Scientific Name: Anguillicola crassus
Other scientific names appearing in the literature of this species:
Common Name: Eel parasite
Taraschewski et al. (1987) describe this adult nematode as having a body covered by a soft, wrinkled outer cuticle. The circular mouth opening is small surrounded by 4 dorsolateral and ventrolateral cephalic papillae and 2 small lateral amphids. The anterior rim of the buccal capsule bears one row of 22, 24, 26 or 28 curcumoral teeth. The oesophagus appears strongly muscular, consisting of 3 lobes, expanding at its posterior half; anteriorly, the oesophagus forms 6 slightly elevated, rounded lobes protruding into the buccal cavity. The valvular apparatus of the oesophagus is well developed. Male worms have 6 pairs of caudal papillae; 2 praeanals, 1 or 2 adanals and 2 or 3 postanals.
Anguillicola crassus is a parasitic nematode of eels (Anguilla spp.) originating in Southeast Asia where it is a natural parasite of the Japanese eel, Anguilla japonica (Kuwahara et al, 1974; Egusa, 1979). Anguillicola crassus is classified in the nematode suborder Camallanata, members of which always use a crustacean as intermediate host, often a copepod (Wang & Zhao, 1980). Many species of copepod and ostracod have been reported to serve as intermediate hosts for A. crassus (Hirose et al., 1976, De Charleroy et al., 1990; Petter er al., 1990, Moravec et al., 1993), but the parasite is also known to use many non-crustacean intermediate hosts. Thirty-three species of fish belonging to 10 families (De Charleroy et al., 1990; Thomas & Ollevier, 1992; Moravec & Konecny, 1994) and at least one snail (Moravec, 1996) also act as intermediate hosts for A. crassus.
Life-cycle of the parasite: Anguillicola crassus is a parasitic swim bladder nematode of eels, Anguilla spp. (Kuwahara et al., 1974). The parasite's life cycle is a simple one which can be completed in less than 2 months under laboratory conditions (De Charleroy et al., 1990). Adult worms infest the swim bladder of its eel host producing thousands of fertilized eggs which pass to the intestinal tract then hatch during or after passage through the digestive system. The free-living larvae attach to the substrate by their hooked tail where they are ingested by copepods (intermediate host) (De Charleroy et al., 1990). The larvae molt to the third stage (the infective stage for eels) within the copepod (Thomas & Ollevier, 1989; De Charleroy et al., 1987). Infected copepods are ingested by young eels (Lecomte-Finiger, 1983; De Nie, 1987) or by other reservoir or paratenic host species such as ruffe (Gymnocephalus cernua), pumpkinseed (Lepomis gibbosus), brown-bullhead (Ictalurus nebulosus) (Thomas & Ollevier, 192; De Charleroy et al., 1990; Moravec, 1996). The larvae remain infective to eels for up to 2 months within paratenic hosts (Moravec, 1996). Larger eels whose diets consist mainly of fish (Tesch, 1977; de Nie, 1988) are likely infected by feeding on reservoir host fish rather than through ingesting copepods. After infecting the eel (final host), the parasite larvae pass through the intestinal wall and body cavity to reach the swim bladder, attach to the swim bladder wall, feed on blood and grow to sexual maturity (DeCharleroy et al., 1990).
Eels infected by A. crassus develop a disease called anguillicolosis, and causes hemorrhagic lesions, fibrosis and collapsed swim bladders (Haenen et al., 1994). VanBanning and Haenen (1989) study of the pathological effects of A. crassus on wild and farmed European eels, indicate that acute inflammatory reactions and fibrosis of the swim bladder occur in response to the parasitic infection.
The overall body length of males is 20-60 um with females measuring 47-71.5 um. Body width of males is .9-2.8 um and 3-5.6 um for females (Taraschewski et al., 1987).
Natural range: Anguillicola crassus Originated in Southeast Asia, and its natural host is the Asian eel, Anguilla japonica (Kuwahara et al., 1974; Moravec and Traschewski, 1988).
