Pain is a clinical expression of tissue damage. Pain management comprises the set of control measures deployed to ease the suffering and to improve the quality of life. To control pain, there are available non­pharmacological, i.e., physical and psychological means, and pharmacological (chemical analgesics, sedatives and anaesthetics) approaches. Pain relief in fish, however, implies use of chemical analgesics and anasthetics and, in very limited instances, physical treatment, e.g., gradual cooling of water.

The presentation will provide an overview of analgesic and anaesthetic agents that have been tested in fish also highlighting the existing gaps in knowledge in this field. Firstly, comprehension of existing necessity and difficulties in applying pain management in fish might be required for a better understanding of the subject matter. Why veterinary practitioners should consider learning about, and applying pain management procedures also in fish?

Fish are companion animals, experimental models, and livestock. For many practical reasons, fish are now in the top most popular pets. According to petfoodindustry.com, in the U.K., fish demand has increased by 17% over the last three years, with 12 million pet fish living indoors and another 18 million currently living in garden ponds. Ornamental fish are also increasingly being used in experimental research for a variety of reasons, such as the high degree of genetic homology between fish and humans, and the need for alternatives to mammal use in experimental settings. A relevant example is the use of zebrafish (Danio rerio) as an in vivo model for the study of vertebrate development, neurological disease mechanisms and preclinical drug screening. Nonetheless, fish are livestock farmed for food and/or breeding purposes. Consequently, as surgery and other potential painful procedures are becoming more common in fish, the use of analgesics and anaesthetics has become a common requirement in aquatic veterinary practices and fishery farms, to help improve fish welfare and minimize the pain associated with the procedures.

Drugs designed to relieve pain and discomfort in fish are analgetics, whereas to induce insensitivity, unconsciousness, and sleep, anaesthetics are used. Anaesthetics are also used as the first step of euthanasia in fish. However, both analgesia and anaesthesia in fish are areas where relatively little in terms of clinical data is known or formally reported. The factors acknowledged as impeding pain management practice in fish as successful as in terrestrial vertebrates are the width of range of species, which makes generalization across species very problematic; analgesic agents tested on fish, as well as licensed anaesthetic products for fish are very limited; the ability to monitor depth of fish anaesthesia is restricted to a few specialist research groups globally.¹ Additionally, the use of pre-anaesthetic (sedation) protocols in fish, in combination with one or more adequate anaesthetics - which ensure avoidance of aversive reactions to anaesthetics in other species, seems not to be yet available in fish.

The choice of the correct methodology of administration of analgesic and anaesthetic agents should take into account three groups of variables: i.) species-related characteristics, such as fish species, strain, body weight, and physiological stress; ii.) environmental conditions, such as water temperature and pH; and iii.) agent related characteristics, such as chemical composition, pharmacological properties, exposure duration, and drug concentration. Generally, in small to medium sized fish, anaesthetics are administered dissolved into the water where fishes are held (equivalent to inhalation anaesthesia in mammals) or introduced, i.e., immersion of fish into a water tank containing anaesthetic.

Administration of anaesthetics via injection is restricted only to large fish species, of over 10 kg. In the case of anaesthesia, monitoring depth of the process by visual assessment, and interpretation of ventilation rates (opercular ventilation rates), posture (body movement and equilibrium), and reflex response to stimuli (e.g., tail or fin pinch) are standard practice in fish.

Fish Analgesia

There are four groups of analgesics in fish commonly referred to into the literature:^1-10 opioids, local anaesthetics, non-steroidal anti-inflammatories, and antidepressants - the latter two, lesser explored in fish.

Experimentally, the opioid morphine (5 mg/kg) has been shown to minimize pain and discomfort in rainbow trout, flounder, and goldfish.⁸ It is assumed that morphine actions pharmacokinetically in fish similar to its action in mammals, by inhibiting neurotransmitter release, thus blocking nociceptors and central transmission (presynaptic effect). Except that in fish, excretion rates are much slower (total elimination time = 56 hours), suggesting that its use would be suitable to chronic conditions or after major surgery.⁸ Other opioids, such as tramadol, dermophine, butorphanol, and buprenorphine were also tested on farmed and ornamental fish species, such as cod, trout, carp, dogfish, and koi carp, but it appeared to have limited effectiveness and poor analgesic properties.⁸

Local anaesthetics block nociceptive transmission. Lidocaine analgesic effects were tested in rainbow trout and zebrafish where, at 1 mg/kg, no side effects were observed, and it was assessed as a very efficient analgesy.⁸ Novocain is known to reduce reflex responses in cod,³ but since it has not been tested in vivo in fish, its use is not recommended.

