Anguillicola crassus

From Fish
A. crassus adult nematodes in the abdomen of a fish
A. crassus adult

Anguillicola crassus is a parasite which originated in East Asia and was introduced into Europe in the 1980s with infected Japanese eels Anguilla japonica. It was reported independently from Germany and Italy in 1982. The species spread to England from the continent in 1987 with infected continental eels A. anguilla and is now widespread, although not yet found in Scotland and Wales. It has now spread throughout Europe and is found in most countries, including Iceland and the Baltic Sea.

Male parasites range from 20-60μm in length with females measuring 47-72μm. Body width ranges from 0.9-2.8μm for males and 3-5.6μm for females.

Life Cycle

A. crassus requires an intermediate arthropod host in its life cycle to become infective to the definitive eel host. Intermediate host specificity is wide with Diaptomus gracilis, juvenile Gammarus and the brackish species Eurytemora affinis all acting as possible intermediates but only Cyclops vicinus and C. albidus allowing the parasite to develop into the infective stage. Higher densities of infection can lead to significantly reduced survival of the host. Transmission to the definitive host is through the food chain and all sizes of eels from glass eels upwards can be infected. Transmission can also occur from eel to eel through predation. A. crassus is found in the swim-bladder of eels and gravid adults can contain up to 0.5x10^6 eggs each. Released eggs pass into the intestinal tract and hatch during or after passage through the digestive system. Free-living larvae tend to aggregate in clumps of 10-30 individuals, attaching firmly to the substrate by their hooked tails.

Despite the fact that this is an introduced species, A. crassus has been able to find a range of suitable host organisms in its new environment. In addition to various species of marine and freshwater fishes, aquatic snails, amphibians and insects can serve as hosts for A. crassus[1]. Intermediate hosts include copepods and ostracods. The European eel also seems to be more susceptible to the parasite than are their original hosts. Once introduced into a lake or river it may spread rapidly among the eel population. Anguillicola crassus is a successful colonizer due to its large production of eggs and low specificity regarding intermediate hosts. Levels of infestation have been recorded to rise from 10% to 50% within a year[2].

Clinical signs

Infected eels develop a disease called 'Anguillicolosis' which causes haemorrhagic lesions, fibrosis and collapsed swim bladders as well as inflammatory reactions. Adverse effects on wild and cultured eels may occur when the level of infestation is high. Damage caused through inflammation of the swimbladder and secondary bacterial infections may decrease growth and increase mortality in the eel[3].

Damage is caused both by larvae, which live in the tissue of the swimbladder, and adults and pre-adults, which suck the blood of the host. Pathological reactions of the tissues include inflammation, necrosis, and scarring. Of all eels examined in a two year German study, 28% showed pathological alterations of the swimbladder[4]. Repeated infestations may eventually reduce the swimbladder to a “non-functional mass of tissues”. Concern has been raised that massive damage to the swimbladder may make the eels unable to migrate back to their breeding grounds in the Sargasso Sea. This could have a severe impact on the recruitment of eels and thus on the future of the European eel population[5].

Through international trade, A. crassus has been 'transfaunated' outside its natural range through the movement of infected stock. Its effects upon European and American eels are severe but how much it interferes with migration to the Sargasso Sea is not understood. Since the 1980s European eel stocks have declined by 80-90%. The ecology and the livelihoods that depend upon eels have been dramatically altered and Anguillicola has, no doubt, played a part.


  1. Moravec, F., & Skorikova, B., (1998) Amphibians and larvae of aquatic insects as new paratenic hosts of Anguillicola crassus (Nematoda: Dracunculoidea), a swimbladder parasite of eels. Diseases of Aquatic Organisms 34:217-222
  2. Belpaire, C., et al (1989) Effects of eel restocking on the distribution of the nematode Anguillicola crassus in Flanders, Belgium. J Appl Ecol 5:151-153
  3. van Banning P., (1991) Swimbladder nematode (Anguillicola crassus) in the European eel (Anguilla anguilla) (ICES Identification Leaflets for Diseases and Parasites of Fish and Shellfish. Leaflet No. 48). International Council for the Exploration of the Sea
  4. Wurtz, J, Knopf, K, & H. Taraschewski. (1998) Distribution and prevalence of Anguillicola crassus (Nematoda) in eels Anguilla anguilla of the rivers Rhine and Naab, Germany. Dis Aquat Organ 32:137-143
  5. Hartmann, F., & Nellen, W., (1997) The status of the European eel, Anguilla anguilla, after the invasion of the introduced swim bladder parasite, Anguillicola crassus, using the eel population in the lower Elbe as an example. - In ICES Annual Science Conference, (Theme Session, 9 pp.). Baltimore, MD (USA), 25 Sep-3 Oct 1997: ICES (International Council for the Exploration of the Sea), Copenhagen, Denmark