Mechanical/thermal sensitivity and superficial temperature in the stump of long-term tail-docked dairy cows

Background. Tail docking of dairy cows is a painful procedure that affects animal welfare. The aims of this study were first to evaluate the response to mechanical and thermal stimulation, and second to determine the superficial temperature of the stump of tail-docked dairy cows. Methods. One hundred and sixty four dairy cows were enrolled. From these, 133 cows were assigned to the tail-docked group (TD) and 31 cows were selected as control animals (C). The following sensory assessments to evaluate pain in tail-docked cows were performed. Sensitivity of the tail region in both groups of animals was evaluated using a portable algometer. Cold and heat sensitivity assessment was performed using a frozen pack (0ºC) and warm water (45ºC), respectively. Pinprick sensitivity was evaluated using a Wartenberg neurological pinwheel. Superficial temperature was evaluated using a thermographic camera. All sensory assessments and superficial temperature were evaluated in the ventral surface of the tail stump (TD) and tail (C). Results. Analysis revealed a significant effect of condition on the pressure pain threshold with significantly lower values necessary to obtain a withdrawal response in TD cows (5.97±0.19 kg) compared to C cows (11.75±0.43 kg). Chi Square test revealed that there was a significant association between heat sensitivity and condition with 29.3% of TD cows responding positively. Nonetheless, no association was found between condition and cold sensitivity. Similarly, after pinprick sensitivity test was performed, 93.2% of TD cows elicited a positive response to stimulation. This sensory testing was significantly associated with the condition. There was a significant effect of condition on superficial temperature, where TD cows had significantly lower temperature (26.43±0.27 ºC) when compared to C animals (29.86±0.62 ºC). Discussion. Pressure pain threshold values in both groups of animals were higher that those previously reported for tail docked pigs, sows and cows. In contrast, pinprick stimulation evaluates the presence of punctate mechanical hyperalgesia/allodynia, usually related to traumatic nerve injury, and this association may reveal that it is possible that these animals developed a disorder associated to the development of a tail stump neuroma and concurrent neuropathic pain, previously reported in tail-docked lambs, pigs and dogs. Thermal sensitivity showed that tail-docked cows responded positively only to heat stimulation. These findings suggest that long-term tail-docked cows could be suffering hyperalgesia/allodynia, which may be indicative of chronic pain. Lower superficial temperature in the stump may be associated to sympathetic fiber sprouting in the distal stump, which can lead to vasoconstriction and lower surface temperatures. Further studies are needed in order to confirm neuroma development and adrenergic sprouting.