The effect of chronic clinical pain on thermal and mechanical thresholds in sheep

Threshold responses were measured to a thermal skin test and a mechanical pressure test in two groups of conscious unrestrained sheep. The first group of sheep were healthy adult females and formed a control sample, the second group were also adult females, but were all suffering from a condition known as footrot. Footrot is a chronic infective lesion affecting usually one foot which appears to cause severe pain in its worst manifestation. These sheep were assessed for the severity of the lesion and degree of lameness and were divided into high and low severity subgroups. Footrot did not alter the threshold to the thermal test but the mechanical pressure threshold was significantly reduced in both footrot sub-groups compared to controls. A local anaesthetic block of the affected foot restored values to close to the control level. After treatment of the affected foot, the mechanical threshold in the low severity sub-group was returned to normal, but in the high severity sub-group it was still significantly reduced compared to the control animals. However, when retested 3 months later these values had returned to the normal control levels.