tests use an immediate response (limb withdrawal) to an acute pain stimulus. Other
models utilise the pain that develops slowly at a site of acute inflammation, with the
peak of hyperalgesia occurring several hours after injection of a pro-inflammatory
irritant substance, such as kaolin or carrageenan.
In addition, many more animal model tests for other pain modalities have been
developed using behavioural responses in laboratory animals. These include
visceral pain induced by injecting irritant chemical agents into the abdomen and
measuring the writhing activity of mice (Siegmund et al. 1957 ), and the injection of
irritant agents into joints to evaluate the efficacy of agents targeted at arthritic
conditions (Coderre and Wall 1987 ). In addition, animal models have been deve-
loped to mimic neuropathic pain states (Bennett and Xie 1988 ). Finally, some
animal pain models are being used to study the neurological basis of pain states
in humans. This includes not only models using genetically modified laboratory
animals (Bo ̈lcskei et al. 2005 ), but also naturally occurring conditions in animals
which parallel human pain states, such as interstitial cystitis (Lavelle et al. 2000 )
and arthritis (Lascelles et al.2007a).
In terms of management, the control of acute pain in animals, when it is either
observed or anticipated, is generally based on the procedures developed for
humans, namely surgical techniques, immobilisation of traumatised regions and
pharmacological intervention. In many situations the prevention or reduction of
predictable pain in animals is addressed, as in humans, by the use of pre-emptive
analgesia. This involves the administration of analgesics before the painful proce-
dure is inflicted to prevent or suppress the development of central or peripheral
sensitisation (Lascelles et al. 1998 ). Initially, there was some difficulty with this
technique gaining acceptance in human medicine, as the original reports were based
on retrospective studies. However, demonstration of the benefits in veterinary
medicine (Slingsby and Waterman-Pearson 1998 ) showed clearly that the concept
was a genuine physiological response to the pain stimulus rather than a placebo
effect. The assumption that animals will not demonstrate a placebo effect has been
fundamental in their use to evaluate analgesic strategies.
The second type of pain that animals are assumed to experience is chronic pain.
Probably because of their particular affinity with chronic pain states in humans,
chronic pain in animals is best studied and understood in dogs, cats and horses.
Additionally, in the last few years there has been an increasing focus on chronic
pain in farm animal species. The causes of chronic pain in animals are similar to
those in humans. Of particular clinical importance are arthritic joint lesions.
However, an appreciation of other chronically painful conditions, like cancer and
spinal injury, has been receiving much more recent attention (Yazbek and Fantoni
2005 ), with particular focus on the dog and cat. Another major factor in relation to
severe chronic unremitting pain in the major veterinary species has been the option
of euthanasia, which in most societies has not been accepted as a pain alleviating
option in humans. That said, there remains a significant need for therapy of chronic
pain conditions in animals. This can be for companionship/sentimental reasons or
for commercial reasons, such as extension of the breeding life of a valuable animal,
as well as the primary consideration of animal welfare. The whole issue of the
166 A. Livingston