This review compiles information on the taxonomy, identification, geographical distribution, life cycles, host ranges, occurrence, development and growth in both intermediate and final hosts, pathogenicity, and control measures of AnguilUcola crassus and A. globiceps, swimbladder nematodes of farmed and wild populations of two species of eels, Anguilla japonica and A. anguilla, in East Asia. Anguillicola crassus is distributed in Japan, Korea, Taiwan, and China, while A. globiceps is reported only in Japan and China. These nematodes use cyclopoid copepods as intermediate hosts. Known intermediate hosts are Eucyclops serrulatus (Japan) and Therirwcyclops hyalinus (Korea) for A. crassus, and Mesocyclops leuckarti, T. hyalinus, T. taihokuensis, E. serrulatus, Acanthocyclops viridis, and Cyclops slrenuus (China) for A. globiceps. Anguillicola crassus shows a seasonal occurrence in T. hyalinus with high prevalence in summer, Paratenic hosts are yet unknown in East Asia. Anguillicola crassus is relatively common in farmed and wild populations of Anguilla japonica in East Asia, but A. globiceps is usually found in wild populations of A. japonica in Japan and China. In culture ponds, A. crassus is more prevalent and abundant in A. anguilla than in A. japonica. Although A. globiceps induces only the thickening of the host’s swimbladder wall, A. crassus gives severe pathological effects in A. anguilla and heavy infection leads to host mortality. Prevalence of A. crassus in A. japonica cultured in Japan and Korea is relatively low in winter, whereas prevalence of A. globiceps in wild populations of A. japonica from Japan is high in winter.
Following the introduction of Anguillicola crassus into Lake Balaton, by 1991 the entire eel population became infected. At the same time, marked differences existed in the prevalence and intensity of infection between different areas of the lake. The highest prevalence values occurred in the eastern basin less densely populated with eels, while in the western basin a large proportion of the fish were free of infection. Helminth-free status accompanied by thickening of the swimbladder wall developed after intensive infections. In 1991, eel mortality could be observed only in the western basin. In 1992, the number of eels with swimbladders having a thickened wall but not containing helminths increased also in the central and eastern areas of the lake. Parallel to this, a mortality less expressed than the one in 1991 occurred in the central part of the lake. By 1993, a host-parasite equilibrium had become established in Lake Balaton.
The endoparasitic helminth communities of the European eel, Anguilla anguilla (L.), were investigated in four meanders, cut off from the rivers Leie and Scheldt in western Handers, Belgium. Six species of helminths (2 cestodes, 2 nematodes and 2 acanthocephalans) were found. The dominant parasite species was the nematode Anguillicola crassus (Kuwahara, Niimi et Itagaki, 1974) infecting 79% of the eel population with intensities up to 112 specimens per fish. At two localities no acanthocephalans could be found, whereas these parasites were very common at the other sites. The prevalence, mean intensity, intensity and abundance, their correlation to the body length, and the frequency distributions were analysed. The site selection of parasites is in relation to food composition and feeding habits of eels, physiological and structural differences in the intestine and possible interspecific competition were discussed.
The swimbladder parasite Anguillicola crassus Kuwahara, Niimi et Itagaki, 1974 (Nematoda: Dracunculoidea) is a well-known pathogenic parasite of the Japanese and European eels. Numerous studies on the life cycle of the parasite have revealed the involvement of a copepod or an ostracod intermediate host and a fish paratenic host, in which the third-stage larvae (Lj) infective to the eel develop. The present study comprised infection experiments with the larvae of A. crassus. These experiments can be divided into three groups: (1) experimental reproduction of the parasite's life cycle via copepod intermediate hosts and fish paratenic hosts, (2) infection of another potential paratenic host with third-stage larvae of A. crassus collected from a paratenic host; (3) study of the ability of larvae damaged by paratenic hosts to infect the final host, the eel. Infection experiments have revealed that larvae which are still viable but have become encapsulated as a result of the host reaction mounted against them by cyprinid paratenic hosts (bleak, Alhumus alhumus) have lost their ability to infect the final host, the eel. At the same time, experimental infection of the eel with larvae derived from other paratenic fish hosts (river goby, Neogobius fluviati-lis: ruffe, Gymnocephalus cemua) showing no or only weak host reaction proved to be successful.