Mechanical nociceptive thresholds in endurance horses

Alteration of limb sensitivity is forbidden in equine sports but difficult to enforce. We aimed to develop an objective field method to assess mechanical nociceptive threshold (MNT) in endurance horses.

A remotely controlled pneumatic actuator (1 mm tip) was used to measure forelimb pastern MNT in 108 endurance horses.

Median (IQR) MNT at rest was 1.9 N (0.9-3.5). Icing had no significant effect on limb sensitivity. MNT measured at weekly intervals increased from week 1 (1.2 N (0.6-1.8)) to week 3 (1.9 N (1.2-2.8)) (P<0.05). In 17 horses without impaired sensitivity, MNT increased from 1.2 N (0.6-2.3) before to 2.4 N (1.2-5.2) after racing (P=0.0017). In desensitised horses, MNT after racing was higher (8 limbs-23.1 N (21.4 to >25)) than in horses without impaired sensitivity (42 limbs-2.2 N (1.2-4.3)) (P<0.0001). Desensitisation with mepivacaine increased MNT to above the safety cut-off (25 N) at 10 minutes; sensitivity return to baseline varied between individuals but was restored by 330 minutes. None of the horses became averse to the technique.

MNT was practical, non-traumatic, repeatable and well tolerated under field conditions in endurance horses. The technique differentiated postracing MNT in horses with normal sensitivity from those with impaired sensitivity.