Effects of Heat Stress on Heart Rate Variability in Free-Moving Sheep and Goats Assessed With Correction for Physical Activity

Abstract

Heart rate variability (HRV) is the heart beat-to-beat variation under control of the cardiovascular function of animals. Under stressed conditions, cardiac activity is generally regulated with an upregulated sympathetic tone and withdrawal of vagal tone; thus, HRV monitoring can be a non-invasive technique to assess stress level in animals especially related to animal welfare. Among several stress-induced factors, heat stress is one of the most serious causes of physiological damage to animals. The aim of this study was to assess the effects of heat stress on HRV in small ruminants under free-moving conditions. In three experimental periods (June, August, and October), inter-beat intervals in sheep and goats (three for each) in two consecutive days were measured. HRV parameters were calculated from the inter-beat interval data by three types of analyses: time domain, frequency domain, and non-linear analyses. The temperature–humidity index (THI) was used as an indicator of heat stress, and vectorial dynamic body acceleration (VeDBA) was calculated to quantify the physical activity of the animals tested. First, we investigated correlations of THI and VeDBA with HRV parameters; subsequently, THI was divided into five categories according to the values obtained (≤ 65, 65–70, 70–75, 75–80, and >80), and the effects of the THI categories on HRV parameters were investigated with and without correcting for the effects of physical activity based on the VeDBA. The results indicated that HRV significantly decreased with increasing THI and VeDBA. For non-linear HRV parameters that were corrected for the effects of physical activity, it was suggested that there would be a threshold of THI around 80 that strongly affected HRV; high heat stress can affect the autonomic balance of animals non-linearly by inducing the sympathetic nervous system. In conclusion, to assess psychophysiological conditions of unrestrained animals by HRV analysis, the confounding effect of physical activity on HRV should be minimized for a more precise interpretation of the results.

Open-Source Wearable Sensors for Behavioral Analysis of Sheep Undergoing Heat Stress

“…Our findings are consistent with the findings from [39], where HS effects in Merino and Omani sheep in an extensive system were evaluated. Similar results for RT were also found by [40] in Merino sheep under HS conditions in an extensive system. Despite the fact that the mean rectal temperatures observed within the present study were within the normal range, the increase in rectal temperature associated with 35 • C does suggest animals were experiencing HS conditions.…”

Heart Rate and Heart Rate Variability Change with Sleep Stage in Dairy Cows

“…The NZ recordings were made outdoors over winter with average an average daytime temperature of 10.2 • C (range 2-18 • C) and overnight average of 8.4 • C (range 2-14 • C). Seasonal thermal stress has been found to affect behaviour, stress and immune response in dairy cows [32,33], and increasing temperature humidity index has been associated with decreased HRV measures in sheep and goats [34]. Although the cows in the winter conditions in NZ had higher HR and lower HRV than cows in summer conditions in Scotland, the environmental conditions could have affected the HR and HRV activity in this study.…”

An Assessment of Stress Status in Fattening Steers by Monitoring Heart Rate Variability: A Case of Dietary Vitamin A Restriction

“…In addition, the covariate of VeDBA was significant for HR, RMSSD, and %DET (P < 0.05). In previous studies, the effect of physical activity quantified by VeDBA was significant for HR and all HRV parameters when HRV of animals was evaluated under free-moving conditions (Oishi et al, 2018;Kitajima et al, 2021). RMSSD: square root of the mean squared differences of successive inter-beat intervals (ms), HF: normalized power of the high-frequency band (n.u.…”

“…As a decrease in HRV can be caused by an increase in sympathetic activity and/or withdrawal of vagal activity, HRV is one of good indicators for assessing autonomic nervous system in response to various stressors (Acharya et al, 2006). Until now, a variety of studies have investigated internal and external stress factors that affect HRV in farm animals: e.g., diseases such as lameness (Kovács et al, 2015), diarrhea (Mohr et al, 2002), and bovine spongiform encephalopathy (Konold et al, 2011), and heat stress (Kitajima et al, 2021). Hence, HRV monitoring has gained notable attention as a non-invasive technique for evaluating the stress levels of animals from the point of view of animal welfare (Kovács et al, 2014).…”