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

Kaho Kitajima; Kazato Oishi; Masafumi Miwa; Hiroki Anzai; Akira Setoguchi; Yudai Yasunaka; Yukiko Himeno; Hajime Kumagai; Hiroyuki Hirooka

doi:10.3389/fvets.2021.658763

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

Front Vet Sci. 2021 Jun 1:8:658763. doi: 10.3389/fvets.2021.658763. eCollection 2021.

Authors

Kaho Kitajima¹, Kazato Oishi¹, Masafumi Miwa², Hiroki Anzai³, Akira Setoguchi¹, Yudai Yasunaka¹, Yukiko Himeno⁴, Hajime Kumagai¹, Hiroyuki Hirooka¹

Affiliations

¹ Laboratory of Animal Husbandry Resources, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kyoto, Japan.
² Division of Grassland Farming, Institute of Livestock and Grassland Science, National Agriculture and Food Research Organization, Tochigi, Japan.
³ Department of Animal and Grassland Sciences, Faculty of Agriculture, University of Miyazaki, Miyazaki, Japan.
⁴ Department of Bioinformatics, College of Life Sciences, Ritsumeikan University, Shiga, Japan.

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.

Keywords: dynamic body acceleration; heart rate variability; heat stress; non-linear analysis; physical activity.