In vitro and in vivo evaluation of quinine in gilthead sea bream, Sparus aurata naturally infected with the ciliate Cryptocaryon irritans

•We evaluated in vitro and in vivo the use of quinine in diseased gilthead sea bream.•Quinine has no cytotoxic effect on gilthead sea bream cellular types.•Dietary quinine can reduce the number of Cryptocaryon irritans.•Quinine is adequately absorbed in circulation of gilthead sea bream.

The use of quinine as a potential antiparasitic compound was evaluated at cellular level and in naturally infected gilthead sea bream, Sparus aurata with the ciliate Cryptocaryon irritans. Quinine concentrations up to 10 μg/ml (30 μΜ) did not exhibit any in vitro hemolytic or pro-apoptotic effects on red blood cells or head kidney macrophages, respectively. Dietary administration of quinine in gilthead sea bream at a concentration of 100 mg/kg fish for 10 days was incomplete due to palatability problems after the fifth day post treatment. A decreasing distribution profile of quinine was evident in the analyzed tissues even during the period where quinine consumption was supposedly ensured. The highest concentrations were measured on day 1 reaching 8.98 ± 3.85 μg/ml (27 μΜ), 1.33 ± 0.45 μg/g and 0.75 ± 0.48 μg/g in plasma, skin and gills respectively. Quinine was rapidly removed from the tissue compartment while it remained at low concentration in fish circulation after therapy. A 100% C. irritans prevalence was accompanied with no mortalities during the experiment. Parasitic intensity, measured as trophont number, was significantly reduced in quinine-treated fish at the first sampling point (3rd day: 8 ± 0.8 vs 11 ± 2.6), while no statistical differences were observed thereafter. A significant reduction in plasma glucose level was also observed at the same time point. Organoleptic evaluation in fillets, showed no difference in the flesh bitterness between quinine-treated and control fish.In conclusion, quinine has no cytotoxic effect on the assessed fish cells and is adequately absorbed in circulation, affecting the glycemic level but its distribution to the targeted tissues by the ciliate tissues is relatively small. Quinine significantly reduced C. irritans' intensity in lightly-infected gilthead sea bream. The sensory attributes of quinine-treated fish are not affected by the treatment.