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.