Non-indigenous range: The parasite was introduced to Europe in the early 1980's. In 1985 the parasite was reported infecting Anguilla anguilla in Belgium (De Charleroy, 1986). Within a short span of time, the parasite was transmitted to most European countries (with up to 100% infectivity in some localities). Both farmed eels and natural eel populations were affected (Paggi et al., 1982; Neumann, 1985; Kennedy and Fitch, 1990). It is thought that A. crassus was introduced to western Europe by infected Asian eels imported for consumption and aquacultural restocking (Peters & Hartmann, 1986; Belpaire et al., 1989a).
A south Texas aquacultural facility rearing eels for the Asian consumer market first reported A. crassus in U.S. waters (1993, 1994). It is thought that elvers (juvenile stock) supplied by an east coast grower carried the parasite. When sampled, the Texas aquaculture population was found to be infected with A. crassus (Fries et al., 1996). (Specific identification of the eel host was not made, so it is not know if the south Texas infestation was of an American, European or Asian eel species.) Investigation of the east coast supplier's stocks found one immature specimen of A. crassus . The infected individual was an American eel species, suggesting that the parasite may have had an opportunity to become established on the east coast (Fries et al., 1996). Subsequent sampling of native eels collected from 5 Texas streams turned up no evidence of the swim bladder parasite (Fries et al., 1996).
Interest to Fisheries:
Worldwide aquacultural production of eel accounts for more than 100 thousand tons and provides the great majority of total eel production (Johnson et al., 1985). The two principle farm species are A. japonica and A. anguilla, both of which are infected by A. crassus. Moller et al. (1991) believe that due to compromised swimming ability caused by A. crassus parasitaztion, fewer eels are able to reach spawning grounds resulting in decreases in natural European eel populations. Anguillicola crassus is thus considered a major threat to European eel fisheries (Hoglund and Thomas, 1992). In the U.S., measures to stem the spread of parasite will affect aquaculturists. Growers, importers and exporters face the potential for monetary loss from quarantine and crop destruction (Fries et al., 1996).
Current Status of this Species in the Gulf of Mexico Ecosystem:
Present distribution of the parasite in U.S. waters is not fully known (Fries et. al, 1996), however at least one limited study indicates that A. crassus may be present on the U.S. east coast (Fries et al., 1996). Specimens of the American eel, A. rostrata were found infected during a Texas Parks and Wildlife Department (TPWD) investigation of a south Texas fish farm (Johnson et al, 1995; Fries et al., 1996). Subsequent sampling of five Texas rivers found no indication of the parasite's presence in wild eel populations (Fries et al., 1996).
It is not known how American eels will react to A. crassus, but infections in European eels suggest that the parasite is more harmful to new hosts than to its native host (Fries et al., 1986; Moravec and Taraschewski, 1988).
The rapid spread of A. crassus in Europe is understandable when taking into account the large quantities of A. crassus eggs typically found in the swim bladder of infected eels (sometimes originating from a single pair of adults) and the short period needed to complete the parasite's life cycle (De Charleroy et al, 1990). Another factor contributing to the rapid spread of A. crassus is that it is not particular about habitat or transfer hosts (Taraschewski et al., 1987). Anguillicola crassus is capable of surviving in eels in freshwater, brackish water, and seawater (Kennedy and Fitch, 1990). Introduction of even a few individuals of the parasite to new geographic through careless import, export and restocking practices can result in exponential spread of the parasite (De Charleroy et al, 1990).
More work should to be done to establish the distribution of A. crassus in U.S. waters and its potential to parasitize native species (Fries et al, 1996). Chemical control measures have proven largely unsuccessful (Traschewski et al. 1987; Hartmann, 1989; Geets et al., 1992). Kennedy and Fitch (1990) suggest that the best approach to limit the spread of A. crassus is through legislation regulating import and export of eels.
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