Can disbudding of calves (one versus four weeks of age) induce chronic pain?

Background: Calf disbudding is a welfare-relevant invasive husbandry procedure. While it is widely recognized that disbudding elicits acute pain, its chronic consequences are currently unknown. Objectives: Primary aim of this study was to investigate whether disbudding leads to chronic pain in calves; secondary aim was to evaluate the influence of age at disbudding (1 week versus 4 weeks). Methods: Thirty-four Holstein bull calves, enrolled in a prospective, controlled, randomized trial, underwent either early disbudding (ED) or late disbudding (LD) or sham treatment (control) at 1 and 4 weeks of age. Pain scoring and quantitative sensory tests (von Frey filaments and algometry) were performed at regular intervals up to 105 days of life; neurophysiological determination of nociceptive reflex thresholds and conditioned pain modulation (CPM) up to 90 days. Data was analyzed with the Brunner and Langer model followed by post-hoc test, with level of significance set at 0.05. Results: Thirty-one calves completed the data collection. Signs of chronic sensitization were observed in 38% of disbudded calves (4 in ED and 4 in LD). Pain scores (until 105 days) and allodynia scores (until 60 days) were significantly higher while pressure thresholds (until 105 days) were significantly lower in both groups ED and LD compared to control. Late disbudded calves showed impairment of trigeminal CPM. Lower laser-evoked trigeminal reflex thresholds were found in clinically painful calves compared to control. Conclusions: The sequelae of disbudding can extend beyond the acute post-procedural phase. Chronic trigeminal sensitization, independently from the age at disbudding can affect individual calves.