Anguillicoloides crassus was first described from Anguilla japonica and Anguilla anguilla in eel farms in Japan by Kuwahara et al. (1974). Around this time, eel farming was expanding in Japan, and in order to increase production non-native eels, especially Anguilla anguilla, were imported from Europe. Most eel farms were concentrated in the south-western provinces of Japan and Anguillicoloides crassus was recorded from Aichi prefecture, Hamamatsu, Shizuoka prefecture and Honshu between 1972 and 1974. It is now known that it was actually present in Japan in 1969, but was then misidentified as Anguillicola globiceps by Egusa et al. (1969). It was almost certainly present at this time in China where it was also confused with Anguillicola globiceps by Wang and Zhao (1980), but the presence of Anguillicoloides crassus in China in Hubei and Fujian was later confirmed by Moravec and Taraschewski (1988) and Zhang (1995). It has subsequently been reported from Korea by Kim et al. (1989) and from Taiwan by Ooi et al. (1996) and Münderle et al. (2006).
In all four countries the native host of Anguillicoloides crassus is Anguilla japonica. It is common and widespread in this species of eel in both farms and natural habitats. It is seldom if ever abundant, and prevalence levels are generally around 20% (between 10% and 40%) and mean intensities around 1.5 (Ooi et al., 1996; Han YuSan et al., 2008). It appears to be harmless to Anguilla japonica in eel farms, as this species shows natural resistance to the parasite (Egusa, 1979; Kim et al., 1989). However, it soon became clear that Anguilla anguilla was far more susceptible to Anguillicoloides crassus than the native Anguilla japonica. Prevalence levels in the European eel in eel farms often reached 100% and maximum intensities of up to 30 parasites per eel were recorded (Egusa, 1979). At such high levels of infection, the swim bladders of infected eels were severely damaged and weakened and this in turn often resulted in death of the host. Published details of economic losses due to the parasite are not available, nor do there appear to have been any trials to treat Anguillicoloides crassus in East Asia. As a consequence of its pathogenicity to Anguilla anguilla in farms, importation of Atlantic eels into Japan declined and farming concentrated only on the native Anguilla japonica Other species of eels were introduced into Japan but only Anguilla anguilla appears to have been so deleteriously affected by the parasite. There are no records of Anguillicoloides crassus infecting the Pacific eel Anguilla marmorata in Japan. However, it was able to infect introduced American eels Anguilla rostrata in farms in Taiwan (Han YuSan et al., 2008). The severe deleterious impact of Anguillicoloides crassus on introduced Anguilla anguilla in Japanese eel farms caused Egusa (1979) to issue a warning about the probable consequences to native eel stocks if Anguillicoloides crassus escaped or was inadvertently introduced to Europe.
This warning was not heeded. Pacific eels were introduced into Europe in order to boost eel production in European eel farms. Introductions of Anguilla australis from New Zealand were responsible for the introduction of Anguillicoloides novaezelandiae into Lake Burano in Italy around 1982, but this species appears to have caused no damage to the native eels (Moravec et al., 1994a). However, in 1982 Anguillicoloides crassus was reported in Anguilla anguilla in Europe for the first time, in the Weser-Ems region in North Germany (Neumann, 1985). From here it has spread throughout Germany, then throughout the whole continent and thence to Africa and North America: effectively, throughout the entire range of the two Atlantic species of eel. It exhibits most of the characteristics of a good coloniser and is now to be found in eel farms, in lakes which received stocked eels and in rivers and lakes which have not been stocked. It has also proved to be just as pathogenic to Anguilla anguilla in its native continent as it was in Japan, and it has caused comparable economic losses in European eel farms and in stocked lakes. Its impact on wild eel populations is very difficult indeed to evaluate, but there can be little doubt that it is at best one more mortality agent threatening the declining stocks of European eels since the damage it causes to the swimbladder affects the behaviour and stamina of eels and so, possibly, the ability of silver eels to migrate to the Sargasso sea for spawning.
The only species deleteriously affected by Anguillicoloides crassus are the Atlantic eels, Anguilla anguilla and Anguilla rostrata. The parasite is harmless to Anguilla japonica and other species of Pacific eel. It appears that the parasite and Anguilla japonica may have co-evolved together such that this eel has acquired a measure of resistance to Anguillicoloides crassus (Egusa, 1979; Kim et al., 1989; Knopf and Lucius, 2008).
As far as is known, all individuals of Anguilla anguilla are equally at risk of infection by Anguillicoloides crassus: small individuals by eating infected copepods and large individuals by eating infected fish. There is no resistance to re-infection (Haenen et al., 1996) and antibodies produced against the parasite have no protective function (Knopf and Lucius, 2008). Once Anguillicoloides crassus has entered a lake or eel farm, its prevalence often rises to 100%, and transmission rates and infection levels may be very high when eel densities are themselves high as in eel farms or shallow lakes (Molnár et al., 1994; Baruš et al. 1999a). Stress in individual eel hosts may predispose them to disease (Gollock et al., 2005a).