Very few non-steroidal anti-inflammatories (NSAI) have been assessed with respect to their analgesic effects in fish, by taking into account NSAls properties observed in other species, which refer to the decrease of inflammatory reaction by decreasing production of thromboxanes and prostaglandins. Ketoprofen, tested in Koi carp and dogfish with doses ranging between 2 and 4 mg/kg, was proven as not effective, whereas carprofen at doses of 1–5 mg/kg, was found to cause deleterious effects in trout, i.e., increased ventilation and depressed activity.^8,10

Fish Anaesthesia

Physical Means

Gradual cooling to 10.3°C was proven as effective in adult zebrafish (D. rerio) to a degree where surgical plane of anaesthesia is achieved. However, it is advised that gradual cooling to be used as a last option and when surfaces and instrumentation can be maintained at 10°C.³

Chemical Means

There are currently over 15 anaesthetics in use in veterinary practices, experimental settings, and aquaculture. The most common anaesthetics are: tricaine methanesulfonate (MS222), benzocaine, lidocaine hydrochloride, clove oil, eugenol, isoeugenol, etomidate, and metomidate hydrochloride, 2-phenoxyethanol (2-PE), quinaldine and quinaldine sulfonate, 2,2,2 tribromethanol (TBE), propoxate, 4-Styrilpyrydine, propofol, and isoflurane. However, only very few of them are approved for use in fish destined for human consumption.

A summary of the anaesthetic agents applied to fish, showing a range of doses, efficiency, and side effects of the anaesthetics in different fish species will be discussed during the lecture. It is important to emphasize that these data will refer to certain species and conditions of use, such as water temperature and pH, body size, and physiological status.

As a general recommendation, in instances where there is no prior experience with the fish, fish strain and/or analgesic or anaesthetic substances, a pilot test using the lowest doses and a small number of fish of different strains to be carried out, to ensure safety and efficacy of the procedure.

References

1.  Readman GD, Owen SF, Murrell JC, Knowles TG. Do fish perceive anaesthetics as aversive? PLoS One. 2013;8(9):e73773.

2.  Nordgreen J, Tahamtani MF, Janczak MA, Horsberg TE. Behavioral effects of the commonly used fish anaesthetic tricaine methanesulfonate (MS-222) on zebrafish/Danio rerio) and its relevance for the acetic acid pain test. PLoS One. 2014;9(3):e92116.

3.  Collymore C, Tolwany A, Lieggi C, Rasmussen S. Efficacy and safety of 5 anesthetics in adult zebrafish (Danio rerio). J Am Assoc Lab Anim Sci. 2014;53(2):198–203.

4.  Collymore C, Banks EK, Turner PV. Lidocaine hydrochloride compared with MS222 for the euthanasia of zebrafish (Danio rerio). J Am Assoc Lab Anim Sci. 2016;55(6):816–820.

5.  Davis DJ, Klug J, Hankins M, Doerr MH, Monticelli RS, Song A, Gillespie HC, Bryda EC. Effects of clove oil as a euthanasia agent on blood collection efficiency and serum cortisol levels in Danio rerio. J Am Assoc Lab Anim Sci. 2015;54(5):564–567.

6.  Sladky KK, Swanson CR, Stoskopf MK, Loomis MR, Lewbart GA. Comparative efficacy of tricaine methanesulfonate and clove oil for use as anesthetics in red pacu (Piaractus brachypomus). Am J Vet Res. 2001;62:337–342.

7.  Gil WH, Ko GM, Lee HT, Park I, Kim SD. anesthetic effect and physiological response in olive rounder (Paralichthys olivaceus) to clove oil in a simulated transport experiment. Dev Reprod. 2016;20(3):255–266.

8.  Sneddon LU. Clinical anesthesia and analgesia in fish. Journal of Exotic Pet Medicine. 2012;21:32–43.

9.  Chervova LS. Pain sensitivity and behavior of fishes. J Ichthyol. 1997;37:98–102.

10.  Mettam JM, Oulton LJ, McCrohan CR, Sneddon LU. The efficacy of three types of analgesic drug in reducing pain in the rainbow trout, Oncorhynchus mykiss. Animal Science, Veterinary Medicine and Zoology. In: Animal Studies Repository. 2011.