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Journal of veterinary internal medicine2026; 40(2); aalag052; doi: 10.1093/jvimsj/aalag052

Relationship between prevalence and severity of exercise-induced pulmonary hemorrhage and environmental factors.

Abstract: Environmental risk factors could contribute to the development of exercise-induced pulmonary hemorrhage (EIPH) in racing Thoroughbreds. Objective: To identify environmental risk factors that might contribute to differences in EIPH prevalence and severity across 12 Thoroughbred racetracks in the United States. Methods: Eight hundred fifteen 2-year-old and 122 >2-year-old Thoroughbred racehorses. Methods: Prospective blinded observational study. Videoendoscopy was performed 30-60 min post-race. Three observers independently assigned an EIPH grade to each videorecording, and prevalence and severity of EIPH were determined. Multivariable logistic regression assessed relationships between EIPH prevalence and severity, respectively, and independent variables including furosemide administration, race distance and surface, and lifetime race number (LRN). P ≤ .05 was considered significant. Results: One thousand one hundred ninety-two videorecordings received EIPH severity grades from 12 racetracks in 10 different states and 3 time zones. Ambient temperature (AT) was negatively associated with EIPH prevalence (OR = 0.82, 95% CI, 0.69-0.98; P = .03) and severity (OR = 0.79, 95% CI, 0.63-0.99; P = .04). Furosemide administration (OR = 0.04, 95% CI, 0.006-0.33; P = .002), turf (OR = 0.64, 95% CI, 0.43-0.95; P = .027) and all weather (OR = 0.03, 95% CI, 0.004-0.26; P = .001) track surfaces, and LRN (OR = 1.07, 95% CI, 1.01-1.12; P = .012) were associated with occurrence of EIPH. Two-year-olds racing in bad air (air quality index [AQI] > 100) had more severe EIPH than those racing in AQI < 50 (OR = 2.78, 95% CI, 1.06-7.29; P = .04). Conclusions: AT, AQI, LRN, and surface type might be associated with EIPH prevalence and severity between American racetracks.
© The Author(s) 2026. Published by Oxford University Press on behalf of the American College of Veterinary Internal Medicine.
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Publication Date: 2026-03-29 PubMed ID: 41904682PubMed Central: PMC13033154DOI: 10.1093/jvimsj/aalag052Google Scholar: Lookup
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Summary

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Overview

  • This research investigated how environmental factors influence the occurrence and severity of exercise-induced pulmonary hemorrhage (EIPH) in Thoroughbred racehorses across various racetracks in the United States.
  • The study identified specific environmental conditions, such as ambient temperature and air quality, and other variables that significantly affect the prevalence and severity of EIPH in racing horses.

Background and Objective

  • Exercise-induced pulmonary hemorrhage (EIPH) is a condition where bleeding occurs in the lungs of horses during intense exercise, such as racing.
  • Environmental risk factors have been hypothesized to contribute to the development and severity of EIPH, but there is limited knowledge about which specific factors play a role.
  • The objective was to identify environmental and other risk factors that contribute to differences in EIPH prevalence and severity at 12 Thoroughbred racetracks in the U.S.

Methods

  • Study Design: Prospective blinded observational study, meaning observations were made moving forward in time without bias from observers knowing key details beforehand.
  • Subjects: 815 two-year-old and 122 older Thoroughbred racehorses participated.
  • Data Collection: Videoendoscopic recordings of the horses’ airways were taken 30-60 minutes after races to detect and grade EIPH severity.
  • Grading: Three independent observers assigned EIPH severity scores to each video recording.
  • Variables Examined:
    • Furosemide administration (a medication used to reduce bleeding risk)
    • Race distance
    • Track surface type (turf, dirt, all-weather)
    • Lifetime race number (LRN) – representing each horse’s racing experience
    • Environmental factors such as ambient temperature (AT) and air quality index (AQI)
  • Statistical Analysis: Multivariable logistic regression was used to examine associations between these variables and both the prevalence and severity of EIPH.
  • Significance Threshold: P ≤ 0.05 was considered statistically significant.

