Vaccination of eels (Anguilla japonica and Anguilla anguilla) against Anguillicola crassus with irradiated L3

The original host of the swimbladder nematode Anguillicola crassus, the Japanese eel (Anguilla japonica) and the recently colonized European eel (Anguilla anguilla) were immunized with 40 irradiated (500 Gy) 3rd-stage larvae (L3) of this parasite and challenged with an infection of 40 normal L3. The immunization induced a significant reduction of the number of adult worms developing from the challenge infection in A. japonica, but not in A. anguilla. The induced resistance (calculated using the relation of the number of adult worms in immunized eels and in non-immunized control eels) in A. japonica was 87.3%+/-30.4%. Following a single infection, the percentage of adult worms found in A. japonica was lower as compared to A. anguilla, and the few adult worms were much smaller, revealing a lower susceptibility of A. japonica to A. crassus in comparison to A. anguilla. Both eel species developed an antibody response against A. crassus, but the level of antibody responses was not positively correlated with the protection against infection, suggesting that the antibody response is not a key element in resistance of eels against A. crassus. This study suggests that the original host of A. crassus is able to mount efficient protective immune responses against its parasite, whereas the newly acquired host seems to lack this ability.     

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.     

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.     

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.     

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.     

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.     

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.     

Population structure of the parasitic nematode Anguillicola crassus, an invader of declining North Atlantic eel stocks

Probably half of all animal species exhibit a parasitic lifestyle and numerous parasites have recently expanded their distribution and host ranges due to anthropogenic activities. Here, we report on the population genetic structure of the invasive nematode Anguillicola crassus, a parasite in freshwater eels, which recently spread from Asia to Europe and North America. Samples were collected from the newly colonized naïve host species Anguilla anguilla (Europe) and Anguilla rostrata (North America), and from indigenous Anguilla japonica in Taiwan and Japan. Using seven microsatellite loci and one mitochondrial marker, we show that the parasite's population structure in Europe mirrors the zoogeographic Boreal–Lusitanian break along the English Channel. Both the north-to-south decline of nuclear allelic diversity and the loss of private alleles in the same direction are consistent with a significant isolation-by-distance pattern based on ρ_ST values. In combination with the specific topology of the distance tree among nematode populations, our data suggest that Europe was invaded only once from Taiwan, and that subsequently, genetic diversity was lost due to random drift. On the contrary, the North American sample shares distinct nuclear and mitochondrial signatures with Japanese specimens. We propose that the genetic structure in Europe was shaped by long-range anthropogenic eel host transfers in the north and a single dispersal event into the southwest. The genetically distinct Brittany sample at the edge of the Boreal–Lusitanian boundary is indicative of natural dispersal of fish hosts since recruitment occurs naturally there and invertebrate host dissemination is interrupted due to oceanic currents.     

Anal redness in European eels as an indicator of infection by the swimbladder nematode, Anguillicola crassus

Anal redness in European eels Anguilla anguilla is related to the prevalence and mean abundance of the swimbladder nematode Anguillicola crassus and may provide a simple, non-invasive diagnostic tool for A. crassus infection.     

Biased sex ratio in the European eel (Anguilla anguilla) swim-bladder parasite Anguillicola crassus, experimentally induced by 11-ketotestosterone

Parasites are intimately connected to the host in which they live, and some may be affected by the polluted environment of their host. The present study describes the effect of a steroid hormone (11-ketotestosterone) on the sex ratio of the invasive hematophagous nematode Anguillicola crassus Kuwahara, Niimi & Itagaki, 1974, when experimentally injected to European eels, Anguilla anguilla. Our results showed that this steroid induced a significant male-biased ratio in the nematode A. crassus infrapopulations, suggesting that the presence of endocrine disruptors in the environment may lead to skewed sex ratios among parasites.     

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'.     

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.     

