Endurance and Endotoxemia

Endotoxemia in horses is usually associated with severe infections, inflammation, and colic. Endotoxin from intestinal bacteria activates release of inflammatory mediators such as thromboxane B2 (TxB2), tumor necrosis factor-alpha (TNF-alpha), prostaglandin F1-alpha (PGF1-alpha), and interleukin-6 (IL-6). These mediators produce malaise, fever, dehydration, cardiovascular collapse, organ failure, and sometimes death. Interestingly, strenuous exercise in horses can also cause signs of endotoxemia. It's possible that the inflammatory response to exercise somehow triggers endotoxemia.

To investigate this further, researchers, including Michelle Barton, DVM, PhD, of the Departments of Large Animal Medicine and Physiology and Pharmacology at the University of Georgia's College of Veterinary Medicine, measured plasma endotoxin in horses competing in endurance rides in order to determine effects of endotoxemia on physical, hematologic (blood), and biochemical parameters in these horses.

Owners of 83 horses filled out questionnaires the day prior to 48-, 83-, and 159-km endurance rides in Asheville, N.C. Horses were weighed and blood samples drawn prior to, during, and after the competition for measurement of plasma endotoxin, TxB2, TNF-alpha, PGF1-alpha, IL-6, and other blood and serum biochemistry tests.

All horses had measurable plasma endotoxin at various checkpoints during the rides. Endotoxemia (endotoxin in the blood) occurred most often in the 48- and 83-km groups (50% of horses). There were no hematologic abnormalities associated with endotoxemia, and the only biochemical abnormality discovered was a mild, but significant, increase in plasma lactate. Barton explains that since elevated lactate concentration (lactic acidosis) increases with intense anaerobic exercise, endotoxemia might follow because lack of oxygen might compromise the intestinal wall. The presence of endotoxemia in these horses didn't correlate with performance during the ride. However, horses with better times had increased TxB2 and PGF1-alpha concentrations, and decreased TNF-alpha concentrations.

Barton concedes that there is still no clear relationship between endotoxemia and performance in endurance horses. However, the presence of endotoxin could become disastrous when endurance horses perform in poor physical condition or become heat-stressed or otherwise over-exerted.

Barton is currently continuing her endotoxemia research on with a focus on colic and septic foals. There is a link to current research projects at www.vet.uga.edu/LAM/.

Barton, M.H.; Williamson, L.; Jacks, S.; et al. American Journal of Veterinary Research, 64 (6), 754-761, 2003.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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