Clinical Anesthesia and Analgesia in Fish

Fish have become a popular experimental model and companion animal, and are also farmed and caught for food. Thus, surgical and invasive procedures in this animal group are common, and this review will focus on the anesthesia and analgesia of fish. A variety of anesthetic agents are commonly applied to fish via immersion. Correct dosing can result in effective anesthesia for acute procedures as well as loss of consciousness for surgical interventions. Dose and anesthetic agent vary between species of fish and are further confounded by a variety of physiological parameters (e.g., body weight, physiological stress) as well as environmental conditions (e.g., water temperature). Combination anesthesia, where 2 anesthetic agents are used, has been effective for fish but is not routinely used because of a lack of experimental validation. Analgesia is a relatively underexplored issue in regards to fish medicine. However, recent studies have investigated opioid agents, nonsteroidal anti-inflammatory drugs, and local anesthetics to determine their efficacy in minimizing pain and discomfort. The opioid morphine and the local anesthetic lidocaine do have significant effectiveness in reducing pain-related responses in rainbow trout (Oncorhynchus mykiss). Studies aimed at developing reliable analgesic protocols should explore a wide range of analgesic drug classes in several fish species. Recent research studies have suggested that fish are capable of nociception or pain perception. These investigations have demonstrated that bony (teleost) fish have nociceptors, receptors to detect potentially painful stimuli (Fig 1), which are very similar to those found in mammals. A variety of species also exhibit adverse behavioral and physiological responses to a potentially painful event that are ameliorated by the use of analgesia (Fig 2). Therefore, it would seem prudent to reduce any pain and discomfort during surgical interventions or when tissue damage has or is likely to occur. Anesthetic drugs have historically been used on fish in aquaculture, experimentation, and veterinary practice. Their use reduces the stressful impacts of handling, especially during routine procedures such as weighing, vaccination, blood sampling, tagging, experimental surgery, and veterinary procedures. Therefore, light anesthesia or sedation can improve the efficiency of rapid procedures. In invasive procedures in which tissue damage will occur, full or deep anesthesia or loss of consciousness is used to improve fish welfare and minimize the pain associated with surgical intervention. Anesthetic administration is usually achieved via immersion, with the drug being dissolved in water where the fish is held or an anesthetic vessel such as a tank or large bucket (also termed inhalation). When induction takes place through immersion, it is paramount to observe the fish and gauge the depth of anesthesia to reduce the incidence of overdose. Uptake of these chemicals is primarily via the gills and possibly the skin. The level of anesthesia is monitored by recording gill ventilation rates, maintenance of equilibrium (upright position), and reflex responses (e.g., swimming response to tail pinch). Research studies have measured rates of induction and recovery, responsiveness to external stimuli and handling, as well as pharmacokinetics of these agents. However, these research investigations often focus on a small range of species, testing only one anesthetic agent or a range of drugs on one species. Analgesia has received much less attention, especially before 2003; historically, fish were thought to be incapable of nociception or pain. However, studies are now trying to show the efficacy of analgesic medication in reducing or ameliorating any detrimental changes in behavior and physiology as a result of damage or noxious stimulation. This article will focus on what is known regarding anesthesia (primarily immersion) and analgesia in fish to provide an up-to-date account that can be referred to by scientists and veterinarians. Using the most efficient means of reducing stress and/or pain is crucial for the welfare of animals treated.