The efficacy of alfaxalone for immersion anesthesia in koi carp (Cyprinus carpio)

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To characterize the physiologic and behavioral effects of a single induction dose and two maintenance doses of alfaxalone delivered by water immersion in the anesthesia of koi (Cyprinus carpio).

Study design

Prospective, within-subject complete crossover design.


Six adult koi (Cyprinus carpio) with a median body weight of 344.5 g (range 292.0–405.0 g).


Koi were immersed in water containing 10 mg L−1 alfaxalone until immobile and then maintained with alfaxalone at either 1 or 2.5 mg L−1 via a recirculating water system. Times for anesthetic induction and recovery periods were recorded. Physiologic and blood gas parameters were evaluated before, during and after the anesthetic trial. Response to noxious stimuli was also assessed.


Median anesthesia induction time for all fish was 5.4 minutes. Median recovery time was 11.8 and 26.4 minutes in the 1.0 and 2.5 mg L−1 doses, respectively, which were significantly different (p = 0.04). Cessation of opercular movement occurred in 0/6 and 4/6 fish exposed to 1.0 and 2.5 mg L−1 dose respectively. No difference was observed in median heart rate over the duration of the anesthetic events. Response to noxious stimulation was 4/6 and 0/6 in the 1.0 and 2.5 mg L−1 doses respectively. Oxygenation and ventilation did not change during the experiment, but there was a significant decrease in blood pH along with an increase in blood lactate concentration.

Conclusion and clinical relevance

Administration of alfaxalone, via water immersion, as an induction and maintenance anesthesia agent provided rapid and reliable anesthesia of koi with no mortality. The maintenance dose of 2.5 mg L−1 was sufficient to prevent response to noxious stimuli but was associated with a clinically relevant depression in opercular rate.

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