Abstract

Tricaine methanesulfonate (MS-222) is the only U.S. FDA approved anesthetic for use in food fish. It requires a 21 day withdrawal period in food fish prior to human consumption, and there is evidence that chronic exposure in fish, amphibians, and humans can cause reversible retinal deficits. An alternative that has been receiving attention recently is clove oil, the active ingredient of which is eugenol (4-allyl-2-methoxyphenol), an alkylphenol. The advantages of eugenol relative to MS-222 are that eugenol is considered an unregulated GRAS (generally regarded as safe) substance by the U.S. FDA, is commercially available, and is less expensive. The objectives of this study were to compare the anesthetic efficacy of eugenol and MS-222 in red pacu (Colossoma brachypomum).

Fifteen cultured red pacu (Colossoma brachypomum) of uniform age and similar weights were used in a within-subjects, complete crossover design. Each subject was exposed to each anesthetic concentration, serving as its own control. The six treatment groups were: MS-222 at 50 mg/L, 100 mg/L and 200 mg/L; and pharmaceutical grade eugenol (Sigma Co., 100% eugenol) at 50 mg/L, 100 mg/L, and 200 mg/L. Each subject had a minimum of 2 weeks between anesthetic exposures. All blood data was analyzed using an iSTAT blood analyzer (SDI/Heska Corp.). Data are presented as mean ±SD. Time to induction was more rapid with eugenol (MS-222 = 507.3 ±134.5 secs.; eugenol = 231.3 ±138.3 secs.), but recovery time was prolonged (MS-222 = 329.1 ± 171.4 secs.; eugenol = 568.7 ± 87.5 secs.). There was a consistent elevation of blood glucose concentrations with anesthesia, for both MS-222 (pre = 58.2 ±10.5 mg/dL; post = 74.8 ±15.5 mg/dL) and eugenol (pre =57.1 ±9.5 mg/dL; post = 71.3 ± 16.6 mg/dL). Mean percent change of mixed venous/arterial P_O2 dropped by approximately 80% with anesthesia, regardless of anesthetic agent or concentration. Mean percent change of mixed venous/arterial P_CO2, on the other hand, consistently increased by approximately 90% with anesthesia. As expected, with an elevation in P_CO2, pH declined with anesthesia (MS-222 pre = 7.73 ±0.06; MS-222 post = 7.46 ±0.11; eugenol pre = 7.76 ± 0.18; eugenol post = 7.62 ±1.15). Hematocrit and hemoglobin values consistently increased with anesthesia as did blood sodium and potassium. Fish anesthetized with eugenol were more likely to react to a needle stick than fish anesthetized with MS-222, regardless of concentration used.

Acknowledgements

This study was funded by a grant from the Department of Companion Animal and Special Species Medicine, North Carolina State University.