The swimbladder nematode Anguillicola crassus in the European eel Anguilla anguilla and the Japanese eel Anguilla japonica: differences in susceptibility and immunity between a recently colonized host and the original host

The swimbladder nematode Anguillicola crassus originates from Asia where it is a parasite of the Japanese eel Anguilla japonica. After its introduction to Europe about 25 years ago, the parasite spread rapidly within the indigenous populations of the European eel Anguilla anguilla and subsequently the prevalence and mean intensity appeared to stabilize. Under experimental and aquaculture conditions the na?ve new host appears to be more susceptible to A. crassus compared to the original host. Both eel species develop a immune response against A. crassus. The antibody response is well characterized for the European eel, but poorly characterized for the Japanese eel. It remains unclear if antibodies have any protective function against A. crassus. Encapsulation of larvae of A. crassus can be observed in naturally infected European eels. However, encapsulation of larvae following experimental infection has not been detected in European eels, but only in Japanese eels. Reinfection experiments and intraperitoneal injection of A. crassus homogenates failed to demonstrate the development of acquired immunity in European eels. Immunization with irradiated third stage larvae provided preliminary evidence for acquired immunity against A. crassus in the Japanese eel, but not in the European eel.     

Occurrence of Anguillicola crassus (Nematoda: Dracunculoidea) in Japanese eels Anguilla japonica from a river and an aquaculture unit in SW Taiwan

The infection by swimbladder nematodes of the genus Anguillicola (Dracunculoidea: Anguillicolidae) was examined in 2 populations of the Japanese eel Anguilla japonica in SW Taiwan. Wild eels from the Kao-Ping river were compared with cultured eels from an adjacent aquaculture unit. Only the cosmopolitan species Anguillicola crassus was present. Among wild eels, prevalence of infection varied between 21 and 62%, and mean intensity between 1.7 and 2.7 for adult worms. Similar intensity values (1.3 to 2.8) were recorded for the larvae. In cultured eels, prevalence as well as mean intensities were higher. In the cultured hosts, mean larval intensities exceeded those of adult worms 2-fold, and maximum larval intensities were 4- to 5-fold higher than in eels from the river. In cultured eels, dead larvae were also more abundant than in wild eels. We conclude that infrapopulations of A. crassus in Japanese eels are regulated by the defense system of this host, intraspecific density-dependent regulation being less likely as the major regulatory mechanism. No influence of the parasite on eel condition was found in either wild or cultured eels, indicating a low or moderate pathogenic effect of A. crassus on this host. This study shows that A. crassus is moderately common in cultured and wild Japanese eels in Taiwan, where the parasite is endemic.     

An enhanced humoral immune response against the swimbladder nematode, Anguillicola crassus, in the Japanese eel, Anguilla japonica, compared with the European eel, A. anguilla.

The humoral immune response in the two eel species, Anguilla japonica and Anguilla anguilla against two fractions of antigens in Anguillicola crassus were studied. Within species, both eel species showed significantly elevated titres compared with controls when immunized with antigens from Anguillicola crassus. In interspecific comparison, Anguilla japonica showed significantly elevated titres in comparison with Anguilla anguilla. Immunization of Anguilla anguilla caused a significantly decrease in the plasma levels of protein in comparison with control fish and all groups of Anguilla japonica. In contrast, Anguilla japonica showed significantly lower plasma levels of Ig in all groups compared with Anguilla anguilla. The different susceptibilities to Anguillicola crassus between the natural host, Anguilla japonica, and the naive, Anguilla anguilla, is partly due to differences in the ability of the two eel species to mount a humoral immune response.     

