Development of mechanical and thermal nociceptive threshold testing devices in unrestrained birds (broiler chickens)

Behavioural signs of pain are difficult to quantify and interpret in animals. Nociceptive threshold testing is therefore a useful method for examining the perception and processing of noxious stimuli underlying pain states. Devices were developed to measure response thresholds to quantified, ramped mechanical and thermal nociceptive stimuli applied to the leg or keel of unrestrained birds. Up to 9N mechanical force was delivered via a single round-ended 2mm pin using a pneumatic actuator at 0.4Ns(-1). Heat was applied through a small copper element at 0.8°Cs(-1) to a maximum of 50°C. The repeatability and reliability of threshold measures were validated using 10-12 broiler chickens (aged 49-66 days) per site and modality. Mechanical threshold, or skin and threshold temperature, were recorded over three sessions across a 36h period. Both stimulus types elicited clear, reproducible behavioural responses. Mechanical threshold means and 95% confidence intervals were 3.0 (2.8-3.2)N for keel and 2.0 (1.8-2.1)N for leg sites. Keel thermal tests gave a mean skin temperature of 39.3 (39.1-39.5)°C, and threshold of 46.8 (46.6-47.1)°C. Leg skin temperature was 35.7 (35.6-35.9)°C and threshold 42.5 (42.2-42.8)°C. Threshold measures were consistent within and across sessions and birds showed individual repeatability across tests within sessions. Individual birds' mechanical keel thresholds were also repeatable across sessions. The apparatus gave reliable, reproducible measurements of thresholds to noxious mechanical and thermal stimuli. The range recorded was comparable with previously published nociceptor thresholds in dissected chicken nerve filament fibres, and the method appears suitable for studying nociceptive processes in broiler chickens.