Tricaine methanesulfonate is an immersion anesthetic approved in the United States for use in fish; it is commonly used to anesthetize fish for procedures such as manually assisted spawning, weighing, measuring, surgery, transportation, and research.1 The anesthetic effects of this drug are thought to result from sodium channel blockade in neural tissue.2 Tricaine methanesulfonate is a preferred anesthetic for use in fish because it is soluble in water and can be administered by immersion and because concentrations are easily adjustable. Additionally, it produces rapid anesthetic induction and recovery (< 5 minutes each).1,3 Tricaine methanesulfonate is absorbed and primarily cleared across the gill epithelium, although there is also biotransformation in the liver and potential renal excretion.4
Although commonly used, MS-222 is reported to have retinotoxic effects in fish, frogs, and humans.2,5–8 A commonly cited clinical report7 describes a 62-year-old ichthyologist who developed vision loss with decreased ERG wave amplitudes after chronic low-level exposure to MS-222 over approximately 30 years. The authors of that report7 based the diagnosis of retinopathy associated with chronic exposure to MS-222 partially on information found in a series of studies2,5,8 that indicated the in vitro exposure of eye cups (ie, all tissues remaining after removal of the cornea, lens, and vitreous from the enucleated eye) to high concentrations of MS-222 resulted in altered ERG waves in retinas of eyes isolated from frogs. Interestingly, 7 months after cessation of exposure to MS-222, the vision of the patient described in that report7 had clinical improvement and his ERG waves had doubled in amplitude.
Electroretinography is considered the gold standard to assess retinal function, and b-wave amplitudes provide evidence of the function of inner retinal components.9 Results of ERG are useful in differentiating retinal from postretinal blindness because a positive ERG wave is indicative only of retinal function, providing no information regarding the remainder of the visual pathway (eg, optic nerve or visual cortex).
Considering that fish are routinely anesthetized with MS-222, it is logical to question whether repeated or prolonged exposure to the drug results in vision loss or retinal damage in these species. In our experience, fish anesthetized repeatedly with MS-222 do not have apparent behavioral changes associated with vision loss, such as difficulty locating food or objects in their environment or avoiding capture. Additionally, to the authors' knowledge, there have not been reports of vision loss or retinotoxic effects attributed to MS-222 exposure in fish.
The purpose of the study reported here was to determine whether repeated exposure to clinically relevant concentrations of MS-222 would alter retinal function or induce histologically detectable retinal lesions in koi carp (Cyprinus carpio). We hypothesized that koi carp anesthetized with MS-222 for a total exposure time of 20 minutes daily on 1 to 13 consecutive days would not have changes in retinal function (as assessed via ERG b-wave amplitudes) or histopathologic retinal changes indicative of toxic effects.
API Stress Coat Aquarium Water Conditioner, Mars Fishcare Inc, Chalfont, Pa.
Blackwater Creek Pelletted Koi Food, Blackwater Creek Koi Farms Inc, Eustis, Fla.
MS-222, Tricaine S, Western Chemical Inc, Ferndale, Wash.
Versa-Clean, Fisher Scientific, Waltham, Mass.
Retiport system, model 32, Acri Tec Inc, Salt Lake City, Utah.
ERG-LED program, Roland Consult, Brandenburg, Germany.
Kooijman active electrode, Roland Consult, Brandenburg, Germany.
Ground electrodes, 27-gauge, stainless steel, subdermal needle electrodes, CareFusion, Middleton, Wis.
Stata, version 10, StataCorp LP, College Station, Tex.
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