Anisakis simplex

From Fish
Anisakis simplex on the liver of Alaska pollack
A. simplex in the muscle of chum salmon.
Numerous live Anisakis from Alaska Pollack

Anisakis simplex is a parasitic nematode of the body cavity, muscle and visceral organs of marine fish. This parasite has been known to infect Chub mackerel (Scomber japonicus), Alaska pollack (Theragra chalcogramma), Pacific cod (Gadus macrocephalus), Chum salmon (Oncorhynchus keta), Japanese common squid (Todarodes pacificus), and many other marine fishes.

Life cycle

Final host is mainly whales, and the parasite develops to the adult inside their stomachs followed by the egg production. Released larva hatched from the egg is ingested by crustacean, e. g. euphausiid. Fishes are infected by eating such krill. Previously, it was considered that Anisakis type I, of which L3 larva causes anisakiasis, is only Anisakis simplex. However, recent studies revealed that type I is composed of A. simplex, A. simplex C, A. pegreffii, A. typical and A. ziphidarum[1]. In a limited sense, A. simplex means only the one species A. simplex, meanwhile, in the broad sense, A. simplex includes A. simplex C and A. pegreffii.

Clinical signs

Infected fish exhibits no external abnormalities. A coiled filiform parasite is observed in the body cavity and on the liver or stomach. In some fishes, this parasite infects the muscle.

Pathogenicity to fish is considered to be low. Live parasite, a causative agent of anisakiasis, penetrates into the gut wall of human and causes the acute gastroenteritis, viz. violent abdominal pain, nausea and vomiting. Therefore, be careful when the host fishes is consumed raw. Heating or freezing to -20 C for 24 hrs is effective for killing the parasite. ‘Ruibe (defrozen salmon sashimi)’ is a traditional dish in Hokkaido, a northern part of Japanto, which seems to be effective to kill the parasite. The parasite elicits some patients allergic reactions such as urticaria[2]. The patient sometimes develops anisakiasis even after eating the processed foods since the allergens remain in such products.

Diagnosis

L3 larva of Anisakis type I has boring tooth at the anterior end, oesophagous, ventriculus and intestine but no caecum. The larva of type I is distinguished from that of type II by the presence of the mucro (tail spine) and the blunt posterior end of type I[3]. However, larvae of type I cannot be classified further only by morphology. Therefore, sequences of rRNA or PCR-restriction fragment length polymorphism (RFLP) mapping are used to identify them [4].

Anisakis simplex parasitizes over 200 species of wild fishes and squids. A. simplex is the most common species of anisakiasis in Japan. It is well known that Mr. Hisaya Mori, a Japanese famous actor, developed anisakiasis after eating ‘Battera (sushi of chub mackerel)’.

References

  1. Mattiucci, S., L. et al (2002) Genetic markers in the study of Anisakis typica (Diesing, 1860): larval identification and genetic relationships with other species of Anisakis Dujardin, 1845 (Nematoda: Anisakidae). System Parasitol 51:159-170
  2. Del Pozo, M. D., et al (1997) Anisakis simplex, a relevant etiologic factor in acute urticaria. Allergy 52:576-579
  3. Koyama, T (1974) I. Anisakidae larvae. 1. Morphology and classification. Fish and anisakis (The Japanese Society of Fisheries Science), Koseisha koseikaku, pp:9-19
  4. Abe, N., K. Tominaga and I. Kimata (2006) Usefulness of PCR-restriction fragment length polymorphism analysis of the internal transcribed spacer region of rDNA for identification of Anisakis simplex complex. Japan J Infect Dis 59:60-62