Effects of habituation to the testing facility on mechanical nociceptive thresholds of pigs

Abstract

Abstract Aims This study aimed to gain methodological knowledge about quantification of mechanical nociceptive thresholds in pigs stimulated on the hind legs. Methods Thirty-two castrated male pigs were used, weighing 50–60kg, 16 weeks of age and housed in standard pens (4.40m×4.40m) with ad libitum access to water and feed. Mechanical nociceptive testing (4 stimulations/pig) was done by an electronic von Frey anesthesiometer (IITC Life Science Inc., CA) with a rigid, hollow plastic tip (cut-off: 1000 gf) and an area of 0.3 mm2. Stimulations targeted caudal aspects of the metatarsus while the pigs were restrained in a cage measuring 35cm×40cm (length×width). During the last 3d before nociceptive testing, half of the pigs were habituated to the experimental set-up twice daily. Results Across the three days, the habituation procedure led to a reduction in time required to approach and release the pigs from the home pen (P<0.05) and to increased acceptance of the confinement (P<0.01). The habituated animals had a lower mechanical nociceptive threshold (median: 495gf (302–675) vs 745gf (479–1000); P<0.05), and a decreased occurrence of censored observations (0 vs 25%; P<0.05) compared to animals tested without habituation. Conclusions The present experiment focussed on methodological aspects of hand-held tests of mechanical nociceptive thresholds in pigs. The results show that the pigs responded to a 3-day habituation regime by increased ease of handling and tolerance of being confined in a test cage. In addition, habituation to the test environment and handling led to reduced mechanical nociceptive thresholds as well as a lowered proportion of censored observations. The current results reiterate the value of habituation in research involving animal behaviour. Further characterization of the methodology is needed to allow its application in the evaluation of clinical conditions in pigs.