Anisakiasis

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http://www.soton.ac.uk/~ceb/Diagnosis/Vol7.htm

 

 

 

Anisakiasis

 

 

 

Introduction

 

Anisakiasis is a collective name for infections belonging to the

genera of nematodes, Anisakis, Phokanema, Tarranova and Contracaecum.

The adult worms from the many species belonging to this family,

Anisakidae, occur in various sea mammals, seals, dolphins, porpoises

and whales, the larvae can cause a serious zoonosis, anisakiasis, in

man. Anisakiasis was first reported in the Netherlands in 1960 and

since then cases have been reported from Japan, North America, Canada,

Chile and the United Kingdom with the increase in popularity of `sushi'.

 

 

 

Two species of nematode have been especially implicated worldwide in

causing human anisakiasis, Anisakis simplex and Pseudoterranova

osculatum. The larvae of P. osculatum can be distinguished from A.

simplex by the possession of a caecum.

 

 

 

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 in the

faeces of these mammals they embryonate and hatch to liberate the

first stage larvae into the sea water. These larvae then become

ingested by crustaceans and moult to become L2 and L3 larvae. If fish

and squid eat the infected crustaceans, the third stage larvae become

liberated and penetrate the intestine or muscles of that host. These

larvae and encyst. Over 100 species of fish can act as intermediate

hosts. 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. (Diag. 2)

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 inhabitation of the stomach or duodenum

but can be found in any part if the alimentary canal or outside the

gut in various viscera. The end result is abdominal pain, nausea, and

sometimes vomiting and diarrhoea, often occurring within 6 hours of

eating an infected meal. 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.

 

 

 

 

 

 

 

 

Diagram 2. Line diagram respresenting the morphology of the third

stage larvae of Anisakis simplex (from fish), it is the causative

organism of anisakiasis in man. (Smyth, J.D, 1994)

 

 

 

Laboratory diagnosis

 

Diagnosis can only be confirmed by endoscopy and the removal of the

worms by biopsy forceps and microscopical identification.

 

 

 

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°C for 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.