Effects of experimental infections with different doses of Anguillicola crassus (Nematoda, Dracunculoidea) on European eel (Anguilla anguilla)

To study the effects of various doses of Anguillicola crassus in primary infections, parasite-free European eels were orally infected with doses of 0, 1, 5, 10, 20 or 40 third-stage (L3) larvae. The eels were either killed and examined for parasites and lesions after 56 days, or reinfected with 20 L3 larvae to study the effect of primary infection on resistance. Reinfected eels were killed and examined on Day 112. Blood samples were collected weekly and examined in an enzyme-linked immunosorbent assay (ELISA) to detect antibodies against adult parasite cuticula antigen. The mean percentage of A. crassus recovered from eel swimbladders ranged from 14 to 20% on Day 56 and from 9 to 26% on Day 112. There was no significant relation between the proportion of recovered parasites and the primary and/or secondary infection dose. Furthermore, the higher the dose, the more severe were the haemorrhages and pigment spots seen in the swimbladder. Reinfected eels had significantly more severely thickened swimbladders, haemorrhages in the swimbladders and congestion of blood vessels in the swimbladders. Results of the ELISA showed that the eels developed no detectable antibody response against A. crassus. We concluded that under the given stressful experimental conditions, although eels develop pathological signs after primary and secondary infection with A. crassus, they do not develop an antibody response or resistance.