Pain is a sensory and emotional experience often caused by intense or damaging stimuli. The International Association for the Study of Pain says pain is a conscious experience involving unpleasantness, i.e. suffering. For a creature to experience pain, by that definition, it must be capable of consciousness and suffering.
The standard measure of pain in a human is that person's testimony, because only they can know the pain's quality and intensity, and the degree of suffering. Animals without human language cannot report their feelings, and whether they are conscious and capable of suffering has been a matter of some debate.
All vertebrates and some invertebrates are capable of nociception, a neural response to intense or damaging stimuli. Nociception can be observed using modern imaging techniques, and a physiological and behavioral response to nociception can be detected but, presently, there is no objective measure of suffering.
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The idea that animals might not experience pain or suffering as humans do traces back at least to the 17th-century French philosopher, René Descartes, who argued that animals lack consciousness.[1][2][3]Researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain.[4] In his interactions with scientists and other veterinarians, Bernard Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain.[4] Some authors say that the view that animals feel pain differently is now a minority view.[1] Academic reviews of the topic are more equivocal, noting that, although it is likely that some animals have at least simple conscious thoughts and feelings,[5] some authors continue to question how reliably animal mental states can be determined.[2][6]
The ability to experience pain in an animal, or another human for that matter, cannot be determined directly but it may be inferred through physiological and behavioral reactions.[7]
Some criteria that may indicate the potential to feel pain include:[8]
Animal protection advocates have raised concerns about the possible suffering of fish caused by angling. In light of recent research, some countries, like Germany, have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish.[9]
Though it has been argued that most invertebrates do not feel pain,[10][11][12] there is some evidence that invertebrates, especially the decapod crustaceans (e.g. crabs and lobsters) and cephalopods (e.g. octopuses), exhibit behavioural and physiological reactions indicating they may have the capacity for this experience.[13][14][15] Nociceptors have been found in nematodes, annelids and molluscs.[16] Most insects do not possess nociceptors,[17][18][19] one known exception being the fruit fly.[20] In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opiate receptors. Opioid peptides and opiate receptors occur naturally in nematodes,[21][22] molluscs,[23][24] insects[25][26] and crustaceans.[27][28] The presence of opioids in crustaceans has been interpreted as an indication that lobsters may be able to experience pain,[29][30] although it has been claimed "at present no certain conclusion can be drawn".[29]
One suggested reason for rejecting a pain experience in invertebrates is that invertebrate brains are too small. However, brain size does not necessarily equate to complexity of function.[31] Moreover, weight for body-weight, the cephalopod brain is in the same size bracket as the vertebrate brain, smaller than that of birds and mammals, but as big or bigger than most fish brains.[32][33]
The question of whether or not crustaceans can experience pain is unresolved. One paper holds that lobsters' opioids may "mediate pain in the same way" as in vertebrates.[30]
Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.[34]
Dolorimetry (dolor: Latin: pain, grief) is the measurement of the pain response in animals, including humans. It is practiced occasionally in medicine, as a diagnostic tool, and is regularly used in research into the basic science of pain, and in testing the efficacy of analgesics. Non-human animal pain measurement techniques include the paw pressure test, tail flick test and hot plate test.
Animals are kept in laboratories for a wide range of reasons, some of which may involve pain, suffering or distress, whilst others (e.g. many of those involved in breeding) will not. The extent to which animal testing causes pain and suffering in laboratory animals is the subject of much debate.[35] Marian Stamp Dawkins defines "suffering" in laboratory animals as the experience of one of "a wide range of extremely unpleasant subjective (mental) states."[36] The United States Department of Agriculture defines a "painful procedure" in an animal study as one that would "reasonably be expected to cause more than slight or momentary pain or distress in a human being to which that procedure was applied."[37] Some critics argue that, paradoxically, researchers raised in the era of increased awareness of animal welfare may be inclined to deny that animals are in pain simply because they do not want to see themselves as people who inflict it.[38] Animal research with the potential to cause pain is regulated by the Animal Welfare Act of 1966 in the US, and research likely to cause "pain, suffering, distress or lasting harm" is regulated by the Animals (Scientific Procedures) Act 1986 in the UK.
To date (2011), eleven countries have national severity classification systems relating to pain and suffering experienced by animals used in research: Australia, Canada, Finland, Germany, The Republic of Ireland, The Netherlands, New Zealand, Poland, Sweden, Switzerland, and the UK. The US also has a mandated national scientific animal-use classification system, but it is markedly different from other countries in that it reports on whether pain-relieving drugs were required and/or used.[39] The first severity scales were implemented in 1986 by Finland and the UK. The number of severity categories ranges between 3 (Sweden and Finland) and 9 (Australia). In the UK, research projects are classified as "mild", "moderate", and "substantial" in terms of the suffering the researchers conducting the study say they may cause; a fourth category of "unclassified" means the animal was anesthetized and killed without recovering consciousness. It should be remembered that in the UK system, many research projects (e.g. transgenic breeding, feeding distasteful food) will require a license under the Animals (Scientific Procedures) Act 1986, but may cause little or no pain or suffering. In December 2001, 39 percent (1,296) of project licenses in force were classified as "mild", 55 percent (1,811) as "moderate", two percent (63) as "substantial", and 4 percent (139) as "unclassified".[40] In 2009, of the project licenses issued, 35 percent (187) were classified as "mild", 61 percent (330) as "moderate", 2 percent (13) as "severe" and 2 percent (11) as unclassified.[41]
In the US, the Guide for the Care and Use of Laboratory Animals defines the parameters for animal testing regulations. It states, "The ability to experience and respond to pain is widespread in the animal kingdom...Pain is a stressor and, if not relieved, can lead to unacceptable levels of stress and distress in animals."[42] The Guide states that the ability to recognize the symptoms of pain in different species is essential for the people caring for and using animals. Accordingly, all issues of animal pain and distress, and their potential treatment with analgesia and anesthesia, are required regulatory issues for animal protocol approval.
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