The Life Cycle of Anisakis simplex
Worms Parasitic in Marine Mammals, Fish, and Sometimes Humans
Aug 20, 2009
Anisakis simplex, also known as herring worm, or whale worm, is just one of a group of so-called anisakid roundworms that are parasites of marine mammals such as whales and seals. Anisakis simplex is the best known because it is the most common cause of disease in humans who eat raw or undercooked fish. Others include Pseudoterranova decipiens, Contracaecum spp., and Phocascaris spp.
Natural History of Anisakids
To complete their life cycle, anisakid worms must pass through three very different hosts:
- Adult male and female worms live in the stomachs of marine mammals. Anisakis simplex is found in whales, porpoises, and seals; other species inhabit seals as well and even birds. Adult worms mate and produce eggs that are passed into the water with the host’s feces.
- Inside the egg, a tiny larva undergoes a period of development before hatching. Eggs containing larvae, or hatched larvae, are eaten by tiny sea animals such as krill (shrimp-like crustaceans) as they sink toward the ocean floor or drift in the current. (Some are eaten only after settling on the bottom.)
- Within the krill or other small host, larvae continue to develop. If the host is eaten by a fish, larvae use a tiny boring tooth near the mouth to work their way through the fish’s tissues.
- In the fish, the larva develops to the stage that’s infective for the definitive host – the sea mammal. Larvae lie coiled up in fish muscle or abdominal organs, surrounded by a protective capsule.
- If the fish dies, the capsule breaks down and the larva moves away from the place where it has lain dormant. If the fish has been eaten by another fish, the larva simply moves to a location in the new host and becomes dormant again. If the fish dies but is not eaten, the larva may move from the abdominal organs into the muscles. If the fish is eaten by a marine mammal, the larva breaks free in the stomach and develops to an adult worm.
- Worms that reach adulthood and mate in the stomach of a marine mammal produce eggs that are released into the water to start the cycle over again.
- Larvae that are accidentally swallowed alive by a human usually cannot mature but typically cause a painful illness called anisakiasis, or anisakidosis. (Anisakiasis refers specifically to infection with A. simplex; anisakidosis is a more general term meaning infection with one of the anisakid worms.) Anisakiasis was first diagnosed in the 1950s.
The larva's ability to move within a dead fish, or from one fish to another has important implications for the spread of the parasite:
- Fish needn’t eat krill or other zooplankton to become infected: large old fish can have numerous encapsulated larvae in their tissues from eating smaller infected fish.
- When fishers don’t clean their catch immediately, it gives larvae in the abdomen time to migrate into the muscles and increases the likelihood of human infection.
Anisakidosis in Sea Mammals and Fish
Whales, porpoises, seals and other marine mammals, and even fish, seem to be relatively unaffected, even when they have very large numbers of anisakid worms living in their tissues or stomachs. Litres of worms have been discovered in the stomachs of dead seals, and individual fish sometimes harbour hundreds of the coiled larvae. The impact of these parasites lies not in harm to the natural hosts, but in the loss of esthetic value in fish for the table, and the threat to human health.
Sources
“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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