Spatio-temporal dynamics of the parasitic nematode Anguillicola crassus in Flanders, Belgium

Despite Egusa's earlier warning of the damage that the parasitic nematode Anguillicola crassus could inflict on the European eel Anguilla anguilla, its introduction in Europe was a fact in the early 1980s. Based on an elaborate dataset on Anguillicola crassus infection of 11 river catchments, this paper presents the results of a detailed study on the dispersal of the parasite in Flanders, Belgium, and the host-parasite relationship. In addition, data from 1986 and 1997 are used for comparative purposes, providing a perspective on the temporal infection pattern over 15 yr. The presence of A. crassus in Flanders was first discovered in 1985; 2 yr later a survey revealed a prevalence of 34.1% and a mean infection intensity of 5.5, based on adult nematodes only, and 10 yr later the parasite was present at all 11 sites sampled. Prevalence had increased to 62.5 % but the mean infection intensity had decreased to 3.9 adults per infected eel. Finally, in the year 2000, a third study revealed that A. crassus was present in 139 of 140 investigated sites; a further increase in prevalence to 68.7% and a decrease in mean infection intensity to 3.4 adults per infected eel was observed. When all larval stages were taken into account, mean prevalence amounted to 88.1% and mean intensity to 5.5 adults. The high infection level in Flanders is thought to be the result of restocking with glass eel and yellow eel, both of which are susceptible to A. crassus. The general infection parameters were similar in all 11 river catchments. It is possible that in Flanders both prevalence and mean infection intensity are stabilizing due to density-dependent regulation of the parasite infrapopulation. Fibrotic swimbladder walls were observed, mainly in large eels, and 20% of the total number of nematodes consisted of encapsulated larvae in the surveys of 1997 and 2000; 8 cases of swimbladder regeneration were observed.     

Regulation of Anguillicola crassus (Nematoda) infrapopulations in their definitive host, the European eel, Anguilla anguilla

The parasitic nematode Anguillicola crassus was recently introduced into populations of the European eel, Anguilla anguilla. We investigated, under experimental conditions, the regulation of A. crassus infrapopulations. We tested the effects of (1) the resource-limited habitat of the parasite and (2) the coexistence of several developmental stages in its niche (the swim-bladder) on the composition of the infrapopulations. The results revealed that the respective effects of these factors differed substantially during the course of the infection. Third-stage larvae (L3s) establishment would not be constrained by the size of the swim-bladder. Their moult to fourth-stage larvae (L4s) would be accelerated as the number of L3s increased. The moulting time of L4s to adults would be reduced by males and would be constrained by the size of the swim-bladder. However, the moult of L4s to adults and their further development would be synchronized with those of the opposite sex. At the time of mating, the number of males and the body weight of adults would depend on the size of the swim-bladder. Soon after the laying of eggs, the developmental constraint on the late L3s would decrease. When adults die, constraints would cease and late larval stages would moult to become adults.     

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.     

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.     

Anguillicola crassus infection in Anguilla rostrata from small tributaries of the Hudson River watershed, New York, USA

We studied the invasion of the exotic nematode parasite Anguillicola crassus in the American eel Anguilla rostrata using tributaries of the Hudson River estuary. Yellow-phase American eels were sampled from 6 tributaries, and their swim bladders were examined for nematode infection. Prevalence averaged 39% with an intensity of 2.4 nematodes per eel. Parasite distribution was not significant along a latitudinal gradient; on the other hand, physical barriers (dams and natural waterfalls) significantly reduced infections upstream. Urbanization may increase the susceptibility of eels to infection; we found significantly elevated infection rates when urbanized lands exceeded 15% of the tributary catchment area. Yellow-phase eel condition was not affected by parasite infection. The invasion of the entire Hudson River watershed is ongoing and therefore will continue to be a management concern. Further analysis of the parasite-host interaction in North America is warranted.     

Spatio-temporal dynamics of the nematode Anguillicola crassus in Northeast Tunisian lagoons

Anguillicola crassus, parasite nematode of the European eel Anguilla anguilla, was recorded for the first time in Tunisia (1999) in the Ichkeul lagoon. Its distribution has since spread toward Bizerte and Ghar El Melh lagoons. The monthly epidemiological survey reveals that A. crassus exists throughout the year in the Ichkeul lagoon. In this lagoon, its prevalence is low in winter (12% in December), with a marked increases in the spring reaching a maximum in March (35%), before it starts to decrease in summer with a minimum in July (4.35%), which in turn is followed by a pronounced new rise in autumn (30% in November). However, mean intensity values do not show such a marked variation. The majority of the values are between 1 and 1.5 parasites per host. In the Bizerte and Ghar El Melh lagoons, the presence of this nematode is limited only to one to three months. Investigations in the Tunis lagoon did not reveal until now the presence of A. crassus. It has been observed that the length of the eel influences the prevalence values: A. crassus becomes less common if the length of the eel increases. Comparatively with the global epidemiological values of A. crassus signalled subsequently (1999) in the Ichkeul lagoon, we note that the present values record a clean increase.     

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.