Swim bladder nematodes (Anguillicoloides crassus) disturb silvering in European eels (Anguilla anguilla)

The introduced parasite Anguillicoloides crassus is thought to play an important role in the decline of freshwater eel (Anguilla spp.) populations. These nematodes are known to negatively affect many fitness-related traits in eels. We used experimental infections to study the effect of A. crassus on the relative size or mass of organs, and the expression of functionally relevant genes (total of 12 parameters) that are involved in the silvering process of Anguilla anguilla. Our results showed that the liver mass, the hemoglobin α-chain, and androgen receptors α expression levels were significantly higher in infected eels, whereas the freshwater rod opsin expression level and the gut mass were significantly lower in infected eels. Our results suggested that infected eels were at a more advanced stage in the silvering process than uninfected counterparts of similar size. These results may be explained by 2 hypotheses. First, A. crassus could trigger physiological mechanisms involved in the silvering process as a side-effect of infection. Second, eels may adjust their life history traits in response to infection. The implications for eel migration and reproductive success may be either negative or positive, depending on whether the response to A. crassus infection results in an additional cost of the parasite or is due to the phenotypic plasticity of the host.     

Humoral immune response of European eel Anguilla anguilla experimentally infected with Anguillicola crassus

A humoral immune response of the European eel Anguilla anguilla elicited by an experimental infection was demonstrated for the first time against the swimbladder nematode Anguillicola crassus. Eels were experimentally infected once or repeatedly and the antibody response was observed over a period of 325 d. Specific antibodies against A. crassus in the peripheral blood of the eels were measured using an ELISA and the immunoblot technique. Anti-A. crassus antibodies were first observed 8 wk post infection, and appeared to be independent of both the number of infective third stage larvae (L3) administered and the frequency of administration. However, individual eels showed great differences in the course of the antibody response. The late appearance of antibodies in the peripheral blood supports the hypothesis that not the invading L3 but rather the adult parasites elicit the production of specific antibodies. A stage-specific antibody response against the L3 was not observed. Main antigens are located in the body wall, especially in the gelatinous outer cuticle, of adult A. crassus.     

Development of Anguillicola crassus (Dracunculoidea, Anguillicolidae) in experimentally infected Balearic congers Ariosoma balearicum (Anguilloidea, Congridae).

The development of Anguillicola crassus in experimentally infected Ariosoma balearicum (Anguilloidea, Congridae) kept in seawater was studied in the laboratory. In parallel trials the effect of water salinity on the development of larval A. crassus in European eels Anguilla anguilla was also investigated using eels kept in seawater of a salinity of 34 per thousand. Both eel species were orally inoculated with L3 larvae of A. crassus and then maintained for up to 3 mo at 18 degrees C in seawater. 110 d post infection, no adult but larval (L3 and L4) stages of A. crassus were detected in the swimbladder wall of Balearic congers, although this period of time was sufficient for the parasites to develop to the adult stage in European eel kept in seawater. The results presented suggest that the definitive host specificity of A. crassus comprises species of the family Anguillidae (i.e. the genus Anguilla), but not members of the Congridae. Theoretically however, A. balearicum might serve as a metaparatenic host. Factors determining the definitive host range of A. crassus remain to be elucidated. Water salinity does not seem to act as a factor affecting definitive host specificity once the parasite has become ingested by the eel.     

Impacts of the swimbladder nematode Anguillicola crassus on Anguilla anguilla: variations in liver and spleen masses

Variations in the liver and spleen masses of the eel Anguilla anguilla were analysed in relation to the parasite load of Anguillicola crassus at autopsy (current infection by swimbladder lumen worms) and in relation to the severity of damage observed in the swimbladder (a way of assessing the intensity of past infections). None of these measures of parasite pressure were shown to account for variation in the relative liver mass, either when controlling for somatic mass or eel age. In marked contrast, a significant increase in spleen size was revealed in eels harbouring many lumen worms and also in eels with severe damage in the swimbladder. Splenic enlargement was nearly two‐fold higher among severely affected eels (harbouring more than seven lumen parasites and showing severe damage in the swimbladder) than among infection‐free eels (no lumen parasites and no pathological signs in the swimbladder). Several possible hypotheses are reviewed before arguing for an adaptive host response involving the haematological and immunological functions of the spleen. Indeed, among eels with no pathological signs in the swimbladder, the relative spleen mass was positively associated with the mass of lumen parasites, which suggests a hyper‐synthesis of blood cells by the spleen in response to the bloodsucking activity of lumen worms. Nevertheless, among eels with no lumen parasites at autopsy, there was still an increase in spleen size in relation to the severity of the swimbladder damage, which also suggests a hyper‐synthesis of splenic immune cells (lymphocytes and macrophages) in reaction to damaged tissues and particularly to larvae in the swimbladder wall.     

