Kirk, R.S. (2000) Review of the literature on the eel parasite 'Anguillicola crassus'. (Technical Report) Huntingdon, U.K. : National Fisheries Laboratory, Environment Agency. 78 p.

Abstract

Anguillicola crassus is a parasitic nematode that is endemic in East Asia in its native host, the Japanese eel (Anguilla japonica). The parasite was introduced into Europe with imported Japanese eels in the early 1980s and has spread rapidly through European eel (Anguilla anguilla) populations. The life cycle is transmitted in Europe via intermediate hosts (ostracods, copepods) and paratenic hosts (a range of eel food organisms). young eels can be directly infected by consuming intermediate hosts. Older eels are infected by predation on paratenic hosts. High transmission levels in the European eel are due to accumulation of larvae in a wide range of fish paratenic hosts. Transmission dynamics in the paratenic host population is influenced by differential host status. Anguilla crassus has excellent natural dispersal abilities and most of the attributes of a successful colonizer. Anthropochore inter- and intra-catchment movements of eels for restocking and the natural movements of eels in freshwaters and coastal waters have accelerated dissemination and extended its range throughout Europe. It has been able to infect three species of eels (A. anguilla, A. japonica, A. rostrata) in four continents. The only known limitations to its dissemination appear to be temperature and salinity. In spite of these environmental constraints, it is spreading throughout Northern Europe and is capable of transmission in brackish environments. Pathological changes caused by the migratory and blood-feeding activities of A. crassus are thought to result in hydrostatic dysfunction of the swimbladder of European eels and reduction in overall fitness which may impair the success of the spawning migration to the Sargasso Sea. Mass mortalities attributable to A. crassus have occurred in farmed and wild populations. Thus the parasite has the potential to impact upon eel populations. The effect of the parasite on simulated migration of silver eels is currently being investigated. The parasite can be controlled by levamisole administered to eels in farmed conditions, but repeated treatments are required due to low sensitivity of larval stages. No treatments are available for wild eels. Areas for future research are recommended. An integrated sampling programme is crucial for assessment of impacts of A. crassus and other adverse factors on eel populations.

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