Measuring piglet castration pain using linear and nonlinear measures of heart rate variability

Authors: BYRD, CHRISTOPHER - Purdue University; RADCLIFFE, SCOTT - Purdue University; CRAIG, BRUCE - Purdue University; Eicher, Susan; Lay Jr, Donald

Submitted to: Animal Welfare
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2020
Publication Date: N/A
Citation: N/A

Interpretive Summary: In the United States, male piglets not intended for the breeding herd are routinely castrated to improve meat quality and reduce the performance of aggressive behavior during postnatal life. While castration is commonly carried out early in life to minimize any lasting negative effects, it is evident that the procedure causes pain and stress to the piglet. In the past, non-invasive behavioral indicators of castration pain have typically been the most beneficial for distinguishing between castrated and non-castrated piglets, whereas physiological measures (e.g. the stress hormones cortisol and catecholamines) may be affected by a number of additional factors including handling. Accordingly, there is a need for additional non-invasive methodologies to evaluate the physiological response to castration pain. Heart rate variability is a non-invasive measure that has been used as an indicator of the stress response in livestock species, however, little work has evaluated its use as an indicator of pain. The purpose of this study was to evaluate the usefulness of linear and nonlinear heart rate variability measures for evaluating castration pain in piglets. Following surgical castration, piglets exhibited nonlinear measures of heart rate variability indicative of greater pain-related stress than sham castrated piglets. However, postural behavior, stress hormone response, and most of the linear heart rate variability measures were unable to distinguish between castration and sham-castration treatments. Therefore, the inclusion of nonlinear heart rate variability measures may be valuable for evaluating potentially painful procedures in future livestock studies.

Technical Abstract: The purpose of this study was to evaluate the usefulness of linear and nonlinear heart rate variability (HRV) measures for evaluating castration pain in piglets over a 3-d experimental period. Thirty individually selected piglets were randomly allocated to 1 of 4 treatments: 1) sham castrated HRV (SHRV; n = 8), 2) surgical castrated HRV (CHRV; n = 7), 3) sham castrated blood collection (SBC; n = 7), or 4) surgical castrated blood collection (CBC; n = 8). Piglets in the SHRV and CHRV piglets underwent a 1 h heart rate variability and postural behavior evaluation on d -1 (baseline), d 0 (castration treatment), d 1, and d 3 of the experimental procedure. Piglets in the SBC and CBC groups underwent blood collection for serum cortisol analysis at -0.5, +1 , +2 , +3 , +24 , +48 , and +72 h relative to castration treatment. No effect of treatment was found for amount of time spent lying (P > 0.05), however, there was a tendency for serum cortisol to be greater in CBC pigs at +1 h (52.1 ± 8.9 ng/mL; t52.4= 2.60; P = 0.08) and +24 h (31.4 ± 6.9 ng/mL; t51.52 = 2.60; P = 0.08) post-castration compared to SBC piglets (+1h: 23.1 ± 6.5 ng/mL; +24 h: 11.3 ± 3.9 ng/mL). Castrated piglets exhibited greater low frequency to high frequency ratio (26.8 ± 3.2 vs. 9.0 ± 1.1 arbitrary units; F10.42 = 7.53; P = 0.02), lower sample entropy (1.47 ± 0.15 vs. 1.97 ± 0.16 bits; F10.4 = 5.33; P = 0.04) and greater percent determinism (54.0 ± 2.1 vs. 38.8 ± 2.0%; F12.07 = 4.93; P = 0.05) compared to SHRV piglets, indicating greater pain-related stress due to the surgical castration procedure. Therefore, the inclusion of nonlinear HRV measures may be valuable indicator of pain in future studies focused on piglet castration.