The objective of this presentation is to describe and highlight what is currently known about pain (nociception) and analgesia (antinociception) in fish. The primary question regarding fish analgesia is whether fish “experience” pain or are fish species merely capable of demonstrating a “reflexive” response to a noxious stimulus (nociception)? Of critical importance is the concept of whether we can recognize pain in fish, and is the perception of pain by a fish equivalent to that of a mammal? We will never be able to fully and objectively answer these questions, because fish simply cannot tell us. Many would argue that fish do not have the same anatomic and/or physiologic capabilities to “process” pain.13 In other words, fish are merely responding and passively reacting to stimuli to which they are exposed, with little or no ability for cognition or self-awareness.13 However, recent research has demonstrated that the transmission of peripheral sensory signals, via the spinal cord, to midbrain and forebrain regions that are homologous to mammalian cortical and limbic structures.2,5,7,15-17 Additionally, the endogenous opioid system, which is activated in response to nociception and contributes to analgesia, is also well conserved throughout vertebrate phylogeny.4,6,15 Thus, the physiologic and anatomic requirements for pain and analgesia appear to be remarkably similar among all vertebrate species, and therefore, there is substantive and compelling evidence from the neuroanatomic, neurophysiologic and behavioral literature to suggest that, at some level, a variety of fish species experience pain under certain contexts. In my clinical experience, both kappa- and mu-opioid agonists appear to be affective in providing pain relief, particularly post-surgically; however, mu-opioid agonists appear to provide fewer deleterious side effects.1,8-10,12
Many veterinary clinicians argue that the administration of analgesics is risky to the patient and may mask behavioral signs of pain, which are considered evolutionarily adaptive for survival. However, veterinarians have an ethical obligation to treat painful conditions in all animals, including fish, as effective pain management reduces stress-induced disruption to homeostatic mechanisms, and also decreases morbidity and mortality associated with trauma or surgery. Several obstacles limit successful analgesic use in fish, including subjectivity in pain assessment, inadequate knowledge of analgesic efficacy across species, pharmacokinetics of analgesic drugs, and the unknown relationship between risks and benefits for specific drugs. It is my hope that future research will help us to determine if fish feel pain. Until then, we must use all available evidence, especially in those species most closely related to the species being studied, to err on the side of animal in subjectively assessing that a procedure considered painful in a mammal, should also be considered potentially painful in a fish species.
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