A close encounter with Anisakis simplex (also known as herring worm or whale worm) usually goes something like this: an unsuspecting human eats one or more live larvae coiled within the muscle tissue of a raw marine fish. Hours or days later, the person experiences nausea and severe stomach or abdominal pain. Victims may have diarrhea, sometimes bloody stool, and they may vomit. Sometimes they pass the larvae or vomit them up, but more often they don’t — because the larvae have burrowed into the wall of the stomach or intestine. This infection is called anisakiasis (also anisakidosis or anisakiosis).
Before the 1950s this scenario played out without a diagnosis — or at least without a correct one. Doctors didn’t know that these parasitic worms, which normally pass the adult stage of their life cycle in the stomachs and intestines of marine mammals like whales and seals, could cause problems in humans. People knew there were sometimes worms encapsulated in marine fish, but they didn’t realize what would happen if they swallowed them alive.
Anisakis simplex is the most common nematode species involved in anisakiasis. Pseudoterranova decipiens (also known as cod worm or seal worm) is the second most common cause (in which case the proper name for the condition would actually be pseudoterranovosis), and a number of other species are also sometimes involved.
1950s Dutch Discovery of Anisakiasis
During the 1950s doctors in Holland resorted to surgery to treat a few people with severe abdominal pain of unknown cause. They found slender worms several centimeters long and about a millimeter wide in pieces of tissue removed from the digestive tract. Although these worms were unknown to medicine, they were not unknown to fishermen and people who cleaned marine fish — and all of the patients had eaten raw lightly salted herring before their symptoms began. The worms were a larval stage of Anisakis simplex.
1960s Anisakiasis and Japan
Dutch discoveries were published in the scientific literature, and doctors in Japan immediately recognized the description of a disease they saw relatively frequently but could not yet explain. The Japanese enjoy a variety of raw fish dishes including sushi and sashimi, and many of the marine fishes used for these dishes are infected with anisakid larvae.
Because of the higher numbers of patients with anisakiasis in Japan, much of the subsequent study of the disease has been done in that country. The parasite can infect people everywhere however. In northern Europe herring is often the source. In Spain, it’s pickled anchovies; in the United States, raw Pacific salmon.
1990s – Anisakis and Allergy
During the 1990s research on allergy revealed that anisakid larvae, both alive and dead, are capable of provoking severe, sometimes deadly allergic reactions. Symptoms include skin rashes, asthma, inflammation of the eyes, and anaphylactic shock. Allergy to anisakids is probably more common than we realize.
People who have experienced anisakiasis sometimes become allergic. So do people who only have skin contact or inhale microscopic particles from the larvae or their byproducts, making this an occupational hazard for people working in fish processing operations. Frighteningly, those who have a severe allergy to the parasite can have a reaction after merely eating chicken when the bird has been fed fish meal that includes the larvae.
Thus, people who believe they are allergic to fish may actually be allergic to anisakid nematodes. When ingestion of the live larvae is combined with allergic reaction, the condition is called gastroallergic anisakiasis.
Anisakiasis Trends
Anisakiasis is becoming more common for a number of reasons:
- an increase in the consumption of raw fish as people enjoy dishes such as sushi worldwide.
- faster cooking methods and a tendency to lightly cook food for health reasons.
- an increase in the number of anisakid larvae found in marine fish. While increasing numbers of sea mammals, particularly seals, are often blamed, it’s likely that depletion of the world’s ocean fish stocks and the resulting changes in the feeding habits of larger fish are also part of the explanation.
Research into anisakid allergies is not yet fully understood and research is ongoing.
Sources
“Anisakis simplex: From Obscure Infectious Worm to Inducer of Immune Hypersensitivity.” Audicana, M. Teresa and Malcolm W. Kennedy.Clinical Microbiology Reviews 21(2), 2008. pp. 360-79. doi: 10.1128/CMR.00012-07
“Anisakis spp.” Berland, Bjorn. In: Parasites of the Colder Climates. Akuffo, Hannah, Ewert Linder, Inger Ljungström, and Mats Wahlgren, eds. London: Taylor and Francis, 2003. pp161-8.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
"Seals and Cod." MacKenzie, Debbie. The Starving Ocean. Dec, 2002. fisherycrisis.com
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