Abstract

Fish are most commonly anesthetized with MS-222 (tricaine methanesulfonate), a sodium-channel-blocking local anesthetic used as an immersion anesthetic, but its mechanism of action as a general anesthetic is uncertain. Alfaxalone is a neurosteroid that acts at the GABAA receptors in both mammalian and non-mammalian species.^1,2,3 Alfaxalone has been evaluated as an immersion agent in koi carp (Cyprinus carpio) and successfully induced and maintained anesthesia via this route.⁴ Additionally, alfaxalone is an effective intramuscular anesthetic in both mammalian and non-mammalian species.^1,2,3,5 It would be useful to have a reliable anesthetic with a known mechanism of action that can be injected intramuscularly in fish that are not easily isolated, or are too large to induce via immersion. The purpose of this study was to investigate the use of alfaxalone as an intramuscular injectable anesthetic agent in koi carp.

Eight adult koi carp, weighing a median of 359 (228–450) g, were utilized in a crossover design with a 1 week washout period. In each trial, six fish received 1 mg/kg, 5 mg/kg, or 10 mg/kg of alfaxalone intramuscularly. Following injection, koi were assessed every 15 min for opercular rate and sedation score. The sedation score was based on a subjective visual scale from 0 to 5, 0 indicating no response and 5 indicating absent righting reflex and successful anesthesia. When anesthesia was achieved, koi were placed on a recirculating system with water directed over the gills. The time to anesthesia was recorded and heart rate via Doppler was assessed and recorded every 15 min until recovery. Time to recovery from anesthesia was recorded.

Anesthesia was achieved in 0/6, 1/6, and 5/6 fish at 1, 5, and 10 mg/kg respectively. Duration of anesthesia for one fish at 5 mg/kg was 2 h. At 10 mg/kg, median anesthesia duration was 6.5 (3–10) h. At 10 mg/kg, prolonged apnea (2–3 h) was observed in 3/6 fish, with 2/3 dying under anesthesia, and 1/3 recovering 10 h post-injection. The two fish that died lost Doppler signal at 8.3 and 8.5 h following injection. Median peak sedation scores were 1.5, 2.5, and 5, at 1, 5, and 10 mg/kg respectively.

At 1 and 5 mg/kg, intramuscular alfaxalone did not reliably induce anesthesia in koi carp. Anesthesia was achieved in a majority of fish at 10 mg/kg of intramuscular alfaxalone, but with 33% mortality. The duration of anesthesia and opercular rate were variable and unpredictable. Due to the unreliable response to the consistent dosage and injection technique, as well as risk of mortality, intramuscular alfaxalone cannot be recommended for anesthesia in koi carp.

*Presenting author

Literature Cited

1.  Bertelsen MF, and Sauer CD. (2011). "Alfaxalone anaesthesia in the green iguana (Iguana iguana)." Vet Anaesth Analg. 38(5): 461–466.

2.  Kischinovsky M, Duse A, et al. (2013). "Intramuscular administration of alfaxalone in red-eared sliders (Trachemys scripta elegans) - effects of dose and body temperature." Vet Anaesth Analg. 40(1):13–20.

3.  Marsh MK, McLeod SR, et al. (2009). "Induction of anaesthesia in wild rabbits using a new alfaxalone formulation." Vet Rec. 164(4):122–123.

4.  Minter LM, Bailey KM, Lewbart GA, et al. (2012) The Efficacy of Alfaxalone for Immersion Anesthesia in Koi Carp (Cyprinus carpio). IAAAM 43rd Conference Proceedings, Atlanta, GA.

5.  Richardson EY, Bailey KM, Posner LP. (2012). "Alfaxalone Anesthesia in Bullfrogs (Rana catesbeiana) by injection or immersion." Merck Merial Summer Scholar Abstract Presentation, Raleigh, NC