Anisakiasis is a collective name for infections belonging to the genera Anisakis, Phokanema, Tarranova and Contracaeecum . Anisakiasis was first reported in the Netherlands in 1960 and since then cases have been reported from Japan and North America.

Life cycle

These nematodes are parasites in a large number of marine animals including seals, sea-lions, whales and dolphins. It is in these mammals that the adult worm is found.  Eggs are passed out into the faeces of these mammals and they hatch to liberate the first stage larvae.  These larvae are ingested by crustaceans and moult to become third stage larvae. Fish and squid eat the crustaceans, the third stage larvae are liberated and penetrate the intestine or muscles of that host.  These larvae grow but do not moult to become adults.  The marine mammals ingest the fish and thus the cycle is completed.  Humans become infected by consuming raw or improperly cooked fish which contain the third stage larvae.

Morphology

The larvae reach a length of 50mm with a diameter of 1-2 mm. Classification of the Anisakids is made by the structure of the digestive tract

Clinical Disease

After ingestion of the raw fish by humans, the larvae penetrate the intestinal wall resulting in abdominal pain, nausea, and sometimes vomiting and diarrhoea.  Abdominal irritation may mimic gastric ulcer, carcinoma, appendicitis or other conditions requiring surgery.  Eosinophilic granulosus may result.  Transient anisakiasis which has been reported in North America, is characterised by some vomiting and distress within a few hours of ingesting the fish and then quickly subsides The larvae can be coughed up a few days later.  Low grade eosinophilia and occult blood in the stool are common.

Epidemiology and prevention

Anisakis larvae are usually found in herring, mackerel and North American salmon.

Pseudoterranova are found in cod, halibut, rockfish, sardine and squid.

Human infections result from eating raw fish, insufficiently cooked or smoked fish, marinated or salted fish  Freezing fish at -20"Cfor a minimum of 5 days kills the larvae and thus the fish is suitable for consumption in dishes like sushi. Smoking the fish at temperatures >65°C renders it suitable for consumption.  Marinating or salting the fish cannot be depended on to kill the larvae, however adequately cleaning the fish can render it safe for consumption  Human anisakiasis has been virtually eliminated from the Netherlands due to the mandatory freezing of herring.