Effect of equine flysheets on fly avoidance behaviours, body temperature, stress level, and heart rate.

BACKGROUND: Flysheet use is common without any published scientific evidence to justify their use. OBJECTIVES: To demonstrate the effectiveness of flysheets at reducing fly avoidance behaviours and measuring changes in body temperature, heart rate, and cortisol levels. STUDY DESIGN: Ten-week alternating crossover design. METHODS: Five horses were enrolled. Flysheets were alternated (ON, OFF) between groups weekly for 10 weeks. Fly avoidance behaviours were recorded for 30 min once weekly. Cortisol samples were collected before flysheet application or removal, and again 10 min later. Star-Oddi data loggers recorded heartrate and temperature every 15 min throughout the study period. RESULTS: Flysheet use significantly reduced fly avoidance behaviours. Mean head tosses decreased from 12.17 (95% CI: 6.76-17.58) to 2.33 (95% CI: 0.64-4.02) (p < 0.0001), and tail swishing fell from 214.67 (95% CI: 131.06-298.28) to 52.54 (95% CI: 11.17-93.92) (p < 0.0001). Mean baseline cortisol levels were lowest without flysheets (2.92, 95% CI: 2.35-3.48 μg/dL), though no time effect was detected (p > 0.05). Heart rate was significantly higher when flysheets were ON (+7.2 bpm, 95% CI: +5.6 to +8.8, p < 0.0001). In contrast, body temperature was significantly lower when flysheets were ON (-7.4°C, 95% CI: -8.9 to -5.9, p < 0.0001). MAIN LIMITATIONS: Sample size was small. Star-Oddi devices were rejected by two horses creating gaps in data collection. Observed cortisol elevations cannot be attributed exclusively to flysheet application or wear. CONCLUSIONS: Flysheets effectively reduced insect avoidance behaviours but were associated with increased heart rate and inconsistent effects on body temperature, suggesting a potential thermal or physiologic cost. Flysheet use may improve horse comfort during insect season, but heat load and physiologic stress should be monitored.