Bleeders Show Higher Cytokine Levels Racing at Sea Level

Horses with a more severe exercise-induced pulmonary hemorrhage (EIPH) show increased amounts of certain immunoregulatory proteins, called cytokines, when they race at sea level rather than higher altitudes, researchers on a South African study concluded.

Although the reason for the increase is not known, the study may help researchers understand the molecular relationship between EIPH and the immune system, said Montague N. Saulez, BVSc MS, PhD, Dipl. ACVIM, associate professor of equine medicine at the University of Pretoria, South Africa.

EIPH can be found in horses that exercise intensely, and it can negatively impact their health, increasing time off the track, and costing owners and trainers money. More than 55% of racehorses that exercise strenuously are affected. The disorder can impair a horse's performance, causing it to become anxious, swallow repeatedly, and cough because of blood that accumulates in the trachea. "Those horses with a severe lung bleed may suffer dyspnea (labored breathing) and struggle to aerate their lung," Saulez said.

Using tracheal endoscopy to examine 97 Thoroughbreds within two hours of racing, one group at high altitude and the other at sea level, Saulez and colleagues graded each horse for EIPH based on the amount of blood in the trachea. Then they collected and analyzed blood from 10 horses in each grade level, looking at the levels of cytokine mRNAs (messenger ribonucleic acids), which are critical for helping regulate cellular growth, development, and activation of the immune system and its response to inflammation.

Saulez and his colleagues found that the horses raced at sea level had a greater expression of IL-6 (a cytokine mRNA), compared with horses that raced at a higher altitude, especially those with Grade 4 EIPH, the most severe grade of EIPH in the study.

But effort and distance could have played a role. "Horses that raced at sea level did so over shorter distances, so they may have overexerted themselves, causing more bleeding, which led to the increase in IL-6," Saulez said.

"To our knowledge, this is the first study in which an association between mRNA expression and EIPH has been reported," he added.

"Studying cytokine expression may help in understanding the process of disease development of EIPH at a molecular level," Saulez said. "In the future, studying traits that may be inherited together due to an association between horses' genes may help determine the predisposition to EIPH by evaluating the balance between pro-inflammatory and anti-inflammatory cytokine expression. Also, more therapeutic options may be available that can interrupt the inflammatory changes, which may decrease the prevalence and severity of EIPH."

The study, "Cytokine mRNA expressions after racing at a high altitude and at sea level in horses with exercise-induced pulmonary hemorrhage," was published in the April American Journal of Veterinary Research.

The abstract is available on PubMed.

About the Author

Rebecca Overton

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