Abstract

Tricaine methanesulfonate (MS-222), a benzocaine derivative, is the most commonly used anesthetic agent in fish, worldwide, and the only United States Food and Drug Administration (U.S. FDA) approved anesthetic for use in food fish.⁴ However, it requires a 21-day withdrawal period in fish prior to human consumption, and there is evidence that chronic exposure in fish, amphibians, and humans can cause reversible retinal deficits.² In an effort to find a safe, unregulated alternative, clove oil, the active ingredient of which is eugenol (4-allyl-2-methoxyphenol), has been demonstrated to have anesthetic properties in several fish species.^1,3,5,6 Advantages of eugenol relative to MS-222 are that it is considered an unregulated substance by the U.S. FDA, is commercially available, and is inexpensive. However, its safety and efficacy as an anesthetic have not been systematically evaluated. The objectives of this study were to compare the anesthetic efficacy and associated physiologic changes of MS-222 and eugenol across three concentrations of each agent in red pacu (P. brachypomus), and to describe analgesic properties of both agents.

Fifteen cultured red pacu (P. brachypomus) of uniform age and similar weight were used in a within-subjects, complete crossover design. Each subject was exposed to each of the following six anesthetic concentrations expressed as mg/L of H2O: MS-222 (Finquel®, Argent Chemical Labs, Redmond, WA, USA) at 50 mg/L (MS50), 100 mg/L (MS100) and 200 mg/L (MS200); and 100% eugenol (Sigma Chemical Co., St. Louis, MO, USA) at 50 mg/L (E50), 100 mg/L (E100), and 200 mg/L (E200). There was a washout period of 2–4 wk between each anesthetic exposure. Mixed venous/arterial blood samples were collected on each subject from the caudal artery and vein, prior to, and immediately after anesthetic induction for comparative purposes, and the following parameters were measured using an iSTAT clinical analyzer (Sensor Devices Inc., Waukesha, WI, USA): sodium, potassium, glucose, ionized calcium, hematocrit, hemoglobin, pH, P_CO2, T_CO2, P_O2, S_O2, H_CO3, and base excess. Rate of respiration, as assessed by opercular movement, was recorded prior to, and at each stage of anesthesia and recovery. Stages of anesthesia and recovery were monitored, and behavioral reactions to the insertion of a hypodermic needle during blood sampling were documented prior to, and during anesthesia, in order to subjectively assess analgesia. Data are presented as mean±SD.

Mean induction times were longer for fish being exposed to MS-222 (MS50=600±0; MS100=572.3±63.6 and MS200=361.3±127.1 sec) relative to eugenol (E50=309.3±135.9; E100=209.7±137.1 and E200=186.3±124.6 sec), and there was a linear trend indicating shorter induction times at higher concentrations of each anesthetic. Time to recovery followed the opposite pattern with more rapid recoveries following exposure to MS-222 (MS50=215.3±117.2; MS100=318.3±155.9 and MS200=452.3±145.9 sec) relative to eugenol (E50=537.9±126.9; E100=568.3±78.0 and E200=600±0 sec), and there was a trend toward prolonged recovery times as anesthetic concentration increased. Subjects exposed to eugenol at higher concentrations required post-anesthesia resuscitation (6 of 15 at E100; 11 of 15 at E200). More individuals reacted to post-anesthesia needle insertion under eugenol anesthesia compared to MS-222. Mean percent change of mixed venous/arterial P_O2 dropped by approximately 80%, while P_CO2 consistently increased by greater than 90% with anesthesia. Mean blood glucose, sodium, and potassium concentrations, as well as hematocrit consistently increased with anesthesia for both MS-222 and eugenol. Mean blood pH consistently decreased with anesthesia for both MS-222 and eugenol.

Anesthesia with either MS-222 or eugenol contributes to hypoxemia, hypercapnia, respiratory acidosis, and hyperglycemia in the red pacu. Relative to MS-222, eugenol is an effective anesthetic compound in red pacu, characterized by rapid inductions and prolonged recoveries, but the margin of safety appears to be narrow. However, care must be taken when using higher concentrations of eugenol for induction, as medullary collapse is possible and may occur rapidly. In addition, it is unwise to assume that eugenol has analgesic properties until further studies are conducted.

Acknowledgments

This project was supported by a grant from the Department of Companion Animal and Special Species Medicine Research Fund, College of Veterinary Medicine, North Carolina State University.

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