The influence of various confounding factors on mechanical nociceptive thresholds in the donkey

Objective: To evaluate a mechanical nociceptive threshold (MNT) testing device in the donkey, and to investigate the influence of potential confounders on MNTs generated. Study design: Prospective, randomised. Animals: Sixteen castrated male donkeys aged 4-9 years, weighing 105-170 kg. Methods: Mechanical nociceptive thresholds were measured using an actuator with three pins placed on the dorsal aspect of the distal limb, connected to a force meter. The pins (surface area 15 mm²) were extruded onto the limb by pressurising an air-filled syringe, until the MNT force (when foot-lift was observed) or 25 N (cut-off force) was reached. Effect on MNT of presence of a companion donkey, the limb tested, rate of application of force, testing location, level of distraction, ambient temperature and hair cover at the test site was evaluated. Long and short-term repeatability of MNT was assessed. Data were analysed using general linear models and Mann-Whitney U tests, p < 0.05 was considered significant. Results: Increasing the rate of force application significantly increased the mean ± SD MNT from 9.2 ± 2.0 N when applied at 0.4 N sec^-1 to 10.6 ± 2.1 N when applied at 1.2 N sec^-1 (p = 0.001). No other factors significantly influenced MNT. Mean MNT remained stable over a 3 week period, however MNTs were significantly (p = 0.006) higher (12.8 ± 3.0 N cf 10.3 ± 1.9 N) after a 12 month interval. Conclusions and clinical relevance: When designing studies measuring MNT in donkeys, rate of application of force must be standardised. Donkeys' MNTs have good short-term stability suggesting this technique is appropriate for short-term analgesiometry studies; however variability of MNTs over the long-term is greater.