Pain mechanisms and their implication for the management of pain in farm and companion animals.

Over the last two decades there has been a dramatic increase in the literature relating to the mechanisms and management of pain in domestic animals. Understanding the mechanisms of pain is crucial for its effective management. This review highlights the current understanding of the neurophysiology of nociception and the plastic changes involved in chronic pain states. Additionally, we describe a range of novel molecules and pathways that offer opportunities for the development of mechanism-based analgesic therapies. Pain management in animals is limited by pain assessment which remains highly subjective, with clinicians relying on indirect measures of pain, using rating scales and (less frequently) quantifiable physiological and behavioural parameters. The need for a systematic approach which would assess different pain components is well justified. Species-specific issues on pain assessment and management in mammalian companion and farm animals are addressed in the later part of this review.

The effect of abdominal surgery on thresholds to thermal and mechanical stimulation in sheep.

Thresholds to noxious mechanical and thermal stimulation were measured in 6 groups of sheep prior to induction of anaesthesia and subsequently for a period of 2 h in the post-anaesthetic period. Groups 1-4 were anaesthetised using thiopentone and underwent ventral midline laparotomy. Four animals (group 5) underwent anaesthesia but not surgery, and a further 6 sheep (group 6) undergoing surgery were anaesthetised using ketamine. Groups 1-3 were intravenously administered the following drugs intra-operatively: flunixin meglumine, carprofen and buprenorphine, respectively. Groups 4-6 received no additional treatment. Thresholds to the mechanical test were not changed in the post-anaesthetic period for any group. There was a significant reduction in the responses to thermal stimulation after surgery for sheep in group 4 (45 and 60 min), while sheep in group 2 had thresholds to thermal stimulation greater than those recorded in the remaining groups at all time points post-operatively. Responses to thermal stimulation in sheep undergoing anaesthesia but not surgery (group 5) were unaltered during the 2 h recording period after anaesthesia ended. These data indicate that abdominal surgery induces thermal but not mechanical hyperalgesia in sheep, which appears to be centrally mediated. Moreover, the absence of mechanical hyperalgesia raises the possibility that central changes in noxious information processing may not be detected using mechanical stimuli in the same time course as thermal stimuli.

Repeated intradermal injection of low-dose carrageenan induces tachyphylaxis to evoked hyperalgesia.

Changes in the thresholds to noxious mechanical and thermal stimulation were monitored in a group of normal sheep at different times (30 min-24 h) following intradermal injection of either low-dose (0.0625%) carrageenan, a widely used mucopolysaccharide irritant, or saline (0.9%) into the lateral aspect of the fore limb. Thresholds to noxious mechanical stimulation were measured on the ipsilateral limb whereas thresholds to noxious thermal stimulation were measured at a site distant to that of the injection, the ipsilateral ear. During the first 2 h after carrageenan injection the thresholds to noxious thermal stimulation fell, while thresholds to noxious mechanical stimulation were unaltered. The evoked hyperalgesia was reversed by the administration of non-steroidal anti-inflammatory agents. Hyperalgesia was not present 4 h after carrageenan injection in control or treated animals. This experiment was carried out on 3 further occasions and a diminishing response to injection of carrageenan was observed, suggesting development of tachyphylaxis to evoked thermal hyperalgesia.

Patent Ductus Arteriosus in the older Dog.

OBJECTIVES: The objectives of this study were to describe the clinical course of Patent Ductus Arteriosus (PDA) in older canine patients (defined as patients greater than 24 months at the time of presentation) with and without intervention. This study was also undertaken to evaluate if adult dogs with persistent flow across the ductus should still be closed, what the benefits are from late closure, and to hypothesise why some animals still continue to deteriorate despite closure. BACKGROUND: Currently accepted therapies for patent ductus arteriosus are surgical ligation and transcatheter coil embolisation. The majority of dogs are diagnosed and treated at young age. Some older dogs survive with few clinical signs and live a normal life span without intervention. Some dogs deteriorate despite intervention. METHODS: The case records of 24 dogs that had reached 24 months of age before diagnosis were reviewed and those animals that had no concurrent congenitaal cardiac diseases were included in the study (n = 21). Those animals that were still alive were requested to participate in a long-term follow-up study. RESULTS: After closure of the ductus (n = 16), the clinical signs disappeared in all but one animal. On follow-up of these animals, there was echocardiographic evidence of left ventricular systolic and diastolic dysfunction in many. Late cardiac death was recorded in 3 animals. In dogs where there was no intervention congestive heart failure was a common, but not an inevitable sequel. Development of mitral valve endocardiosis was a common feature. CONCLUSIONS: Older animals with PDA follow an individual course, independent of pre-existing heart failure. Irreversible left ventricular dysfunction is common, however it does not seem to affect the clinical course. These data show a favourable outcome in a high percentage of adult dogs diagnosed with PDA that undergo closure. Outcome without intervention was less favorable.

Preliminary evaluation of a behaviour-based system for assessment of post-operative pain in horses following arthroscopic surgery.

OBJECTIVE: To develop a method for objective assessment of equine post-operative pain. STUDY DESIGN: Prospective nonblinded clinical study. ANIMALS: Twelve adult horses: Group 1 (G1, n = 6), admitted for arthroscopy (under general anaesthesia, with multimodal analgesia); Group 2 (G2, n = 6), 'pain free' controls. MATERIALS AND METHODS: Horses were filmed continuously (CVI, time-lapse video recorder) over 72 hours, from 24 hours pre-surgery (PS) to 48 hours post-recovery (PR) (G1), and over 24 hours (G2). Activity budgets were determined from 24 to 0 hours PS, 0-24 and 24-48 hours PR (G1) and for 24 hours (G2). Using direct observation (DO), active behaviours and postures were recorded at set time points PS and PR (G1) and at two time points (morning/evening) (G2). Heart rate (HR) and respiration rate (RR) were recorded simultaneously. Statistical analysis investigated within-group and between-group time-related changes in behaviour, HR and RR. RESULTS: There was no difference in HR or RR between G1 and G2 at any time point. Anaesthetic 'hangover' and hunger-related activity modulated behaviour from 0 to 6 hours PR, when abnormal postures and locomotion occurred in all G1 horses, but no G2 horses. Compared with G1 (0-24 hours PR), G2 spent more time eating (t = -3.34, p < 0.01), more time at the front of the stable (t = -2.42, p < 0.05), and less time exhibiting 'abnormal' behaviour (U = 56, p < 0.01). Comparing PS and PR behaviour, G1 spent less time exploring from 24 to 48 hours PR (t = 3.49, p < 0.05), less time in sternal recumbency (t = -3.8, p < 0.05) and less time moving (t = 3.19, p < 0.05). Horses tended (p < 0.07) to spend less time positioned in the front of the stable PR (less from 24 to 48 hours PR than from 0 to 24 hours PR). Comparing PR (evening) behaviour, G2 spent more time with head above withers (U = 21.5, p < 0.01), and ears forwards (U = 22, p < 0.01). G1 showed time-related changes (all p < 0.05) in time with lower lip tense (S = 15.8), eating (S = 17.08) and with head positioned above withers (S = 18.04). No differences in event behaviours were observed between G1 and G2. Within G1, only olfactory behaviour varied significantly with time (S = 14.52, p < 0.05). CONCLUSIONS: Changes in equine behaviour suggestive of post-operative discomfort were identified using both DO and CVI. Analysis of activity budgets may be a more sensitive method of identifying behavioural changes indicative of equine discomfort than repeated DO of specific events and postures.