The influence of severity of gastric ulceration on horse behavior and heart rate variability

Despite the high prevalence of gastric ulceration in horses, little is known about the behavior and heart rate variability (HRV) indices associated with the severity of this condition. This study examined the effect of severity of gastric ulceration on behavior and HRV indices associated with pain in 8 mature University teaching horses, in which ulcers were induced for a coinciding trial. Horses were divided into 2 groups (n = 4) by the severity of gastric ulceration: severe ulcer group (S; scores 3-4), and a mild group [M; scores 0-2]. Horses were housed in 3.05 × 3.65m stalls and fed 0.5% body weight (BW) long-stem hay in slow-feeder nets and 0.25% BW concentrate twice daily with ad libitum access to water. Dietary modifications were made after endoscopy and horses were administered supportive treatment immediately post-data collection. Horses received a 24-hour rest period post-endoscopic examination before data sampling began. Behavior and HRV were collected for 3 consecutive days to account for day-to-day variation and optimize data validity. Behavior was recorded at three 2-hour time points per day (Morning, Noon, and Evening) and analyzed in 2 15-minute increments an hour, totaling 9 hours of data per horse. Polar® Equine V800 Heart Rate Monitors were used to record heart rate and HRV, later analyzed using Kubios HRV and MATLAB® software. Mean heart rate and HRV indices, for example standard deviation of beat-to-beat intervals (SDRR) and root mean square of successive beat-to-beat differences (RMSSD), the low (LF; sympathetic tone) and high frequency (HF; parasympathetic tone) component of HRV, and their ratio (LF/HF; index representing the sympatho-vagal balance) were calculated and expressed as response values. The effects of gastric ulcer severity between groups on behavior and HRV were assessed using GLMMs with Tukey's Post hoc test applied to significant results (α=0.05) in R (version 3.3.1). Horses with severe ulcers (S) showed higher heart rate (63.3 beats) and LF/HF ratio (5.0%) and reduced SDRR and RMSSD (155.6, 39.73) when compared with M horses (51.08, 3.25, 199.0, 52.57), respectively, (all P ≤ 0.05). S horses had more frequent eating bouts than M horses (26.6, 12.1 bouts; P = 0.03). S horses displayed more abdominal kicks, tail switching, tongue activity and pawed more than M horses (5.26, 10.0, 10.35, 6.87 versus 1.32, 3.69, 2.07, 1.38 times, all P ≤ 0.05). Therefore, horses with severe gastric ulcers showed a more stressed pattern of behavior and HRV indicators, suggesting these may be reliable in determining severity of gastric ulcers in horses.