Research PaperChallenges of thermal nociceptive threshold testing in the donkey
Introduction
The donkey has a reputation of stoicism with more subtle behavioural pain expression than horses. Current pain assessment techniques in animals depend heavily on behavioural cues, making assessment of pain difficult in donkeys. Nociceptive threshold testing (NTT) is a technique used to quantify nociceptive thresholds and analgesic efficacy, and has been used in various species of animals including horses. In equidae, three modalities of noxious stimuli have been applied; mechanical (Love et al. 2012) thermal (Robertson et al. 2005; Love et al. 2012) and electrical (Spadavecchia et al. 2003). Delivery of more than one type of noxious stimulus is recommended in order to fully evaluate the functional status of the nociceptive pathways (Nielsen et al. 2009).
In laboratory animals, thermal NTT is widely employed in classical experimental tests such as the tail flick test, tail withdrawal, paw withdrawal or hot plate test (Le Bars et al. 2001). Either the time taken (latency) for an animal to respond to an applied constant temperature, or the temperature at which an animal responds (if there is a ramped change in temperature) is measured. Thermal nociceptive threshold (TNT) testing has not been evaluated in the donkey previously, although the modality has been used in horses to evaluate the effectiveness of alpha-2 adrenoceptor agonists (Wegner et al. 2010), local anaesthetics (Robertson et al. 2005) and opioids (Love et al. 2012).
An ideal NTT stimulus should fulfil certain criteria. The stimulus should be easy to apply and repeatable, the behavioural response should be clear and easily identifiable and the stimulus should produce no lasting harm to the animal (Beecher 1957).
In this study we aimed to evaluate a thermal probe-based TNT device that had been developed for horses (Wegner et al. 2010; Love et al. 2012), in the donkey. The initial probe location to be evaluated was the withers (Study 1), and subsequently the limb (Studies 2 and 3). We investigated whether experimental factors such as whether right or left side of the body (laterality) (Studies 1 and 2), limb tested (Study 2), presence or absence of a companion (Study 2), level of distraction (Study 2), rate of heating (Study 2) and ambient temperature range experienced (Study 3) influenced the TNTs. Data collected in two groups of donkeys separated by 12 months were also compared. Certain experimental conditions (i.e. a companion present, limb tested, rate of thermal probe heating) were standardized as per Study 2, although location and level of prior training of the donkeys were not identical.
Section snippets
Ethical approval
This study received ethical approval from the University of Bristol (UB/10/019) and Ross University Institutional Animal Care and Use Committee.
Animals
Two groups of eight adult castrated male donkeys were studied. Group 1 donkeys were aged between four and eight years, and weighed between 105 and 160 kg. This group was studied in November and December 2010. Group 2 donkeys were aged between four and nine years and weighed between 152.5 and 170.5 kg. Group 2 donkeys were studied in November 2011.
Study 1: Evaluation of different cut-out temperatures and the influence of laterality and whether a companion was present or absent on TNTs measured over the withers
In three donkeys the thermal probe was heated to 53 °C, and in two of these donkeys, small raised scabs, similar in size and shape to the thermal probe, developed over the sites of application within 18 hours of testing. The lesions persisted for 14 days. Consequently, all subsequent TNT tests used a cut-out temperature of 51 °C and no skin lesions were observed.
Mean ± SD ambient temperature was 30.7 ± 2.1 °C, and mean baseline skin temperature was 34.9 ± 1.5 °C. Neither presence or absence of
Discussion
This study is the first to investigate TNT testing in the donkey and evaluation of the device followed methods used by researchers testing thermal probe based systems on horses (Robertson et al. 2005; Love et al. 2012). From Study 1 it was clear that a thermal stimulus applied to the skin over the withers of the donkey does not meet the ideal characteristics of a nociceptive stimulus for analgesiometry (Beecher 1957): a cut-out temperature of 53 °C led to the development of skin lesions
Conclusion
The difficulty in interpreting end-point behaviours when heating skin over the withers and the development of skin lesions when the skin was heated to 53 °C suggests that the withers is not an appropriate site for TNT testing in the donkey. Decreasing the cut-out temperature to 51 °C and heating skin over the dorsal aspect of the limb produced a more useful model, with a clear end-point behaviour of a foot-lift, and no skin lesions produced. However, when the limb site was tested in a second
Acknowledgements and conflicting interests
The authors would like to thank Drs Polly Taylor and Mike Dixon of Top Cat Metrology Ltd and Dr Tammi Krecek and the research assistants at RUSVM for their support.
