Causing agent

The Anisakids are roundworms belonging to the class of Nematodes. They are also known as herring worms or cod worms but the can also be found in many other fish species. The larvae (Figure 1) measure approximately 2 centimeters long, are often whitish or rarely reddish and encyst into the fish fillets or viscera. They are able to infect human beings and are then responsible of a parasite infection, named Anisakidosis. Anisakis simplex (here, the induced infection is named Anisakiosis) and Pseudoterranova decipiens are the most frequently encountered species in Man.

Figure 1. Larvae of Anisakis sp. (© Anses – Fishing products laboratory, Boulogne-sur-mer)

Life cycle

Anisakidosis is the result of an accidental human infection with the larval stage of several anisakid nematode species found in raw or undercooked marine fish (or more rarely squid) flesh (Figure 2, blue arrow).
In Man, the parasite remains at the larval stage and does not survive very long (from few hours to 3 weeks approximately). The natural hosts of Anisakidae are the marine mammals, in which the nematodes reach the adult stage in the stomach; they reproduce and lay eggs that are shed in the marine environment with the mammals feces. A free-living larva hatches from the egg and is ultimately ingested by a crustacean, intermediate host of the anisakid worm.
Then, the infected crustacean is ingested by a fish or a squid. These hosts become in turn carriers of nematode larvae that can encyst again in another bigger fish that fed on infected fish/squids. At the end of the food chain, the larvae are passed onto the marine mammals, which are predators of fish/squids, animals that are frequently infected by anisakid nematodes.

Figure 2. Biological cycle of the Anisakidae (Center for Disease Control & Prevention)

Prevalence

The prevalence of Anisakidosis is high in countries where the population frequently consumes raw or insufficiently cooked fish. There are approximately 20,000 cases reported in humans worldwide.
Japan is the industrialized country where Anisakidosis is the most prevalent with close to 2500 cases per year. In Europe, countries where Anisakidosis is commonly reported are Spain, Norway, The Netherlands, Germany, United-Kingdom and Italy. The exact incidence is difficult to estimate but seems to reach a mean of 20 cases per country and per year. In France, a report (by the national institute of healthcare monitoring, InVS, 2003) estimated an incidence of 8 cases per year based on data collected during 1985-1987. There have also been increasing reports from New Zealand, Canada, Brazil, Chile, and Egypt during the past 2 decades.
The increase in the prevalence of Anisakidosis cases can be explained by a trendy food habit that consists in consuming raw fish, the protection of marine mammals and the progress made in the field of parasite diagnosis by endoscopy.
The food that is commonly implicated in the transmission of Anisakidosis to human beings are fish or cephalopods harboring contaminating anisakid larvae, if there are consumed raw, not sufficiently cooked, or conserved in low salt/acetic acid content preparations. Here are few examples of fish dishes that can be at risk for transmission: sushi and sashimi, ceviche, salted or smoked herring, bagoong, pickled anchovies, boutargue (smoked or dried fish egg preparation), etc.

Clinical signs

The symptoms depend on the localization of the anisakid larvae in the digestive tract. If the larva is localized in the stomach, symptoms appear shortly (few hours) after the contaminating meal. They include violent stomach pain that can be associated with nausea and vomiting. The larva can then be naturally expelled or removed with the endoscope. In that case, the pain ceases within few days. If the larva persists, the symptoms can become chronic, pseudo-ulcerous and may last several weeks or months. Intestinal symptoms such as abdominal pain can also occur 5 to 7 days following the contaminating meal.
Some rare cases of allergy or sensitization to antigens of Anisakis sp. have also been reported. The clinical manifestations range from urticaria, Quincke oedema or anaphylactic choc. These symptoms can be elicited by alive or even dead larvae of Anisakis sp., as antigens are stable at high temperature.

Prevention

To protect against the risk of parasitism by anisakid nematodes, one must recommend to the consumer to cook through fresh marine fish. For raw fish connoisseur, it is advisable to freeze fish during 7 days in a domestic freezer. Cutting the fish in thin slices (carpaccio) allows the detection and the eviction of some eventual occurring larvae.
It is not recommended to consume fish eggs, soft roe or viscera (liver) if uncooked; chances of finding parasites in visceral cavity are more important than finding them in the fillets. To rapidly eviscerate a fresh fish is also recommended in order to limit the migration of the larvae toward the fillets. Nevertheless, the evisceration does not completely prevent the risk since anisakid larvae can also encyst in the muscles when the fish is alive.
The eviction of allergens is the sole efficient measure to prevent from the risk of development of allergy to Anisakis sp.
The anisakids are naturally present in all oceans worldwide and no wild fish species seems to be free of nematode parasites. The farmed fish fed with controlled food (ex: farmed salmon) are quasi-free of parasites in their fillets.

References

Hochberg NS, Hamer DH. 2010. Anisakidosis: perils of the deep. Clin Infect Dis. 51(7):806-812.
Pravettoni V, Primavesi L, Piantanida M. 2012. Anisakis simplex: current knowledge.  Eur Ann Allergy Clin Immunol. 44(4):150-156.

Useful links

CDC (Center for Disease Control & Prevention, Atlanta, USA)
Data sheet on Aniskidae
Community reference laboratory for parasites (in particular Trichinella, Echinococcus and Anisakis), Istituto Superiore di Sanità (ISS), Roma, Italy.
Food and Agriculture Organization of the United Nations (FAO) - Assessment and Management of Seafood Safety and Quality.
French Institute of Public Health Surveillance (InVS) - Morbidité et mortalité dues aux maladies infectieuses d’origine alimentaire en France (Juin 2003).
European Food Safety Authority (EFSA) Panel on Biological Hazards (BIOHAZ) ; Scientific Opinion on risk assessment of parasites in fishery products. EFSA Journal 2010 ; 8(4):1543. [91 pp.]. doi:10.2903/j.efsa.2010.1543.