Assessment of chronic stress in sheep (part II): Exploring heart rate variability as a non-invasive measure to evaluate cardiac regulation

Abstract

Ovine footrot is a clinical disease condition which is a significant cause of lameness in sheep and a serious threat to animal welfare. Heart rate variability (HRV) is a promising approach for evaluating stress and emotional states in animals, and the aim of this study was to assess stress and pain associated with ovine footrot using different methods of HRV analysis in the linear and nonlinear domain. The study was performed as a single foot inoculation using a canine rubber boot. The right hind foot of 24 lambs in four experimental groups was inoculated with four different strains of Dichelobacter nodosus, and HRV was recorded for 30min every other day for two weeks (Day 9–22). Symptoms of footrot developed in all experimental groups. The animals were monitored daily to assess lameness and pain, and were treated with gamithromycin subsequent to the trial period (Day 23). The parameters RMSSD and SDNN decreased between Day 9 and Day 17 with a subsequent increase from Day 17 to 21, indicating a temporary reduced vagal tone and a chronic stress response due to the maceration caused by the boot and the developing infection. Also, a significant increase in the LF/HF ratio, indicating a change in the sympatho-vagal balance towards sympathetic dominance, was found from Day 9 to Day 13 which was sustained until Day 17. We found an increase in the detrended fluctuation scaling exponent α (DFA) between Day 9 and Day 15 and from Day 17 to 21, possibly due to a decreased vagal outflow and an increased sympathetic tone. Although the clinical signs were found to be mild, the maceration and the developing infection had a significant impact on all the measured HRV parameters. Based on the results, we suggest that the measurement of HRV is an approach that can give a valuable contribution to our understanding and assessment of adverse welfare during various clinical diseases.