This project was generously funded by Top Cat Metrology Ltd, RUSVM Research Committee, Langford Trust, Association of Veterinary Anaesthetists Educational Trust, Vetronics, Pfizer, Boehringer Ingelheim and Vetoquinol.
References (27)
- et al.
Effect of detomidine on visceral and somatic nociception and duodenal motility in conscious adult horses
Vet Anaesth Analg
(2009) - et al.
Thermal and mechanical nociceptive threshold testing in horses: a review
Vet Anaesth Analg
(2011) - et al.
Thermal and mechanical nociceptive threshold testing in pregnant sheep
Vet Anaeth Analg
(2014) - et al.
Individual differences in pain sensitivity: measurement, causation, and consequences
J Pain
(2009) - et al.
Antinociception in the rat induced by a cold environment
Brain Res
(1990) - et al.
Gender and laterality differences in thermosensation throughout the perceptible range
Pain
(2003) - et al.
Effect of flunixin meglumine on the thresholds to mechanical stimulation in healthy and lame sheep
Res Vet Sci
(1995) - et al.
Nociceptive responses to high and low rates of noxious cutaneous heating are mediated by different nociceptors in the rat: electrophysiological evidence
Pain
(1996) - et al.
Low but not high rate noxious radiant skin heating evokes a capsaicin-sensitive increase in spinal cord dorsal horn release of substance P
Brain Res
(1997) The measurement of pain; prototype for the quantitative study of subjective responses
Pharmacol Rev
(1957)
Refinement of thermal threshold probe to prevent burns
Vet Anaesth Analg
Chapter 23 The forelimb of the horse
Chapter 24 The hindlimb of the horse
Cited by (11)
Donkey behaviour and cognition: A literature review
2021, Applied Animal Behaviour ScienceHeat hypersensitivity at a site proximal to a surgically induced osteochondral fracture in horses
2021, Veterinary Anaesthesia and AnalgesiaCitation Excerpt :Thus, the statistically significant difference found in ambient temperature is not expected to influence thermal testing results (Gozalo-Marcilla et al. 2020). Skin temperature was similar to that previously obtained from the coronary band and the metacarpus (Poller et al. 2013a; Grint et al. 2015). Most mean TNTs were between 47 °C and 52 °C and were similar to baseline TNTs from thoracic limbs of equids (Poller et al. 2013a; Grint et al. 2015; Luna et al. 2015) but were lower than those reported by Lebelt et al. (1998; 60.5 °C).
Comparison of sedation and mechanical antinociception induced by intravenous administration of acepromazine and four dose rates of dexmedetomidine in donkeys
2017, Veterinary Anaesthesia and AnalgesiaCitation Excerpt :However, only one study described antinociception in donkeys after administration of dexmedetomidine (3.5 μg kg−1) intravenously (IV) (Lizarraga & Janovyak 2013). Furthermore, there is evidence that the disposition of drugs in donkeys differs from that in horses (Lizarraga et al. 2004; Grosenbaugh et al. 2011) and, possibly, also the neural processing underlying nociception (Grint et al. 2015a, b). The aim of this study was to investigate the sedative and mechanical antinociceptive effects of four IV dosages of dexmedetomidine in donkeys.
Analysis of Behaviors Observed During Mechanical Nociceptive Threshold Testing in Donkeys and Horses
2017, Journal of Equine Veterinary ScienceCitation Excerpt :The findings of Regan et al [1] and Olmos et al [2] do suggest that donkeys may exhibit a wider repertoire of pain behavior than previously described in the literature [3], although the behaviors appear to be more subtle than those exhibited by other equidae. To compliment behavioral assessments, nociceptive threshold testing (NTT) has been evaluated in the donkey [4–7], aiming to objectively measure the functional state of the nociceptive system. Nociceptive threshold testing is an objective method for investigation of threshold responses to different noxious stimuli and evaluates the somatosensory system in its entirety, including nociceptors, peripheral nerves, the spinal cord, brain stem, thalamus, and cortex [8].
Morphine in donkeys: Antinociceptive effect and preliminary pharmacokinetics
2023, Equine Veterinary Journal