Red means stop

Discolored urine, common in performance horses, often a reason for concern
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Jul 01, 2008

In the Dr. Seuss classic Go Dogs Go, we learn a very important lesson that remains with us throughout our lives: Red means stop.


Photo 1: Urine from a heavily muscled horse that developed exertional rhabdomyolysis at an endurance ride. The deep red color is being produced by myosin pigment cleared through the kidney and into the urine. This horse required intravenous fluid therapy for recovery. Less serious hematuria might also look like this, but the clinical signs would be helpful in determining the correct diagnosis.
It is a very simple concept, and surprisingly is just as applicable to real horses performing in athletic competition as it is to storybook dogs racing cars around the unusual landscapes of Dr. Seuss's books.

Red means stop, and the presence of red-colored urine in an equine athlete generally is a cause for concern. Bloody or discolored urine actually is one of the most common findings among equine and human athletes. This condition is called exercise-induced hematuria; according to Dr. Robert Gambrell, a sports medicine physician in Augusta, Ga., "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."

Incidence in humans and horses


Photo 2: This is a series of urine samples (earliest on the left and most recent at right) from another horse that tied up at an endurance event. This horse has been on intravenous fluids and the samples show the value of "flushing" the dangerous myoglobin molecules from the kidneys. These horses are considered at risk until the urine again appears clear and light yellow.
The incidence of exercise-induced hematuria in humans is between 11 percent and 100 percent, depending on the type and amount of exercise and the athlete's state of hydration. It has been known as sports hematuria, runner's hematuria or 10,000-meter hematuria, and was first reported in humans in 1700.

The number of sports and activities associated with discolored urine in humans is very large and includes everything from traditional football, hockey and boxing to swimming, track, lacrosse, soccer and even snowmobiling, bike riding and rowing.

The incidence of exercise-induced hematuria in horses is much harder to determine because the actual discolored urine often is not observed, depending on the nature and duration of the particular equine sport.


Photo 3: Strenuous exercise is associated with hematuria in equine athletes and its severity is associated with the type of activity, speed involved, length of time competing and the hydration status of the horse. Attention to all of these factors will help reduce or eliminate hematuria in these horses.
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. Hal Schott, David R. Hodgson and Warwick Bayly of the Department of Clinical Sciences, College of Veterinary Medicine at Washington State University, reported that grossly discolored urine was noted in 100 percent of horses exercising on a treadmill at speeds using both 60 percent and 95 percent of the maximal oxygen consumption (V02max).

Seven out of eight horses exercising at only 40 percent 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.


The colors: what they mean
Exercise-induced hematuria is classified either as gross (visible in the urine by eye) or microscopic (clear urine but presence of red blood cells noted on testing). Microscopic hematuria is by far the more common, again leading to probable under-recognition in equine athletes. The cause of sports hematuria is not well understood, however, and might possibly be different depending on the type and duration of exercise.