Chronic neurophysiological and anatomical changes associated with infra-red beak treatment

McKeegan, D. (2009) Chronic neurophysiological and anatomical changes associated with infra-red beak treatment. Project Report. Defra.

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In order to reduce poor welfare associated with feather pecking and cannibalism in laying hens, part of the beak is commonly removed in a procedure described as beak trimming. Infrared (IR) beak treatment in day old chicks is now widely used as an alternative to the more traditional hot blade beak trimming, but there is little scientific evidence regarding its long term welfare consequences. This study examined the long term consequences of IR beak treatment by examining changes in beak nerve function (neurophysiology) and anatomy over a range of ages. In IR treated and control birds (intact beaks) that were 10, 30 or 50 weeks of age, the responses of single sensory nerve fibres were recorded from small nerve bundles of the intramandibular nerve, which provides sensation to the lower beak. The beaks of the birds (as well as further groups aged day old and 4 weeks of age, both too small for neurophysiological studies) were then subject to post mortem microscopic and radiographic examination. The responses of 386 nerve fibres were recorded and by their characteristics identified as different receptor types; mechanoreceptors (sensitive to touch and pressure), thermoreceptors (sensitive to temperature change) and nociceptors (responsive to a noxious or potentially painful stimulus). Receptive fields extending to the inner and outer beak tip were observed in treatment and control birds at all ages, indicating that all areas of the lower beak were sensitive to each type of stimulus, regardless of age or treatment. There was some evidence in IR beak treated birds for an increase in mechanoreceptor sensitivity. This does not support the notion that beak treatment works by reducing sensitivity, and does not concur with previous reports in which hot blade trimming was found to have no effect on mechanoreceptor sensitivity. The effects seen here may therefore be related specifically to IR trimming, to the specific age groups concerned, or may be artefactual. Nociceptors were relatively rare but were found in both IR treated and control birds in all of the age groups examined. Some of these receptors responded to painful mechanical and heat stimulation (polymodal nociceptors) and their thresholds and response curves closely matched those reported previously. There was no evidence of a treatment effect on either mechanical or thermal nociceptive thresholds at any age. This indicates that IR beak treatment is not associated with an increased sensitivity to pain (hyperalgesia) nor does it result in a pain response to a normally non-painful stimulus (allodynia) after 10 weeks of age. In a previous study of beak neurophysiological responses in hot blade trimmed birds, a large number of abnormal nerve fibres were observed which were producing action potentials spontaneously, i.e. in the absence of stimulus. The cause was attributed to activity arising from traumatic neuromas, proliferations of regenerated but unregulated nerve cells which sometimes form following nerve damage. Neuromas have been associated with increased pain sensitivity in humans and other animals. In the current study, a small number of unresponsive spontaneously active units were observed but these were rare and were encountered equally in both control and treatment birds. It is therefore considered unlikely that these represent abnormal neural activity arising from neuromas, which is supported by the results from the microscopic examination of the beak tip. Beak measurements at day-old demonstrated that application of the IR treatment at day old affected on average 36% of beak area (using area forward of the nostrils as a basis for comparison). Detailed beak measurement data indicated that the IR treatment had resulted in a 44% reduction in overall (upper) beak length when compared with control birds by 4 weeks of age. The extent of the trim in terms of beak length reduced with age as beak regeneration occurred but IR treated beaks were still significantly shorter than controls at all ages. There was a trend for a beak step (lower beak protruding by up to 30%) in treated birds; the functional consequences of this (if any) are not clear. Radiographs of control and treatment birds at all ages produced no evidence of adverse or pathological change, the only effect seen was marginal thickening of the palatine bone in some IR treated birds at 50 weeks of age, which is unlikely to be of clinical significance. Microscopic evaluation of beak tip anatomy revealed that by 4 weeks of age there was limited nerve regeneration in IR treated beaks, including repopulation of mechanoreceptors in some birds. There was evidence of beak healing, which included reepithelialisation, fibrovascular hyperplasia and bone remodelling, in all birds. This supports the electrophysiological findings which provided evidence of mechanical sensitivity in regrown beak tips from 10 weeks of age onwards. In older birds, repopulation of mechanoreceptors and innervation were visible along with evidence of fibrous scarring. Importantly, no neuromas or abnormal proliferations of nerve fibres were observed in the beaks at any age. The absence of neuromas concurs with the lack of abnormal spontaneous activity observed in the electrophysiological studies. Collectively, the results suggest that IR beak treatment of day old chicks does not result in chronic adverse consequences for sensory function, nor does it demonstrate evidence of chronic pain associated with the procedure.

Item Type:Research Reports or Papers (Project Report)
Glasgow Author(s) Enlighten ID:McKeegan, Dr Dorothy
Authors: McKeegan, D.
Subjects:S Agriculture > SF Animal culture > SF600 Veterinary Medicine
College/School:College of Medical Veterinary and Life Sciences > School of Biodiversity, One Health & Veterinary Medicine

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