It would appear that Anguillicoloides crassus was originally restricted to East Asia throughout which it is probably widespread and endemic, especially in China, Japan, Korea and Taiwan (there are not many records of its being definitely identified in China, probably because as a non-pathogenic species it is likely to be considered unimportant, and because of its similarity to Anguillicola globiceps). It is likely that it is present in other countries in this region and it has been reported from Thailand (Moravec, 2006) but there have been very few or no published reports of eel parasites from many parts of Eastern Asia. Its natural range probably coincides with that of its preferred host Anguilla japonica.
Factors limiting distribution
Although Anguillicoloides crassus is potentially able to infect Atlantic eels throughout their entire range and has a wide tolerance of environmental conditions, it also exhibits a preference for some physico-chemical conditions, notably of temperature and salinity.
Although present in the Baltic Sea, Anguillicoloides crassus was slow to colonise the Nordic countries of Norway and Sweden, and Finland is still believed to be free of infection. When it was first reported from Sweden, it was from coastal areas where water temperatures were raised by thermal effluents from power stations (Hoglund et al, 1992). Only much later was it reported from inland waters (Hoglund and Thomas, 1992).
The temperature tolerance of Anguillicoloides crassus was investigated experimentally by Knopf et al. (1998) who infected Anguilla anguilla and maintained them over a range of temperatures for 4 months. They found that larval development was significantly retarded at low temperatures. Larval L3 could survive 4 months at 4° C, but were unable to invade the swimbladder wall. Adults exhibited decreased growth and reproduction rates and increased mortality compared to those kept at 18° C. These data thus support the conclusions drawn from field studies that the spread of the parasite in boreal regions may be restricted by temperature.
Field data have indicated that although Anguillicoloides crassus is primarily a freshwater species, it can nevertheless tolerate enhanced salinity levels. Køie (1988, 1991) found that whilst the parasite could survive enhanced salinity in the Baltic sea, it was normally absent from sea water eel farms. Reimer et al. (1994) also noted that the distribution of Anguillicoloides crassus in the Baltic sea indicated that it was tolerant of, and able to be transmitted, under conditions of enhanced salinity. The parasite has also been reported to be able to survive in eels in the tidal reaches of the River Thames (Pilcher and Moore, 1993) and in Italian coastal lagoons with slightly enhanced salinity, although not in the most saline ones (Kennedy et al., 1997; Di Cave et al., 2001).
In experimental laboratory studies Kennedy and Fitch (1990) showed that the proportion of eggs hatching decreased with increasing salinity such that only 6% hatched in 100% seawater. The survival of L2s also declined with increasing salinity but even though 50% survived 10 days in 100% seawater, larval infectivity declined and none survived longer than 15 days. Similar findings were reported by De Charleroy et al. (1990).
Kennedy and Fitch (1990) also reported that there was no loss of viability of adults or L4s or eggs of Anguillicoloides crassus in Anguilla anguilla maintained in full sea water for up to 4 weeks. More detailed studies on survival of Anguillicoloides crassus in saline conditions were undertaken by Scholz and Zerbst-Boroffka (1994), who found that its body fluids were isosmotic with those of the eel host (Anguilla anguilla), but differed in ionic composition, and by Kirk et al. (2000, a, b; 2002). These authors found that adult parasites tolerated seawater by osmoconformation with the blood plasma of their eel hosts. The majority of parasites (90%) osmoconformed with hosts after 2 weeks in sea water following acute transfer and survived for a long period, up to and exceeding 3 months. A small proportion of pre-adults and adult parasites were unable to withstand the osmotic stress and died, but the reasons for this are not known. These same authors concluded that whilst Anguillicoloides crassus was unable to be transmitted to eels in seawater, due to the absence of suitable copepod hosts, it could be transmitted in estuaries using species such as Eurytemora affinis and it could survive the transition to sea water in eels already infected in freshwater, and continue to survive and reproduce in them for up to 6 months. It was thus possible for the parasite to be disseminated naturally by eel movements in coastal waters.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
The nature and detail of the host’s reaction varies to some extent with the host species, but all fish species respond in a similar basic way that has been described by Székely et al. (1996). When the L3 move out of the fish’s intestine, mononuclear cells appear around the migrating larvae. The cells adhering to the larvae assume an epithelioid shape and form a granuloma. Within the granuloma the larvae and epithelioid cells undergo necrosis whilst the granuloma becomes surrounded by a connective tissue capsule. In chronic infections the inside of the parasite nodule becomes filled with amorphous tissue and debris from the decaying parasite larva.
In brief, in less suitable hosts a cell reaction by the host forces the larvae into foci and destroys them. However in the more suitable hosts, such as Perciformes, the number of larvae not affected by the host reaction outnumbers those that are affected, and the unaffected ones may then go on to moult into the L4 stage.
