he had an encounter with the parasitic worm Anisakis that ended up nearly killing him. He believes that people should be better informed about the risks of eating raw or even lightly cooked fish, particularly salmon, rockfish, herring, cod and halibut.
Public health experts say his ordeal, while very uncommon, serves as an object lesson. “It’s not just [undercooked] beef and pork and chicken that we have to be careful with,” said Judy Sakanari, a parasitologist at UC San Francisco. “Fish are wild animals. They carry all kinds of organisms.”
In February, California added Anisakis infection to the list of food-borne infectious diseases that must be reported to the health department. Although no cases have yet been reported, few physicians seem to be aware that the disease is now “reportable”--some medical researchers suspect that the problem may be on the rise.
That is partly because sushi, ceviche and the rare-cooked fish of nouvelle California cuisine remain popular. Moreover, in many places along the coast, sea lions and other marine mammals, which also harbor the parasite, have increased in number, providing more opportunities for fish to acquire the worm, biologists say.
Maxwell’s case is a reminder that the back-to-nature movement in contemporary cuisine, for all its celebrated freshness and wholesomeness, sometimes has a downside. And it illustrates that our food supply can be as ecologically complex as that noisome parasite is unappetizing.
Maxwell was admitted to the hospital before dawn Sunday, July 7. As a general surgeon, he pored over the X-ray and CT scan films that his doctors made of his sore, swollen, inflamed abdomen. And he had to agree with them: The dense mass obstructing his small intestine looked an awful lot like a tumor.
“I was very sobered by those findings,” Maxwell said.
A tumor like that would be life-threatening, and there was no indication that the mass was shrinking. His surgeon decided to perform an exploratory operation that day. At 2 p.m., doctors cut through Maxwell’s abdomen and removed the golf ball-sized mass along with a section of intestine.
So unusual was Maxwell’s condition that his doctors still thought they were handling a tumor when they sent the tissue to the pathology lab. They tried to reassure him by saying it was probably benign.
In a harrowing development, blood flowed from his internal wound the day after surgery, and he had to have two units of blood transfused. But the internal bleeding stopped, and he left the hospital Friday--five days after being admitted, 10 after first eating the gravlax.
It was not until the pathologist cut through the specimen that the doctors began to realize what they were up against: a skinny whitish nematode worm, about an inch long. And the “tumor” was mostly a clump of white blood cells that Maxwell’s immune system had summoned to battle the invader.
Eventually, it was identified as Anisakis simplex, one of the three species of so-called anisakid worms that have been known to cause human disease.
What often happens when someone swallows an Anisakis worm whole is that the creature tries to escape--”tingling throat syndrome,” as parasitologists delicately refer to the worm’s attempt to crawl out a day to two weeks after being swallowed.
Occasionally, though, it burrows into the unwary victim’s digestive tract, using a tiny spike-like proboscis--a “boring tooth”--to fasten onto, say, the intestinal lining. The havoc that can ensue, as in Maxwell’s case, results mostly from the body’s frantic attempts to get rid of the pest.
The acute inflammatory response is typically mistaken for appendicitis, an ulcer, gastritis or a fast-growing tumor. But in spite of all the trouble the worm can cause people, it is not really at home in the human gut. It cannot develop or reproduce there, so it will eventually die.
Sakanari suggests that many instances of Anisakis ingestion lead to transient gut pain. Those cases, along with “tingling throat syndrome,” are not likely to be reported to a physician or health department, she says, causing a systematic underestimate of human encounters with the parasite.
Anisakis has a complex life cycle. The fully developed adult worm resides in the gut of marine mammals, including whales, dolphins, sea lions and seals. (In 1988, a biologist reported finding 10,000 worms in a gray seal’s stomach.) The worms lay eggs, which are excreted into the seawater, hatch into larvae and are picked up by tiny crustaceans such as krill.
The worms develop further in the flesh of fish (like salmon) that eat infected krill; this intermediate stage worm is the one that can trouble people (like Maxwell). In any event, from the Anisakis’ point of view, a trip down the human gullet is a disastrously wrong turn. The adolescent larva reaches adulthood only when it reaches a marine mammal’s stomach.
To avoid becoming what biologists call an “unintended host,” Sakanari recommends cooking or freezing the fish. Heating to 145 degrees for 10 minutes kills Anisakis. So does freezing to minus 4 degrees for seven days, which is recommended by the Food and Drug Administration. Another preventive measure is thorough chewing.
Lest diners be completely deterred from eating raw fish, consider that Sakanari occasionally indulges in sushi with her husband, Michael Moser. He is a marine biologist at UC Berkeley. They frequent only top-notch places.
Maxwell is not quite up to that. “I’m not sure I’m going to eat fish again,” he said. “I’m feeling a little funny about it.”
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A Parasite’s Path
Anisakis simplex worms, sometimes called herring worms or seal worms, are roughly inch-long parasites that infect marine mammals such as dolphins and sea lions. Here is a look at how the worms make their way through the food chain to infect human beings:
1. The adult Anisakis worm infects the stomach of marine mammals. One scientist reported more than 10,000 adult roundworms in the stomach of a gray seal.
2. Anisakis eggs pass into the seawater with the mammal’s feces.
3. Free-swimming larvae emerge from the eggs to infect samll crustaceans such as krill.
4. If the infected crustacean is eaten by fish or squid, the larvae can develop into a third stage, which can then infect marine mammals and people.
5. When people eat worm-infected fish that have not been frozen or throughly cooked--steps that kill the worm--they may become infected. They may have no symptoms, or, in severe cases, may develop an intestinal blockage requiring surgery. But human beings are not the worm’s “natural” host, and the parasite will die without reproducing within weeks. Only in the stomach of a marine mammal does the worm fully mature into an adult.