Influence of Anguillicola crassus (Nematoda) and Ichthyophthirius multifiliis (Ciliophora) on swimming activity of European eel Anguilla anguilla

We investigated the swimming activity of 70 European eels Anguilla anguilla in relation to natural infection with 2 parasite species: the eel-specific swimbladder nematode Anguillicola crassus and the non-specific skin and gill protozoan Ichthyophthirius multifiliis. We measured how long individual eels exposed to a water current in a swimming channel with a steady-stream profile could withstand the water current. The parasites affected the swimming behaviour of eels in different ways. The maximum period of time the fish were able to swim against the current was not correlated with infection by A. crassus. In contrast, infection with I. multifiliis reduced the swimming time. The protozoan has a higher pathogenicity than the swimbladder nematode, at least in closed systems, where I. multifiliis is able to spread within a few days. Reduction in swimming capacity after infection with the ciliate averaged 47 % compared to capacity prior to infection. Thus, our results do not support the previously suggested strong negative relation between swimming activity of eels and intensity of A. crassus infection, at least in the short-term. However, there are indications in the literature that the pathological effects of A. crassus on the eel swimmbladder may involve a higher energy demand, possibly manifested in a prolonged spawning migration. As a result, eels heavily infected with this parasite may arrive too late at the spawning site to participate in mating. This could ensure a selection of 'good genes'.     

First record of Anguillicoloides crassus (Nematoda) in American eels (Anguilla rostrata) in Canadian estuaries, Cape Breton, Nova Scotia

In the summer of 2007, American eels, Anguilla rostrata, from 2 localities on Cape Breton Island, were found to be infected with the swim bladder nematode Anguillicoloides crassus. This is the first documented report of this highly invasive parasite in Canadian waters. More than half of the yellow eels in Mira River (6 of 10), and 1 eel (of 5) from Sydney Harbour were infected. Parasite intensity ranged from 1 to 11 worms per eel. The occurrence of A. crassus at these 2 localities suggests the need for a more extensive survey on the distribution of this exotic parasite in eel populations throughout Cape Breton Island.     

First record of ostracods as natural intermediate hosts of Anguillicola crassus, a pathogenic swimbladder parasite of eels Anguilla spp

The ostracod Physocypria nipponica (Ostracoda: Candonidae) was found (prevalence 14.2%) to be the only intermediate host of the nematode Anguillicola crassus (Nematoda: Anguillicolidae), a pathogenic swimbladder parasite of eels, in a greenhouse-heated culture pond at Isshiki, Aichi Prefecture, Japan. Japanese eels Anguilla japonica from the same pond were found to be infected by adult A. crassus (prevalence 71.8%, intensity 1 to 6). This indicates that A. crassus could complete its life cycle under conditions of modern eel-culture technology where copepods were absent due to the unfavorable water quality for them, by utilizing ostracods as the intermediate host.     

The swimbladder nematode Anguillicola crassus in American eels (Anguilla rostrata) from middle and upper regions of Chesapeake Bay.