On glass eels
L3 migrate to the developing swimbladder wall, the connective tissue of which starts to thicken, and the labyrinth of gas cells is reduced (Nimeth et al., 2000). Resting oxygen consumption is higher in infected individuals at 4-5 months and tissues are also affected. There is no difference in critical swimming speed or swimming performance between infected and uninfected glass eels, nor is aerobic metabolism affected during swimming.
On adult eels
It is agreed that Anguillicoloides crassus does no serious damage to Anguilla japonica. Its effects upon its native host are in fact negligible (Münderle et al., 2006; Han YuSan et al., 2008) if the stock ponds are well managed: prevalence levels are consistently lower than in introduced Anguilla anguilla, in which the parasite can cause severe damage and morbidity in Japan (Egusa, 1979; Nagasawa et al., 1994).
On the swimbladder tissues
The effects of Anguillicoloides crassus on the swimbladder of Anguilla anguilla have been documented by several authors including: Boon et al. (1990a,b); Molnár et al. (1991, 1993); Hoglund et al. (1992); Würtz et al. (1996) and Würtz and Taraschewski (2000). In heavy infections, the swimbladder undergoes a number of characteristic changes. Its wall thickens and its internal surface becomes folded. Inflammation becomes evident, white cells migrate, fibrosis becomes apparent and there is a change in epithelial cells. These proliferate and form hyperplastic tissues, adopting cubic, spherical and columnar shapes, but many of them lose contact with the blood vessels. The lamina propria shows severe fibrosis and infiltration of white blood cells. Haemorrhages and congestion of blood become evident in the swimbladder. Very severe infections lead to a complete loss of the swimbladder lumen which becomes filled with live and dead parasites and fluid (Molnár et al., 1993). Haenen et al. (1996) have also shown that whilst there is no significant relationship between the proportion of parasites recovered in primary and secondary infection doses, the higher the dose the more severe are the haemorrhages.
Whilst in the swimbladder wall, L3 and L4 feed on swimbladder tissue, which may result in parasitic nodules (Molnár, 1994) and they possess proteolytic enzymes that assist migration and penetration into the lumen. They may be surrounded by cell debris and evidence of local necrosis. When they move into the lumen and commence blood feeding, they possess proteinases (Polzer and Taraschewski, 1993).
On eel physiology
It appears that Anguillicoloides crassus has no significant impact on the length-weight relationships of infected eels (Baruš and Prokeš, 1996), but it does have effects on other host tissues. Boon et al. (1989,1990b) have reported decreased plasma protein levels and lower haematocrit counts in infected eels, due to a decrease in both red and white blood cells, and this decrease has been confirmed by Höglund et al. (1992) and Paliková and Navrátil (2001). The levels of some amino acids are also significantly lower in infected eels (Baruš et al., 1998) as are levels of Ca, P, Fe and Mn (Baruš et al, 1999b).
At the hormonal level, no major differences were found in hormonal, metabolic or osmoregulatory status of infected and uninfected Anguilla anguilla by Kelly et al. (2000). Although they examined plasma cortisol and glucose levels as indicators of stress, as well as haematocrit values and chloride levels, they could find no consistent differences and concluded that wild eels could adapt to parasitism. A later study by Gollock et al. (2004, 2005a,b) confirmed that cortisol responses and recovery as well as plasma glucose levels were similar in infected and uninfected Anguilla anguilla, but they suggested that there were differences in the dynamics of glucose mobilisation and utilisation with increased Anguillicoloides crassus levels in eels. They also confirmed that infected and uninfected eels showed similar endocrine stress responses to netting confinement, aerial exposure and temperature stress. However, eels infected with Anguillicoloides crassus showed a more pronounced stress response to severe hypoxia than controls (Gollock et al., 2005a) as infected fish showed higher levels of plasma cortisol, but not plasma glucose. They suggested that the greater corticosteroid stress response to hypoxia in infected eels might also involve them in a higher metabolic cost.
The survival of infected Anguilla anguilla in an experimental system which was devoid of a fresh oxygen supply was investigated by Molnár (1993) in relation to the eel mortalities in Lake Balaton. He found that the eels most severely infected with Anguillicoloides crassus and with the most damaged swimbladders died first, whereas those with lower parasite intensities and less severely expressed pathological lesions could tolerate sublethal oxygen levels for longer. His findings were similar over the normal range of water temperatures, but at 27/28 °C eels required a higher oxygen content in the water to survive. Reduced survival was related to the thickening of the swimbladder wall and not just to the number of Anguillicoloides crassus present. Würtz et al. (1996) studied the actual composition of the gas in the swimbladder in relation to the level of infection of Anguilla anguilla with Anguillicoloides crassus, and found a significant correlation between the proportion of oxygen in the swimbladder and the intensity of infection by adult parasites. The contribution of oxygen to the gas content of the swimbladder was reduced by 36-63% in natural infections and by 11-57% in experimental infections. In uninfected eels, freshly secreted gas in the swimbladder contained 70 vol% oxygen but in heavily infected eels this declined to a level around 23 vol%. Levels of carbon dioxide were variable and did not differ significantly between infected and uninfected eels. Larval Anguillicoloides crassus numbers had no effect on the swimbladder gas composition, but the decline in oxygen levels was considered to be of considerable physiological significance in relation to eel survival and swimming activity.
