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Equine veterinary journal2026; doi: 10.1002/evj.70302

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

Abstract: Flysheet use is common without any published scientific evidence to justify their use. Objective: To demonstrate the effectiveness of flysheets at reducing fly avoidance behaviours and measuring changes in body temperature, heart rate, and cortisol levels. Methods: 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  0.05). 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). Conclusions: 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.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2026-08-13 PubMed ID: 42595736DOI: 10.1002/evj.70302Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated whether flysheets reduce fly avoidance behaviors in horses and examined their impact on horses’ body temperature, heart rate, and stress levels.

Study Purpose and Background

  • Flysheets are commonly used on horses to protect them from flies, but there is little scientific evidence supporting their effectiveness or understanding their physiological effects.
  • The study aimed to objectively measure how flysheets influence fly avoidance behaviors and physiological parameters such as heart rate, cortisol levels (stress indicator), and body temperature.

Methods

  • Five horses participated in a 10-week alternating crossover study design.
  • Each horse experienced weekly periods with the flysheet on (ON) and off (OFF), alternating between groups.
  • Fly avoidance behaviors (head tossing and tail swishing) were recorded once per week during a 30-minute observation period.
  • Salivary cortisol samples were collected immediately before flysheet application/removal and 10 minutes afterward to assess stress level changes.
  • Continuous data on heart rate and body temperature were collected every 15 minutes throughout the study using Star-Oddi data loggers attached to the horses.

Key Findings

  • Flysheets significantly reduced fly avoidance behaviors:
    • Head tosses decreased from an average of about 12 per 30 minutes to about 2.3 when the flysheet was on.
    • Tail swishing similarly dropped from approximately 215 to 53 tail swishes on average.
  • Heart rate was significantly higher with flysheet use, increasing by an average of 7.2 beats per minute, suggesting some physiological stress or increased arousal.
  • Body temperature was significantly lower with flysheet use by an average of 7.4°C, though the paper notes inconsistent effects, so this needs cautious interpretation.
  • Cortisol levels showed elevations, but these could not be conclusively linked to the flysheet application or removal alone, indicating other factors might influence stress.
  • Two horses rejected the wearable data loggers, causing gaps in heart rate and temperature data for those individuals.

Conclusions and Implications

  • Fly sheets effectively decrease the behaviors horses use to fight off flies, indicating an improvement in insect-related comfort.
  • The increased heart rate during flysheet use may point to a physiological or thermal cost, possibly because of the extra layer on the horse’s body or discomfort from the sheet.
  • The inconsistent body temperature results and cortisol findings mean further research is necessary to fully understand the thermal effects and stress implications.
  • The study was limited by a small sample size and technical issues with data loggers, which may affect the generalizability of the results.
  • Horse owners and caretakers should monitor heat load and signs of physiological stress when using flysheets, especially during warm weather or extended wear periods.
  • Overall, flysheets may improve horse comfort during fly season but should be used thoughtfully considering the potential for increased heart rate and thermal stress.

Cite This Article

APA
Graves M, Sun X, Fecteau K. (2026). Effect of equine flysheets on fly avoidance behaviours, body temperature, stress level, and heart rate. Equine Vet J. https://doi.org/10.1002/evj.70302

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Graves, Meggan
  • Department of Large Animal Clinical Sciences, University of Tennessee College of Veterinary Medicine, Knoxville, Tennessee, USA.
Sun, Xiaocun
  • Office of Information Technology, University of Tennessee, Knoxville, Tennessee, USA.
Fecteau, Kellie
  • Department of Biomedical and Diagnostic Sciences, University of Tennessee College of Veterinary Medicine, Knoxville, Tennessee, USA.

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