To most veterinarians and people working closely with animals, from both their
physical and emotional responses, there is no question that animals feel something
equal or very akin to the human experience of pain. From anatomical and physio-
logical perspectives, all animals have the necessary receptors, nerves,
neurotransmitters, and comparative central nervous system anatomy to be able to
experience what humans describe as pain.
Histological and neurochemical evidence indicates that the neurotransmitter
receptors associated with pain perception are very similar in most animals; for
instance opioid receptors of all subtypes are present in the central nervous system
with a very similar distribution to humans in all animal species. One major
challenge to the evaluation of pain in animals derives from the enormous diversity
of what is described under the broad heading of “pain” in humans. Apart from the
temporal descriptions of pain, such as acute, chronic and intermittent, there is the
anatomic location of the pain, or the type of pain, such as throbbing, burning or
stabbing or a combination of all of these. In addition, perhaps the greatest challenge
in the evaluation of pain in animals is the human concept of neuropathic pain
(Mersky and Bogduk 1994 ), where the absent or inappropriate lesions give no
indication of the real source of the pain perceived.
Other criteria have been used for characterising human pain, such as inflamma-
tory pain and non-inflammatory pain, or more specifically physiological pain,
where the pain can be beneficial to the subject (in terms of avoidance) and
pathological pain where the actual pain experience is detrimental to the subject
(such as arthritic pain) (Woolf and Chong 1993 ).
2 Animal Pain
2.1 Significance of Pain Evaluation in Animals
In the absence of verbal communication, those involved in the assessment of pain in
animals have to rely on other strategies to establish the nature and intensity of the
painful or nociceptive experience. One approach is to consider the strategies
involved in evaluating the broad groups by which human pain is characterised, as
this often interacts with the pain control strategy utilised, but the lack of verbal
communication means that the evaluation will be based on observational strategies.
These strategies fall into four main groups. Firstly, the observation or measurement
of physiological parameters, such as pulse, temperature, respiration and feeding and
drinking patterns may be useful. Secondly, the behaviour of the animal, as altered
from the norm, is a widely used and powerful tool, particularly in the hands of an
experienced person. Thirdly, the physical response to exacerbation of the pain
stimulus, by palpation or manipulation and the increased response to this, is very
informative. Finally, alteration of the presumed pain by application of analgesic
strategies, utilising the three previous evaluators, is widely used.
In view of the complexity of the interacting evaluations and the species varia-
bility, a number of visual analogue scales (VAS) have been developed, but these are
always species-specific and indeed often situation-specific as well (vide infra).
Pain and Analgesia in Domestic Animals 161