On eel swimming
There have been several investigations into the effects of Anguillicoloides crassus on the composition of the gas in the swimbladder of eels and on the behaviour and swimming ability of infected eels. Sprengel and Luchtenberg (1991) tested the maximum swimming speed of infected and uninfected Anguilla anguilla in experimental tanks. They found that the maximum swimming speed could be reduced by as much as 18.6% in infected eels when compared with uninfected ones. The extent of the reduction related to the intensity of the parasite infection and this value was observed in eels with more than 11 parasites in the swimbladder. In the case of eels with only a single parasite, the reduction could be as low as 3.0%. Münderle et al. (2004) conducted a similar investigation, but with a different experimental regime. Using Anguilla anguilla naturally infected with Anguillicoloides crassus, they determined how long individual eels could maintain their station against a steady stream water current in a channel. They found that there was no significant correlation between the time the eels swam and the intensity of infection with Anguillicoloides crassus. Although these results were unexpected in the light of the earlier investigations, they suggested that they were not inconsistent with the parasite having an effect on eel swimming as it was possible that the presence of parasites caused the eels to make a higher demand on their energy reserves to maintain their swimming speed.
The damage to the swim bladder wall and the energy drain due to blood feeding by Anguillicoloides crassus and its likely impact on eel swimming performance were also investigated by Palstra et al. (2007). They tested the effects of the parasite on swimming performance of large silver eels exposed experimentally to currents of various speeds and concluded that infected eels showed lower cruising speeds and a higher energy cost of swimming than uninfected ones. They also found that uninfected eels with damaged swimbladders showed similar responses to infected eels and almost half of those tested stopped swimming at lower aerobic swimming speeds. They therefore concluded that reduced swimming performance was associated with swimbladder dysfunction, whether due to the presence of Anguillicoloides crassus or to other causes.
One of the first investigations into the possibility of eels becoming immune to infection with Anguillicoloides crassus was undertaken by Haenen et al., (1996). They infected a batch of Anguilla anguilla with 40 L3 each, and after 56 days killed half of them and re-infected the other half with 20 L3 each. These were then killed after 112 days. Larval recovery ranged from 14-20% on day 56, and 9-26% on day 112, and there was no significant relationship between the proportion of parasites recovered and the primary or secondary infection dose. Reinfected eels showed more severe haemorrhages and pigment spots in the swimbladder, but there was no evidence of resistance to re-infection and no antibodies to the parasite were shown by ELISA.
Humoral responses to Anguillicoloides crassus infections in Anguilla anguilla have been investigated on several occasions. Buchmann et al. (1991) reported a humoral immune response to Anguillicoloides crassus, and the antigens to this response were identified by Nielsen (1999). Békési et al. (1997) attempted to determine whether a humoral response in eels was involved in the heavy infections and mortalities in Lake Balaton but decided that it was not. Knopf et al. (2000a, b) confirmed the existence of a humoral immune response in experimental infections of eels by detecting antibodies against Anguillicoloides crassus 8 weeks post infection. The response was only to antigens produced by adult parasites and was independent of the number of infective L3 present in the host. The response could be suppressed by polychlorinated biphenyls; after treatment with these compounds, antibodies were first detected only after 61 days (Sures and Knopf, 2004a, Sures et al., 2006).
More recent investigations have focused on a comparison between Anguilla anguilla and Anguilla japonica as hosts of Anguillicoloides crassus in view of the fact that the parasite appears to be less pathogenic to the former species (Egusa, 1979; Nagasawa et al., 1994). Knopf and Mahnke (2004) experimentally infected both species of eels with the same dose of 30 Anguillicoloides crassus larvae per eel. Examination of the eels after 98 days at 23° C revealed a big difference in the susceptibility of the two species to the parasite. The recovery rate of parasites from Anguilla japonica was less than half that from Anguilla anguilla (13.8% as opposed to 33.2%). Of the parasites found in Anguilla japonica, 60% were dead, as encapsulated and necrotic larvae in the swimbladder wall, whereas no dead parasites were found in Anguilla anguilla. Development of the parasites was also slower in Anguilla japonica. Clearly the original host of Anguillicoloides crassus, Anguilla japonica, has a more effective defence mechanism. The authors still failed to find any evidence of protective immunity in Anguilla anguilla or to demonstrate that antibodies had any protective function. Sures et al. (2001) considered that Anguillicoloides crassus was an extremely potent stressor of Anguilla anguilla and suggested that the strong cortisol response it evoked in this species might be suppressing any immune response. Attempts to immunise the two species of eel with irradiated L3 larvae of Anguillicoloides crassus were made by Knopf and Lucius (2008). They infected each eel with 40 irradiated larvae and later challenged it with 40 normal (i.e. non-irradiated) larvae. This treatment induced a significant reduction (of 87%) in the number of adult parasites derived from the challenge in Anguilla japonica, but no reduction in Anguilla anguilla. Both species of eels developed an antibody response, but its level was not correlated with protection in either case. Clearly Anguilla japonica is able to mount an efficient, protective response against Anguillicoloides crassus, but Anguilla anguilla lacks this ability. The two host species clearly differ in their susceptibility to the parasite, as Knopf (2006) suggested, but in neither is there yet evidence that this relates to humoral antibody production.
