Investigations on the effectiveness of Levamisol as a medication against the eel parasite Anguillicola crassus (Nematoda)

Two different dosages, 2 and 5 mg l-', of Levamisol (anthelmintic), applied in a water bath, were tested over a long term against various developmental stages of the swim bladder parasite Anguillicola crassus in the European eel Anguilla anguilla. Almost all preadult and adult nematodes, which live in the lumen of the swim bladder, show signs of paralysis 6 h after therapy. During the following weeks, many are killed off. However, some nematodes are able to regenerate after suffering sublethal damage. The mortality rates of preadult and adult parasites were similar at both dosages. The final juvenile stages of the parasite, which live in the swim bladder wall, are less sensitive to the medication. The highest mortality rate observed among this group was 48 %. It was mainly the larger juveniles, from 3 to 5 mm, that suffered lethal damage, especially at the higher dosage. About 3 wk after treatment, the infestation rate with intact nematodes in the swim bladder lumen begins to increase again, due to the recovery of preadult and adult nematodes and the migration of surviv~ng juveniles into the lumen of the swim bladder. The eggs and newly hatched larvae (L2) of A. crdssus, which are released in the lumen of the swim bladder, show no reaction to the medicine.