Influence of flumequine and oxytetracycline on the resistance of the European eel against the parasitic swimbladder nematode Anguillicola crassus

The effect of the antibacterial drugs flumequine (FQ) and oxytetracycline (OTC) on the defence system of the European eel (Anguilla anguilla L., 1758) was investigated using an experimentally induced infection with the parasitic swimbladder nematode Anguillicola crassus. Eight weeks after oral administration of infective larvae, the mean recovery of the parasites in FQ-treated eels was lower than in non-medicated control animals, and significantly lower than in OTC-treated eels. Mean numbers of peripheral blood granulocytes and B-lymphocytes, as well as the total number of circulating lymphoid cells, showed a significant increase as a result of the infection, while drug treatment merely affected the quantity of the lymphoid cells. The difference in protection against the parasite after FQ or OTC administration points to a modulation of the fish resistance as a result of the drug treatment. The results favour a modulation of the cellular rather than the humoral response, as no specific antibodies were found.