Anisakiasis
Anisakiasis
Disease History, Characteristics, and Transmission
Introduction
Anisakiasis is an infection in humans caused by ingesting the larvae of nematodes (parasitic roundworms with long, cylindrical bodies) in raw or undercooked saltwater fish. When the larvae infect humans, the anisakiasis infection causes discomfort to the stomach and intestinal areas. According to the U.S. Food and Drug Administration's Center for Food Safety and Applied Nutrition, Anisakis simplex (herring worm) and Pseudoterranova (Phocanema, Terranova) decipiens (cod or seal worm) are linked to human infections in North America.
Before ingestion into the human body, anisakiads travel through a complex life cycle involving the ingestion by various marine and anadromous fish (those that breed by returning from the sea to the water bodies where they were born) and crustaceans.
Usually, marine life infected with Anisakidae larvae are only found in seawater because larvae need to grow within waters of higher salinity. It is also uncommon in areas where cetaceans (large ocean mammals like whales) are not found, such as waters in the southern North Sea.
Disease History, Characteristics, and Transmission
Human anisakiasis was first reported in Japan during the middle part of the twentieth century. Anisakis simplex and Pseudoterranova decipiens is found frequently inside saltwater fish. P. decipiens is found typically in temperate and arctic environments.
The characteristics of anisakids include a long, cylindrical body shape (what is called vermiform, or worm-like). It does not contain segments. The posterior part narrows to a cavity (pseudocoel), with the anus somewhat off-centered. The mouth is encircled by projections, which are used for sensing and feeding.
Transmission of the adult Anisakis simplex and Pseudoterranova decipiens begins in the stomach of marine mammals, specifically in the mucosa (mucous membranes). The eggs of female anisakids are expelled as feces of infected mammals. The eggs develop into embryos in seawater, where first-stage larvae are formed. The larvae then molt and become second-stage larvae. Upon hatching, free-swimming larvae are ingested by crustaceans, turning into mature, third-stage larvae.
Infected crustaceans are eaten by fish and squid, who become intermediate hosts. Inside fish, anisakids are coil-shaped. When uncoiled, their average length is 0.8 inches (2 centimeters). When these fish and squid die, the larvae move into muscle tissues. Anisakids transfer between fish when larger fish eat smaller ones. During these times, larvae are infective to humans and marine mammals. Sometimes, larvae are ingested by humans when infected seafood is eaten raw, is under-cooked, or improperly prepared. After humans ingest third-stage larvae, the larvae attach themselves to, or burrow into, stomach or intestine tissues.
When third-stage larvae are digested by marine mammals, the larvae molt two times and develop into adult worms. These parasites are longer than two centimeters when uncoiled, with a thicker and sturdier body than when inside fish. The adult worms produce eggs that are expelled by marine mammals.
WORDS TO KNOW
ANADROMOUS: Fish that migrate from ocean (salt) water to fresh water, such as salmon, are termed anadromous.
INTERMEDIATE HOST: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.
NEMATODES: Also known as roundworms; a type of helminth characterized by long, cylindrical bodies.
PARASITE: An organism that lives in or on a host organism and that gets its nourishment from that host. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.
Scope and Distribution
Anisakiasis is found worldwide. However, it is more common in are as where raw fish is eaten such as Scandinavian countries, the Netherlands, Japan, and along the Pacific Ocean coast of South American countries. As of 2007, according to the FDA, Japan has the highest incidence of infection. The incidence of anisakiasis in the United States is unknown, but is thought to be fewer than 400 cases per year. Fish and marine mammals most affected include cod, crabs, cuttlefish, halibut, herring, mackerel, porpoises, rockfish, salmon, seals, sea lions, squid, tuna, and whales.
Treatment and Prevention
Diagnosis cannot be accomplished from stool specimens. Instead, it is made by x-ray images and medical examinations of the patient's stomach and intestines using a flexible endoscope. In addition, microscopic examination of tissue can be made in which larvae are removed through biopsy or during surgery. Anisakis simplex and Pseudoterranova decipiens cannot survive in human hosts. They eventually die while inside the inflamed tissue.
In some case, invasive treatments may be attempted. Endoscopy may be used for the removal of larvae, especially in emergency cases involving obstruction or rupture of the bowel. Also, nasogastric suction (suction through a tube inserted through the nose and into the stomach) may be used, followed by drugs that target parasitic worms. If such action fails, worms can be removed surgically.
Surgical procedures sometimes may be avoided by drug treatments, including albendazole (marketed under Albenza®, Eskazole®, and Zentel® brands). As of March 2007, albendazole has not been approved by the U.S. Federal Drug Administration (FDA) for treatment of the infection in the United States.
Anisakiasis infection can be prevented by heating seafood to a temperature higher than 122°F (50°C), or freezing it to at least −4°F(−20°C) for at least 24 hours. Such actions kill the larvae. If fish or shellfish is to be consumed raw or semi-raw, the FDA recommends that the food be blast frozen to −31°F (−35°C) or below for 15 hours, or regularly frozen to −4°F(−20°C) for seven days.
Impacts and Issues
When anisakid worms can infect humans, within several hours of ingestion they can produce severe sickness that affects the stomach and intestines. Sometimes the larvae are vomited or coughed up. Symptoms include vomiting, diarrhea, nausea, and severe abdominal pain that may resemble appendicitis, so cases are often misdiagnosed. With the increasing popularity of raw seafood dishes, government and medical organizations have made efforts to educate physicians to consider the possibility of anisakiasis in patients with these symptoms.
If larvae pass into the bowel, major symptoms may occur within one to two weeks due to tissue inflammation. They can also produce a minor chronic disease that causes stomach or intestinal irritation, which may last between weeks and years. These symptoms resemble stomach ulcers and tumors, or irritable bowel syndrome.
Fish and shellfish are important foods to maintain a healthy lifestyle. They are high in protein and other essential nutrients. However, the growing international popularity of eating such raw seafood dishes as sushi, sashimi, ceviche, and pickled herring, has produced an increase in the number of cases of anisakiasis, a trend that health authorities expect to continue upward.
See AlsoCancer and Infectious Disease; Food-borne Disease and Food Safety; Helminth Disease; Host and Vector; Parasitic Diseases; Tropical Infectious Diseases.
BIBLIOGRAPHY
Books
Adley, Catherine C., ed. Food-borne Pathogens: Methods and Protocols. Totowa, NJ: Humana Press, 2006.
Guerrant, Richard L., David H. Walker, and Peter F. Weller. Tropical Infectious Diseases: Principles, Pathogens, and Practice. Philadelphia, PA: Elsevier Churchill Livingstone, 2006.
Parker, James N., and Philip M. Parker, eds. The Official Patient's Sourcebook on Anisakiasis: A Revised and Updated Directory for the Internet Age. San Diego, CA: Icon Health Publications, 2002.
Web Sites
Center for Food Safety and Applied Nutrition, Federal Food and Drug Administration.“Anisakis simplex and related worms.” <http://www.cfsan.fda.gov/mow/chap25.html> (accessed March 1, 2007).
Centers for Disease Control and Prevention. “Anisakiasis.” <http://www.dpd.cdc.gov/dpdx/HTML/Anisakiasis.htm> (accessed March 1, 2007).
ProMED Mail, International Society for Infectious Diseases. “Anisakiasis—Israel: suspected.” <http://www.promedmail.org/pls/promed/f?p=2400:1202:16245428003054921509::NO::F2400_P1202_CHECK_DISPLAY,F2400_P1202_PUB_MAIL_ID:X,23022> (accessed March 1, 2007).