It has been shown recently that paratenic hosts can respond immunologically to the parasite. Unger et al. (2009) have demonstrated in experimental infections that Anguillicoloides crassus was able to provoke an immune response in ruffe Gymnocephalus cernuus, which was able to effectively reject the neozoic parasites.
Effects on eel migration
Whilst Anguillicoloides crassus can cause damage to eels in farms, and so a direct economic loss, a major concern is the impact that it may have on wild eels and especially on their spawning migration (Kirk, 2003; Sures and Knopf, 2004b; Taraschewski, 2006; Fazio et al., 2008b). It is currently not known for certain what impact the parasite may have on this migration, but the swimbladder is known to be of major importance during the migration (Tesch, 1977, 1999). Although it has not been shown directly that Anguillicoloides crassus impairs the function of the swimbladder as a hydrostatic organ, the circumstantial evidence that it does so is very strrong. The damage to the swimbladder caused by heavy infections of the parasite and the changes in the gas composition within it (Würtz et al., 1996) would suggest that it cannot function normally. There is also concern that the effects of the parasite in reducing the swimming speed (Sprengel and Luchtenberg, 1991; Palstra et al, 2007) of eels will prolong the duration of migration, as will any drain on the energy reserves of the eel as a direct consequence of the parasites, and this may result in heavily infected eels being unable to reach the spawning grounds or arriving there too late. It is not known how many spawning eels are necessary to maintain the population, and so it is not known how significant the failure of some to spawn may be. In the course of their study, Palstra et al. (2007) simulated eel migration in laboratory trials, and showed that failure to migrate successfully was indeed likely. The only actual study of the parasite in migrating eels was carried out in the coastal waters of the Baltic Sea. No differences were found in condition, body fat or estimated migration speeds between infected and uninfected marked Anguilla anguilla, but intensity of infection was significantly correlated with time and distance between release and re-capture of eels (Sjoberg et al., 2009). The more heavily infected eels were more vulnerable to recapture: it seemed likely also that their vertical migrations were inhibited and that the infected eels migrated in shallow coastal waters close to shore. There must therefore be serious concern that infected eels are inhibited in their spawning migration, and that Anguillicoloides crassus may therefore have contributed to the decline in eel populations in recent years, although it cannot be solely responsible for it since the decline commenced in Europe and North America before Anguillicoloides crassus reached each continent. Effects on the migration of Anguilla rostrata are suspected from similarities in the pathogenicity of the parasite in this host and by analogy with the effects on Anguilla anguilla.
Live infected Anguilla anguilla can seldom if ever be recognised as such with any degree of certainty. Aberrant swimming behaviour may suggest infection, but it is rarely observed and may well have other causes. The only certain method of diagnosis is post mortem examination of the swimbladder. Infected swimbladders may be recognised as such if the intensity of infection is high, but only finding of adults in the swimbladder lumen or of L4s in the swimbladder wall can confirm infection.
Like all anguillicolids, Anguillicoloides crassus is ovoviviparous and eggs in the uterus of the adult contain L2 larvae. A single female may contain from 120 000 to 800 000 eggs (Ashworth, 1994). The eggs may hatch there or be released into the swimbladder and hatch there to release the L2s, which escape through the pneumatic duct to the intestine (Moravec, 2006). Hatching can take place over the temperature range of 10 – 30° C (Thomas and Ollevier, 1993). The free living L2s aggregate and attach to the substrate by their tails. They undulate, presumably to attract the attention of copepods, and the rate of undulation increases with temperature. Survival of L2s is dependent on physico-chemical conditions in the water. They have been reported to live for longer than 42 days at 4° C but for only 15 days at 30° C (De Charleroy et al., 1990; Kennedy and Fitch, 1990; Thomas and Ollevier, 1993). They can survive in 50% seawater for up to 82 days, but only 50% are infective after 40 days. The larvae can tolerate a range of pH, but survival is poor at pH 9.2 whereas 95% survived for 120 days at pH ranges of 4-7 and some remained viable at 160 days (Kennedy and Fitch, 1990).
