By Kenneth Marcella, DVM

In the Dr. Seuss classic beginner book, “Go, Dog. Go!” we learn a very important lesson that remains with us throughout our lives: red means stop! It is a very simple concept and as surprisingly applicable to storybook dogs racing cars around the unusual landscapes of Dr. Seuss books as it is to real horses performing in athletic competition.

Red means stop and the presence of red-colored urine in an equine athlete is generally a cause for concern. Bloody or discolored urine is actually one of the most common findings among both equine and human athletes.

This condition is called exercise-induced hematuria and, according to Dr. Robert Gambrell, a sports medicine physician in Augusta, Georgia, “Although most cases of discolored urine following strenuous exercise are mild and not associated with serious disease, hematuria in the athlete must still be differentiated from other potentially more serious conditions.”

Dr. Martha Terris, assistant professor of urology at the Stanford School of Medicine, goes even further, warning, “Pink or red urine should prompt an immediate visit to your doctor.”

A common problem

The incidence of exercise-induced hematuria in humans is between 11% and 100%, depending on the type and amount of exercise and the athlete’s state of hydration. It has also been known as “sports hematuria,” “runner’s hematuria,” or “10,000 meter hematuria” and was first reported in humans in 1700.

Sports and activities associated with discolored urine in humans includes everything from football, hockey and boxing to swimming, track and soccer, and even includes snowmobiling, bike riding and rowing.

The incidence of exercise-induced hematuria in horses is hard to determine because the actual discolored urine is often not observed, depending on the nature and duration of the equine sport. Abnormal color of a normal bodily fluid is understandably much more noticed and reported in humans.

Research studies done in horses, however, do show a very high correlation with the incidence of exercise-induced hematuria seen in human athletes. Drs. Schott, Hodgson and Bayly of the Department of Clinical Sciences, College of Veterinary Medicine at Washington State University, reported that grossly discolored urine was noted in 100% of horses exercising on a treadmill at speeds utilizing both 60% and 95% of the maximal oxygen consumption (V02max).

Seven out of eight horses exercising at only 40% V02max still showed hematuria if the urine sample was centrifuged and examined via reagent strip analysis to detect the presence of red blood cells. These researchers concluded that exercise-induced hematuria (along with significant proteinuria) is very common in equine athletes as well, even if gross hematuria is not observed.

Exercise-induced hematuria is actually classified as either gross (visible in the urine by eye) or microscopic (clear urine but presence of red blood cells is noted on testing). Microscopic hematuria is by far the most common type, again leading to probable under-recognition in equine athletes.

Five causes of hematuria

The cause of sports hematuria is not well understood, however, and may possibly be different depending on the type and duration of exercise. Five general causes have been proposed that address the variations in sporting activities and anatomical location for red blood cell loss.

Traumatic renal hematuria is thought to occur from damage to the renal vasculature. This trauma may be from a direct blow or contact (hockey, boxing and football in humans and polo or rodeo sports in horses) or from indirect trauma from shaking and jostling (all types of racing, endurance, jumping, reigning, cutting, driving and most other equine activities).

Non-traumatic renal injury is thought to occur because of hypoxic renal nephron damage due to reduced kidney blood flow. “During exercise, blood is preferentially shifted to the skeletal muscles, heart and lungs,” explains Dr. Gambrell. This shifting of blood volume results in a decreased renal blood flow which is in proportion to the intensity and duration of exercise and affected by the hydration status of the athlete. Studies have shown that swimmers and runners in shorter events have much less hematuria than those athletes competing in longer distance events.

“Decreased renal blood flow results in hypoxic damage to the nephron which increases glomerular permeability,” according to Dr. Gambrell, “which theoretically allows excretion of red blood cells into the urine along with significant protein leakage.”

Trauma to bladder lining. Another possible cause of exercise-induced hematuria is trauma to the lining of the bladder. As the human or equine runner moves, the bladder is constantly flapping forward and back and bruising can occur. The anatomy of the horse is somewhat different from human athletes, making horses more capable of running with reduced bladder trauma over the short distances required to elude predators.

The horse, however, was not designed to run for the distances required with endurance sports or to repetitively jump large obstacles. These sports-related movements likely stress the bladder lining in the horse, causing bruising and leading to hematuria (also know as “bongo drum hematuria”).

