Invasive Alien Species in Northern Ireland

Anguillicola crassus, Swim-bladder nematode

Anguillicola crassus Kuwahara, Niimi & Itagaki 1974
 
Introduction
Anguillicola crassus is a parasitic bloodsucking nematode indigenous to East Asia where it is widespread amongst its native host the Japanese eel Anguilla japonica. Found in the swim bladder of eels inhabiting primarily fresh waters, the parasite causes little or no pathological damage to its native host. Thought to have been introduced accidentally into Europe with Japanese eel from Asia in the early 1980s, A. crassus was transmitted to populations of the European eel which were highly susceptible to the parasite and suffered various forms of pathogenic effects. Its spread across Europe has always been associated with the trade in live eels and it is believed this mode of vector was responsible for its accidental introduction to Ireland (the Lough Erne system) in 1998 (Evans and Matthews, 1999).
 
Description
Morphology is a typical of a large parasitic nematode with worm—shaped segmented body though, rather than being white, A. crassus is dark brown in colour highlighted with white internal organs. A transparent membranous sheath is commonly found surrounding freshly removed specimens. Separate sexes, with adult male reaching length of 23mm and females 36mm. The female exhibits a long white convoluted uterus running the length of her body.
 
Country of origin
East Asian region.
 
Current distribution
This nematode has spread across all European countries, ranging from the Mediterranean to the Baltic. It has recently been found in Canada and the United States.
 
Location in Ireland
Anguillicola crassus was first documented in eels from the Erne system in Northern Ireland and Ireland (Lower Lough Erne and Lough Oughter respectively) (Evans and Matthews, 1999) and its subsequent spread through the Erne system was noted in the following years (Evans et al., 2001). Records from the late 1990s document its presence in the Shannon system, Loughs Corrib and Mask and River Barrow in south-eastern Ireland (McCarthy pers. Comm.). It was recorded in Lough Neagh for the first time in 2003 (Evans and Rosell, 2004).
 
Life cycle
Eggs are laid in the swim bladder and along with L2 larvae (larvae from newly hatched eggs) pass through the pneumatic duct into the digestive tract of the eel and are expelled with the faeces. Released eggs and L2 larvae are consumed by many invertebrates including cyclopoid copepods, cladocerans and other crustaceans. L3 larvae develop in the intermediate hosts, which are eaten by the eel. The last stage, L4, bores through the intestinal wall of the eel to the swim bladder where it completes the final moult into an adult.
 
Wildlife and habitat impacts
Anguillicolosis (the pathology attributed to A. crassus infections) in adult European eels can induce thickening or rupture of the swim bladder wall, resulting in emaciation, increased vulnerability to bacterial infections and significant mortalities in eel farms and lakes. Eels may suffer acute inflammatory reactions including fibrosis and formation of fibrotic conglomerates in the swim bladder. Under adverse environmental conditions such as low dissolved oxygen levels and raised water temperatures, the impacts of A. crassus on its European eel host are greatly increased. This raises concerns associated with climate change and the survival of infected eel populations. The pathological damage caused to eel swim bladders may also prevent the successful spawning migration of eels across the North Atlantic.
 
Human impacts
This parasite poses no threat to human health as its final host is a fish. However, it does pose a threat to the economic value of the eel fisheries infected with it, given its potential effects as a serious fish pathogen. This is of even greater importance for Northern Ireland given that Lough Neagh is the largest producer of wild caught eel in the world and 100 per cent of its eels are now known to be infected with A. crassus.
 
Key vectors
The introduction of this species has always been associated with the movement/trade in live eels where the larvae are released from contaminated water and, occasionally, the release of infected eels.
 
What you can do as an individual
Few people are involved in the commercial trading of eels so it is unlikely that people in general will be involved in spreading this parasite. Legislation exists in Northern Ireland to prevent the movement of fish without prior approval (DCALNI Inland Fisheries Branch – Fish Moving Permit) which should prevent people from translocating eels from known infected regions to other aquatic systems.
Large volumes of water should not be removed from infected areas (may contain planktonic copepods infected with L3 larvae) and released into water bodies where A. crassus is not known to exist.
 
Advice to specific groups relevant to this species
Anglers And Boaters:
As outlined above, the movement of any fish species including eels is regulated and legislation exists in Northern Ireland to prevent the movement of fish without prior approval (DCALNI Inland Fisheries Branch — Fish Moving Permit) which should prevent people from translocating eels from known infected regions to other aquatic systems. Permission should always be sought before moving eels to different waterbodies. Veterinary health certificates must be issued with elvers being translocated within the UK following screening for A. crassus. Large volumes of water should not be removed from infected areas (may contain planktonic copepods infected with L3 larvae) and released in waterbodies where A. crassus is not known to exist.
 
Management measures
The only management measures in place to prevent the spread of A. crassus are those outlined above. No biological or pharmaceutical anthelminthic controls exist for this parasite in the wild.
 
Management groups
No formal groups exist.
 
Further information
Links
www.environment-agency.gov.uk
www.dcalni.gov.uk
www.defra.gov.uk
 
Literature
Evans, D.W. and Matthews, M.A. (1999). Anguillicola crassus (Nematoda: Dracunculoidea); first documented record of this swim bladder parasite of eels in Ireland. Journal of Fish Biology 55: 665-668.
Evans, D.W., Matthews, M.A. and McClintock, C.A. (2001). The spread of the swim bladder nematode Anguillicola crassus through the Erne System, Ireland. Journal of Fish Biology 59: 1416-1420.
Evans, D.W. and Rosell, R.S. (2004). Introduction of Anguillicola crassus into the European eel population of Lough Neagh, Northern Ireland. Proceedings of the IX European Multicolloquium of Parasitology, Valencia, Spain pp 564-565.
 
Text written by:
Dr Derek Evans