The patterns of infection of American eels Anguilla rostrata, with the introduced swimbladder nematode Anguillicola crassus, in tributaries of middle and upper Chesapeake Bay are described. A total of 423 subadult eels was collected from 8 Bay tributaries from spring 1998 to fall 1999. Also, 30 elvers were collected from Ocean City, Maryland, in spring 1998. The numbers of juvenile and adult specimens of A. crassus in the swimbladder wall and lumen were counted. No elvers were infected. In subadult eels, prevalence of adult and juvenile stages combined ranged from 13% to 82%; mean intensity ranged from 2.6 to 9.0 worms per eel. Infection levels were highest for Susquehanna River eels (northernmost river) and lowest in the southernmost sites: St. Jerome's Creek and the Pocomoke River. Although eels from these 2 localities were larger, the low infection rates there are most likely due to reduced transmission in higher salinity water and not to eel size. Eels with both adult and juvenile stages of A. crassus were more common than expected by chance. This might be explained by inhibition of juveniles migrating into the swimbladder lumen when adults are already present there.     

Physiological responses to acute temperature increase in European eels Anguilla anguilla infected with Anguillicola crassus

The swimbladder parasite, Anguillicola crassus has infected, and spread rapidly, through European eel Anguilla anguilla (L.) populations over the past 20 to 25 yr. Our aim in the present studies was to elucidate whether the presence of A. crassus in these eels alters their rapid physiological responses to an acute temperature increase, compared to the response of uninfected fish. Both infected and uninfected fish showed significant increases in plasma cortisol after 2 h at a raised environmental temperature with increased plasma glucose after 6 h. However, infected eels exhibited a slight lag in glucose mobilisation, which may be due to the metabolic cost of harbouring a sanguiverous parasite. Both infected and uninfected fish showed a significant increase in haematocrit after 6 h of temperature elevation, but only uninfected fish exhibited a significant increase in haemoglobin at this point. However, there were no significant changes in mean erythrocyte haemoglobin concentration in either group. Our results suggest that acute temperature increase alone is unlikely to cause significant mortality of A. crassus-infected European eels; however, the effects of chronic increases in temperature in combination with other factors such as toxicants and hypoxia requires examination.     

An invasion record for the swimbladder nematode Anguillicoloides crassus in European eel Anguilla anguilla in a deep warm-monomictic [corrected] lake, from invasion to steady state

This study is the first account of the establishment and development of the neozoic nematode parasite Anguillicoloides crassus in its host, the European eel Anguilla anguilla, in a deep, warm-monomictic [corrected] lake. A 21 year study of A. crassus took place in Upper Lake Constance (ULC), Europe's second largest pre-alpine lake. The study included two extensive surveys, one in 1991 during the initial parasite invasion phase and the second in 2006 when the infection was well established. The subtropical swimbladder nematode A. crassus was first recorded in A. anguilla in ULC in 1989. Prevalence reached 60% in 1992 and remained at this level until 2007. In 2008, prevalence decreased to 48%. Infection intensity peaked in 1993 at a mean value of 16 adult parasites per host fish. Around 90% of all A. anguilla examined displayed swimbladder lesions, with a significant trend to increasing severity over time. Moreover, heavy swimbladder lesions were seen in c. 10% of A. anguilla ready to migrate to their spawning habitat. Both ruffe Gymnocephalus cernuus and sunfish Lepomis gibbosus serve as paratenic hosts for A. crassus in ULC. Gymnocephalus cernuus seems to be the main vector, and infection is especially frequent in spring possibly caused by reduced immune system efficacy of G. cernuus during winter. In 1991, hypochromic anaemia was prevalent in ULC A. anguilla acutely infected with A. crassus, whereas in 2006 blood values were indicative of chronic infection. The growth and survival rates of A. anguilla during their continental phase were not noticeably altered in infected fish, but damage to the swimbladder probably impairs migration potential and thus the subsequent breeding success of the oceanic phase.     

The spread of the eel swimbladder nematode Anguillicola crassus through the Erne system, Ireland

The spread of Anguillicola crassus was documented and showed an increase in both prevalence (9·9%) and mean intensity (6·7). Infected eels Anguilla anguilla were recorded 30 km downstream of the first recorded sites of infection. Migrating silver eels from commercial nets near the outlet of the Erne during October-December 1999 were also infected. Continued spread of A. crassus through Ireland's major wild eel fisheries now appears likely.     

