Relating Post-Mortem Hippocampal Neurogenesis to Longitudinally Recorded Severity of Keel Bone Fractures in Adult Laying Hens

In commercial flocks of laying hens, keel bone fractures (KBFs) are prevalent and associated with signs of poor welfare, including behavioural indicators of pain. However, whether their impact is severe enough to induce a depressive-like state of chronic stress is unknown. As levels of adult hippocampal neurogenesis (AHN) in the brains of mammals and birds are suppressed by the chronic experience of stress, this measure may be applicable as a neural biomarker of subjective welfare state. Radiographs obtained longitudinally (at 11 timepoints) from Lohmann Brown laying hens housed in a commercial multi-tier aviary were used to score the severity of naturally occurring KBFs between the ages of 21 and 62 weeks. Individual mobility data was collected using an infrared tracking system to record transitions between aviary zones. Immunohistochemistry was used to stain immature neurons expressing the protein marker doublecortin (DCX). Focal hens with severe KBFs at the final timepoint (3-4 weeks prior to sampling, n=15) had lower densities of immature DCX⁺ multipolar and bipolar neurons in the hippocampal formation (HF) than focal hens with minimal fractures at this time (n=9). KBF severity scores at this time also negatively predicted DCX⁺ cell numbers across the whole HF on an individual level, while the duration of time since a hen had acquired their first fracture tended to negatively correlate with numbers of DCX⁺ neurons in the caudal HF. Activity levels at the final timepoint (3-4 weeks prior to sampling) were not associated with AHN. KBFs thus appear to present a source for chronic stress to laying hens, inducing a negative affective state that lasts at least 3-4 weeks. Management steps to reduce their occurrence are therefore likely to have significant benefits for welfare.

Ethical approval for the study was obtained from the Veterinary Office of the Canton of Bern in Switzerland (approval number BE31/15) and the the Animal Welfare and Ethical Review Body at Newcastle University (Project ID #549). The experiment complied with Swiss regulations regarding the treatment of experimental animals.