For further development the larvae require to be eaten by an intermediate host: generally a freshwater copepod crustacean, although ostracods can also serve as intermediate hosts (Moravec and Konecny, 1994). A wide range of species are suitable intermediate hosts, especially species of the genus Cyclops, of which eleven have been identified as suitable hosts (Moravec, 2006), including C. vicinus. The larvae move into the haemocoel of the copepod and there they moult into L3s. Prevalence levels of up to 96% can be achieved in this host species in laboratory infections, with up to 23 larvae per individual, and even brackish water species of Eurytemora can be infected (Kennedy and Fitch, 1990). Copepod survival is reduced at higher infection densities (Ashworth et al., 1996) and infectivity declines with age (Thomas and Ollevier, 1993). The moult to L3 may commence as early as 10 days post infection; most will have mounted to L3 by 20 days at 20° C (Haenen et al., 1989; De Charleroy et al., 1990; Moravec et al., 1993).
If an infected copepod is eaten by an eel, the contained L3 will migrate to the swimbladder wall and there moult to L4. If the infected copepod is eaten by another species of fish in Eastern Asia, Anguillicoloides crassus cannot survive, but in Europe the probability is high that the fish will be able to serve as a paratenic host. It appears that practically every species of freshwater fish has the potential to serve as a paratenic host on this continent, and so function as a second intermediate host, yet paratenic hosts are unknown in Asia (Nagasawa et al., 1994; Münderle et al., 2006). In the Baltic sea, the more saline tolerant species of black goby, Gobius niger, can serve as a paratenic host to facilitate transmission in brackish waters (Hoglund and Thomas, 1992). Paratenic hosts can be considered as an ecological phenomenon facilitating transmission when there is only a single species of definitive host (Nagasawa et al., 1994). A very wide range of species can serve as paratenic hosts in any single locality (Pazooki and Székely, 1994; Székely, 1994, 1996; Thomas and Ollevier, 1992a) and altogether some 33 species of fish from 10 families have been identified as suitable paratenics (Moravec, 2006). The L3s are generally to be found in the body cavity of the paratenic host, especially in cyprinids (De Charleroy et al., 1990) and their importance as a route in transmission of Anguillicoloides crassus to eels will vary from locality to locality (Haenen and Van Banning, 1990, 1991; Moravec and Konecny, 1994) depending on the size of the paratenic hosts and their density.
Not all species of fish are equally suitable as paratenic hosts. Cyprinid fish are the least favoured compared to Perciformes in which prevalence and intensity levels of Anguillicoloides crassus are always higher. Moreover, in ruffe, Gymnocephalus cernua, and perch, Perca fluviatilis, the larvae moult to L4s and even pre-adults although they never develop into sexually mature adults (De Charleroy et al., 1990; Thomas and Ollevier, 1992a; Székely et al., 1996). Amphibia, snails and a range of freshwater insects have also been identified as paratenic hosts (Moravec, 1996; Moravec and Škoriková, 1998) but it is not clear whether they play an important role in transmission of the parasite to eels.
Eels of all sizes from glass eels upwards can be infected with Anguillicoloides crassus following ingestion of an infected copepod or fish, or even a smaller eel (Nimeth et al., 2000). The L3s migrate to the swimbladder wall within 17 h, and later moult to L4 while still in the swimbladder wall; pre-adults move into the swimbladder lumen when they start to feed on blood (Haenen et al., 1989). At 20-22 °C development of L3 to L4 took 3 weeks, but some larvae were retarded up to 3 months. Young adults have been reported as early as 1 month, and adults with eggs after 6-7 weeks (De Charleroy et al., 1990), but it appears that the pre-patent period is normally around 3 months and females remain patent for 1 month, after which they die (Haenen et al., 1989; Moravec et al., 1994b).
Pattern of infection
Once Anguillicoloides crassus has reached a new locality, infection levels may build up very rapidly indeed. The parasite was first detected in the River Trent (UK) in summer 1987 at a prevalence level of 27% but by August the same year prevalence had risen to 77%. By the following April prevalence was 68% with a mean abundance of 2.5, but by July prevalence had reached 100% and abundance 5.3, and by autumn 1988 prevalence was still 100% and mean abundance 6.7. Thereafter levels remained steady with no evidence of a seasonal pattern in infection levels (Kennedy & Fitch, 1990). Similar rapid rises in infection levels have been reported from other localities. In the Selby Dam (UK), prevalence reached a maximum of 91% and mean abundance 4.2 within one year (Ashworth, 1994); in the Albert Canal (Belgium), prevalence increased to 100% and mean abundance to 5.1 within a year (Thomas and Ollevier, 1992b); and in Slapton Ley (UK) over a year prevalence attained 100% and mean intensity peaked at 14.1 (Kelly et al., 2000). After attaining the maximum infection level both prevalence and intensity of infection generally decreased, and tend to stabilise around 80% and 5-7 mean abundance (Køie, 1991; Thomas and Ollevier, 1992b, 1993; Haenen et al., 1996).