Identification of Anguilla anguilla (L.) and Anguilla rostrata (Le Sueur) and their hybrids based on a diagnostic single nucleotide polymorphism in nuclear 18S rDNA

The variability of the nuclear 18S rDNA was examined for 10 species of freshwater eels: Anguilla anguilla, A. australis, A. dieffenbachii, A. japonica, A. marmorata, A. mossambica, A. nebulosa labiata, A. obscura, A. reinhardtii and A. rostrata. We observed that a single nucleotide polymorphism (SNP) separated A. anguilla and A. japonica from the remaining taxa. While this SNP marker may be used to identify hypothetical hybrids of A. japonica and sympatric eel species, we focused on Atlantic eels to evaluate the potential for this diagnostic chromosomal marker to discriminate between individuals of A. anguilla, A. rostrata and their hybrids.     

Evaluation of an ELISA and immunoblotting for studying the humoral immune response in Anguillicola crassus infected European eel Anguilla anguilla

The applicability of an enzyme-linked immunosorbent assay (ELISA) for the detection of anguillicolosis in feral eels was examined using a crude antigen preparation from the body wall of adult Anguillicola crassus. The screening consisted of samples from 100 feral European eels Anguilla anguilla. As a reference the actual status of infection was determined by dissection of the eels' swim-bladders. The ELISA results were compared with a background value calculated from the results obtained from 43 non-infected farm eels. The screened samples had a high prevalence of A. crassus (83 %); however, the specificity and the negative predictive value of the ELISA were low compared to the high positive predictive value. Nonetheless, the reproducibility (precision) of the test was satisfactory, and for the non-infected reference group specificity was 97.7 %. Although the ELISA, as used in the present study, is not applicable for diagnostic purposes, it represents a useful tool for the investigation of the specific humoral immune response of eels against A. crassus under controlled experimental conditions. Immunoblots using crude antigen preparations from different parts of adult A. crassus as well as a crude somatic third-stage (L3) antigen preparation illustrated that only antigens associated with the body wall of adult A. crassus are potentially suitable for diagnostic purposes. Despite the fact that antibodies against Raphidascaris acus cross-reacted with 3 body wall antigens of A. crassus, the most encouraging results were obtained with the antigen preparation from the outer cuticle of adult A. crassus which yielded a conspicuous, broad band at about 100 kDa.     

Spatial and temporal variation in Anguillicola crassus counts: results of a 4 year survey of eels in Mediterranean lagoons

We present the results of a survey of anguillicolosis in the Rh6ne River delta. From January 1997 to December 2000, a total of 13,319 eels (Anguilla anguilla from elver to silver phase) were examined, in which we found 22,227 swimbladder nematodes (Anguillicola crassus adults and pre-adults). A generalised linear model (GLM) framework was used to explore the relative contribution of various factors to the occurrence, intensity and abundance of the parasite. We reveal a major influence of the month of sampling, and we document the existence of a seasonal pattern with regular peaks in early summer and late winter. In contrast, the year of sampling is of secondary importance, and no particular trend in the development of the infection can be detected. More than a decade after the first record of A. crassus in the Rh?ne River delta, anguillicolosis has thus attained a constant infection rate of nearly 50%, with a mean number of 3 or 4 macroscopic lumen worms per infected eel. The eel length strongly influences the intensity and the abundance of the nematode, but has little if any effect on the probability of being infected. There exists a linear relationship between eel size and the number of parasites, but not between eel size and prevalence. We observe a decrease in the proportion of infected individuals among elver eels. We discuss this result in relation to the possible mortality of heavily infected individuals and/or a change in the eels' alimentary diet.     

Infection status of the swimbladder worm, Anguillicola crassus in silver stage European eel, Anguilla anguilla, from three different habitats in Danish waters

The infection status of silver stage eels, Anguilla anguilla , infected with Anguillicola crassus from three locations, ranging from a lake to seawater, were investigated. Data show a significant difference in mean intensity of Anguillicola crassus in silver stage eels according to the salinity of the habitats.