Results

  • Sample Size: 1,192 video recordings from 12 racetracks located across 10 states and 3 U.S. time zones were analyzed.
  • Ambient Temperature (AT):
    • Negatively associated with EIPH prevalence (Odds Ratio [OR] = 0.82) and severity (OR = 0.79).
    • Interpretation: Lower ambient temperatures were linked to a higher chance and more severe cases of EIPH.
  • Furosemide administration:
    • Strongly associated with reduced EIPH occurrence (OR = 0.04), indicating it was effective at lowering risk of hemorrhage.
  • Track Surface:
    • Turf track surfaces were associated with lower EIPH occurrence (OR = 0.64) compared to dirt.
    • All-weather surfaces similarly showed decreased EIPH prevalence (OR = 0.03).
  • Lifetime Race Number (LRN):
    • Positively associated with EIPH occurrence (OR = 1.07), suggesting horses with more racing experience had higher likelihood of EIPH.
  • Air Quality Index (AQI):
    • For 2-year-olds, racing in poor air quality conditions (AQI > 100) was linked to significantly more severe EIPH compared to low pollution conditions (AQI < 50) (OR = 2.78).
    • This highlights air pollution as an environmental risk factor for the severity of EIPH.

Conclusions

  • Environmental factors such as ambient temperature and air pollution, as well as race-related factors like lifetime races and track surface type, are associated with both the prevalence and severity of EIPH in American Thoroughbred racehorses.
  • Lower ambient temperatures and poor air quality increase the likelihood and severity of EIPH.
  • Medication with furosemide and racing on turf or all-weather surfaces may help reduce the occurrence of EIPH.
  • These findings suggest environmental conditions should be considered in the management and prevention strategies for EIPH in racehorses.

Cite This Article

APA
Pinnell E, Shoemaker S, Wang Y, Tang Y, Sellon D, Leguillette R, Gold J, Sanz M, Bayly WM. (2026). Relationship between prevalence and severity of exercise-induced pulmonary hemorrhage and environmental factors. J Vet Intern Med, 40(2), aalag052. https://doi.org/10.1093/jvimsj/aalag052

Publication

Journal of veterinary internal medicine
ISSN: 1939-1676
NlmUniqueID: 8708660
Country: England
Language: English
Volume: 40
Issue: 2
PII: aalag052

Researcher Affiliations

Pinnell, Erin
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.
Shoemaker, Sierra
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.
Wang, Yuan
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.
Tang, Yanan
  • Department of Mathematics and Statistics, Washington State University, Pullman, WA 99164, United States.
Sellon, Debra
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.
Leguillette, Renaud
  • Faculty of Veterinary Medicine, University of Calgary - VCDS, Calgary, AB T2N 4Z6, Canada.
Gold, Jenifer
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.
Sanz, Macarena
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.
Bayly, Warwick M
  • Veterinary Clinical Sciences, Washington State University, Pullman, WA 99164, United States.

MeSH Terms

  • Animals
  • Horse Diseases / etiology
  • Horse Diseases / epidemiology
  • Prevalence
  • Horses
  • Physical Conditioning, Animal / adverse effects
  • Prospective Studies
  • Hemorrhage / veterinary
  • Hemorrhage / etiology
  • Hemorrhage / epidemiology
  • Male
  • Female
  • Lung Diseases / veterinary
  • Lung Diseases / etiology
  • Lung Diseases / epidemiology
  • Risk Factors
  • United States / epidemiology
  • Furosemide
  • Environment

Grant Funding

  • Breeders' Cup Limited
  • Churchill Downs Incorporated
  • Florida Horsemen's Benevolent Protection Association
  • Kentucky Horse Racing Commission
  • Kentucky Thoroughbred Association
  • New York Racing Association and The Stronach Group

Conflict of Interest Statement

Authors declare no conflict of interest.

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