However, the increase in infection levels may in a few localities be much slower. In the River Test (UK), even 3 years post-introduction, prevalence had only risen to 28% and mean intensity to 5.9, though eventually levels comparable to those in other localities were attained (Norton et al., 2005). The authors believe that this slow increase in infection levels reflected the high pH of the water, which was sub-optimal for larval survival, and the absence of ruffe (Gymnocephalus cernua) from the river as they appear to be the most suitable paratenic host in terms of assisting transmission of larvae to eels. Both prevalence and mean abundance levels were slow to rise in the lower, estuarine, regions of the River Thames (UK), and Norton et al. (2005) believed that this reflected lower transmission rates in tidal waters.
Analysis of these population changes suggested three possible causal mechanisms. The first of these involves parasite-induced mortality of infected copepods. It has been shown experimentally that the 50% survival time of infected copepods relates directly to the parasite intensity level in the copepods: for example, uninfected Cyclops viridis can survive for 31 days, but after exposure to 5 L2 per individual, 50% survival time declined to 14 days and after exposure to 50 L2 each it declined to 3 days (Ashworth, 1994; Ashworth et al., 1996). Thus the parasite-induced host mortality was acting in a density-dependent, regulatory manner. The second method of regulation involves adult gravid female numbers in eels. Ashworth (1994) found that the number of larvae per adult female Anguillicoloides crassus was not related to the number of gravid females present in the swimbladder, i.e. there was no crowding effect on fecundity. However, the mean intensity of gravid females stayed constant over time even when the total number of adults in the swimbladder increased, so the proportion of gravid females in the adult subpopulation decreased as the overall size of the adult subpopulation increased. Ashworth and Kennedy (1999) then identified a third mechanism. When the adult infrapopulation size in the swimbladder of Anguilla anguilla was high, movement of L4 larvae from the swimbladder wall to its lumen appeared to be inhibited such that Anguillicoloides crassus larvae appeared to be arrested in development in a density-dependent manner. Thus, regulation of Anguillicoloides crassus infrapopulation sizes could be responsible for the stability of the suprapopulations. By extending these earlier studies, Fazio et al. (2008a) confirmed that density-dependent regulation did exist. They stressed that such regulation was possible because the habitat, i.e. the swimbladder, was resource-limited, and because several developmental stages co-existed in eels. By contrast, Münderle et al. (2006) considered that populations of Anguillicoloides crassus in its normal definitive host Anguilla japonica, to which it is better adapted and in which it is not pathogenic, were not regulated by any intra-specific density-dependent mechanism but rather by the defence mechanisms of the host itself.
Mass mortality of eels
In a very few localities, levels of Anguillicoloides crassus in an eel population did not stabilise but continued to rise. The best known example of this was in the shallow, eutrophic Lake Balaton in Hungary (Molnár et al., 1991, 1993, 1994; Székely, 1994, 1996). The lake had been subjected to eel stocking at a high rate to the extent that the eel (Anguilla anguilla) population was overstocked and in the summer of 1991 there was a massive eel mortality in the western basin of the lake, followed by a severe, though lesser, mortality in the eastern basin in 1992. Mortality was confined to eels and did not involve any other species of fish. Infection levels of Anguillicoloides crassus in eels were exceptionally high, with 30-50 adults and over 200 L3/L4 per eel. Swimbladders showed extensive damage and were filled with eggs, larvae and fluid, and the eels showed behavioural irregularities. Initially Anguillicoloides crassus was considered the direct and sole cause of eel mortality, but it came to be recognised that it was the combination of very high eel densities, very high parasite population densities in eels and paratenic hosts, exceptionally high water temperatures (up to 26° C) and low oxygen levels that was responsible, and by 1993 eel and parasite abundances had dropped to more normal equilibrial levels.
Similar mass mortalities of eels were reported from the Vranov Dam and two other small reservoirs in the Czech Republic in 1994. In describing and analysing these, Baruš et al. (1999a) stressed that such high mortalities only occurred in closed systems when populations of eels, paratenic host fish and copepods were unusually high over summer, when Anguillicoloides crassus prevalence levels remained at 100% and when water temperatures were in the region of 27° C, i.e. conditions very similar to those in Lake Balaton. However, similar conditions in other lakes and in other years did not result in mass mortalities of eels, which were neither regular nor inevitable (Bálint et al., 1997). Thus, the precise reasons why the regulatory mechanisms ceased to operate on the parasite populations are still unknown. Conditions in the Neusiedler See in Austria have at times been similar to those in Balaton, but there have been no mass mortalities of eels (Schabuss et al., 2005). In this lake eel stocking was prohibited as early as 1993, but there was no obvious trend in the population levels of Anguillicoloides crassus over the following 8 years. There must clearly have been a great deal of illegal eel stocking, as prevalence levels remained around 63% and intensities around 3.4 and there was no evidence of any decline in the parasite population. No evidence has ever been forthcoming to show that Anguillicoloides crassus causes mass mortalities of eels in rivers.