Prostratic or urethral issues. Hematuria has also been reported due to prostatic (not found in the horse) or urethral origin in humans, but these causes are not believed to be a significant factor in horses.

Myoglobinuria is the other significant cause of urine discoloration in the performance horse. Myoglobin is the major portion of the muscle sarcoplasm and functions as an oxygen storage molecule. Horses experiencing muscle damage from sprains, strains, and tears all the way to complete metabolic muscle dysfunction in a case of exertional rhabdomyolysis (tying up) will show varying degrees of myoglobinuria as this pigment is released from damaged muscles and cleared through the kidneys into the urine.

Tying-up horses typically show a reluctance to move along with hard, swollen muscles of the back, rump, hips and upper rear legs. The urine from horses with exertional rhabdomyolysis is dark brown to red to coffee-colored. Discolored urine of this nature, along with the typical clinical signs of tying up, should always be treated seriously. Myoglobin is damaging to the kidney and often untreated exertional rhabdomyolysis cases develop kidney failure. Fluid therapy is crucial in these cases along with appropriate pain relief, anti-inflammatory, vasoactive drugs and supportive care.

Treatment and prevention

Fortunately, most cases of exercise-induced hematuria resolve within days of cessation of exercise and do not develop into more severe conditions in both humans and horses. Bladder infections, kidney and bladder stones, various cancers and other serious conditions can also cause red urine, so persistent cases of red urine must be treated seriously. Affected horses should be subjected to a complete diagnostic workup including blood work, urine analysis, cystoscopy, and radiographs.

Recommendations for reducing the incidence and severity of exercise-induced hematuria include maximal hydration before exercise to ensure a full bladder which will help to reduce bladder wall trauma and keep renal blood flow as nearly normal as possible.

Better fitness and conditioning can also reduce the severity of hematuria since an efficient cardiovascular system and non-fatigued muscles require less blood flow, which protects kidney function.

Veterinarians working endurance rides and other equine competitions are often placed in a difficult position when presented with a horse with red urine. A horse experiencing simple exercise-induced hematuria is not in distress and could realistically continue to compete without risk. Research would suggest that the vast majority of equine athletes already routinely experience some degree of exercise-induced hematuria whether riders or veterinarians actually observe or document it.

A horse exhibiting red urine due to myoglobinuria, however, is in danger and should be removed from competition and treated aggressively. Muscle enzymes, urinalysis and clinical examination should help the clinician differentiate between these two conditions.

Spinning the urine sample is helpful as well. Free hemoglobin produces a pink serum which will be positive for red blood cells when tested with a strip assay. “Myoglobin is cleared more efficiently by the kidneys leaving a clear serum which will test negatively,” says Dr. Terris.

When in doubt, it is better in these situations to err on the side of caution. Clinicians need only remember those lessons you learned early on: red is for stop!


Colorless: highly dilute, recent fluid consumption

Pale yellow: normal

Yellow: normal

Amber: normal, concentrated urine

Deep yellow: concentrated sample, liver problems (jaundiced or icteric), riboflavin, dehydration

Orange: some antibiotics (rifampin), phenacetin, sulfasalazine, bilirubin, increased Vitamin C, Increased carrot intake, riboflavin, dehydration

Pink: hemoglobin, myoglobin, phenytoin, phenothiazines, phenolphthalein (laxative in ExLax) chronic lead poisoning, chronic mercury poisoning, beet pigment, rhubarb

Red: hemoglobin, myoglobin, phenytoin, phenothizines, lead poisoning, mercury poisoning, beets, rhubarb, ExLax, blackberries, porphrin, bladder infection, bladder or kidney stones, excess B vitamins

Green: oxibilirubin (biliverdin), Clorets, pseudomonas bacterial infection

Blue: high calcium, methylene blue dye, indomethacin, cimetidine

Brown: bilirubin, myoglobin, hematuria, methemoglobin, liver disease, copper poisoning, fava beans, rhubarb, furazolidin, nitrofurantoin methocarbomol, metronidazole, cascara laxatives, sorbitol

Gray: furazolidone, nitrofurantoin

Black: melanin, malogen, L-dopa

White: phosphates, increases white blood count, massive infection

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