Haematological effects of 2-phenoxyethanol and etomidate in carp (Cyprinus carpio L.)

Objective To evaluate the effects of handling alone versus handling under anaesthesia with 2-phenoxyethanol or etomidate on haematological parameters in carp. Study design Prospective, randomized, laboratory experiment. Animals Seventy-two juvenile carp (Cyprinus carpio) weighing 35.9 ± 10.4 g were divided into six groups of 12 fish. Methods Either 2-phenoxyethanol or 2% etomidate were administered to induce deep anaesthesia (0.3 mL L−1 and 0.6 mL L−1, respectively) or deep sedation (0.15 mL L−1 and 0.3 mL L−1, respectively). Fish were handled with and without sedation. Blood was sampled at 1 hour and 1 week post-treatment. Phagocyte oxidative activity [nitrotetrazolium blue reduction test (NBT)] and differential erythrocyte [red blood cell (RBC)] and leukocyte (white blood cell) counts were evaluated. Results At 1 hour after the induction of anaesthesia, haematocrit (Ht) and haemoglobin (Hb) were increased in fish anaesthetized with 2-phenoxyethanol, and mean corpuscular haemoglobin (MCH) was increased in fish anaesthetized with etomidate. At 1 week, an increase in RBC, erythroblastosis, erythrocyte damage, lymphopenia, neutrophilia, monocytosis and thrombocytosis occurred in both groups. Red blood parameters did not change 1 hour after handling alone, but after 1 week Ht, Hb and mean cell volume decreased, whereas MCH concentration (MCHC) and abnormal erythrocytes increased. Lymphopenia, neutrophilia, monocytosis, thrombocytosis and a decrease in NBT occurred. Fish handled under sedation showed an increase in Hb and MCHC followed by a decrease at 1 week in Ht, Hb and MCH, erythroblastosis and increased abnormal erythrocytes. Lymphopenia and neutrophilia were less pronounced than in fish handled without sedation, but a decrease in NBT was noted at 1 week post-treatment. Conclusions and clinical relevance Deep anaesthesia with 2-phenoxyethanol or etomidate induced significant haematological alterations in juvenile carp. Deep sedation reduced the immediate immunosuppressive effects of handling but did not eliminate longterm effects. These anaesthetics should be avoided during experimental procedures involving haematological measurements. In contexts that require the short-term handling of carp, these drugs should be used with caution in view of their possible side effects.