Despite many reports and references to the damage caused by Anguillicoloides crassus to Anguilla anguilla in eel farms in the Far East, especially in Japan, and to eels in Europe, there is little or no information actually published to quantify the damage in economic terms. Køie (1991), for example, refers to mortalities of 15-65% in a Dutch eel farm and increased mortalities from 10-20 % in other eel farms, but without placing any economic value on these losses.
In the cases of the eel mortalities attributed to Anguillicoloides crassus in European lakes, the only figures available relate to mortalities. Thus, Molnár et al. (1991, 1994) state that in 1991, 200 tonnes of eels died in Lake Balaton, and in 1992 40 tonnes. Baruš et al. (1999a) similarly quote a figure 3.5 tonnes of eels killed in the Vranov Dam in 1994, but no financial values have been attached to these losses. It must be presumed that there was a severe economic loss to professional eel fishermen as a consequence of the mortalities but no figures are available to support this presumption.
More recently, Taraschewski (2006) has made the point that as eel farming declines throughout Europe, even in Italy, in response to increasing energy and labour costs and as the price of ever scarcer elvers increases, Anguillicoloides crassus no longer poses so much of a threat and so will have a greatly reduced economic impact.
Impact on biodiversity
The effects of A. crassus on wild eel populations are more difficult to study than those on captive ones, but experiments suggest that infected eels are more likely to die in adverse conditions (Didziulis, 2006). Mass mortalities have been reported among wild eels, but only when there have been particular combinations of unfavourable circumstances (for more details, see the Epidemiology section). Concern has been expressed that severe damage to the swimbladder may reduce the ability of eels to migrate to their spawning grounds in the Sargasso Sea, which could have a severe impact on the future of the population (Didziulis, 2006); for more details, see the Pathology section. Such a decline could have significant effects on ecosystem community structure.
There is no danger of Anguillicoloides crassus ever infecting or being transmitted to man, as it is so specific to eels.
Since only the swimbladder of eels contains the parasite, infected eels are perfectly safe for human consumption. On aesthetic grounds, removal of the swimbladders (as is generally done before eels are smoked, for example) can be recommended.
Although Anguillicoloides crassus is capable of causing disease in Anguilla anguilla both in eel farms and in natural habitats, no method of control or treatment of natural infections in lakes or rivers has yet been shown to be practicable. Such attempts at treatment as have been made have focused instead upon treatment in eel farms. There appear to have been no trials to treat Anguillicoloides crassus in Anguilla japonica in east Asia, as the parasite is not considered to be pathogenic (Nagasawa et al.1994), especially if the eel ponds are well managed (Han YuSan et al., 2008). Unsuccessful attempts were made in Japan to prevent infection by reducing levels of copepods in eel farms (Egusa and Hirose, 1983). Pressure to find a treatment declined as cultivation of Anguilla anguilla in Japan was abandoned because of the parasite.
In Europe, there have been some trials and interest in possible drug treatment of eels. Given that there was no known chemotherapy, Taraschewski et al. (1988) investigated a range of drugs of known nematicidal impact. The most effective of these proved to be levamisole and metrifonate [trichlorfon or trichlorphon], administered in freshwater baths. The mode of application was important and in neither case was there a drug-specific reaction. Subsequent studies (e.g. Hartmann, 1987; Fontaine et al., 1990; Geets et al., 1992) have produced general agreement that levamisole is the most effective drug, especially if administered in baths. It can be administered in food, but its effectiveness is then rather less certain.
No method of control or treatment of natural infections in lakes or rivers has yet been shown to be practicable. Such attempts as have been made have focused instead upon treatment in eel farms. When Anguillicoloides crassus was recognised as an important pathogen of imported Anguilla anguilla in eel farms in Japan, attempts were made to reduce levels of copepods in the farms by increasing the flow of water through them, but this was not often feasible and even less often effective. The use of chemicals such as trichlorfon to kill copepods in eel ponds was considered to be relatively ineffective as well as being undesirable in view of the certain environmental contamination of the water in the pond and in its outflow (Egusa and Hirose, 1983). Pressure to find a treatment declined as cultivation of Anguilla anguilla in Japan was abandoned because of the parasite.
The recent demonstration by Knopf and Lucius (2008) that Anguilla japonica can mount an effective protective response against Anguillicoloides crassus led them to suggest that it may be possible to vaccinate this host against the parasite. However, there would be little point in this given that the parasite is not pathogenic in this host. As yet there has been no development of a vaccine for Anguilla anguilla against Anguillicoloides crassus.