Five general causes have been proposed that address the variations in sporting activities and anatomical location for red
blood cell loss.
1. Traumatic renal hematuria is thought to occur from damage to the renal vasculature. This may be from a direct blow and 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).
2. 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 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 Gambrell, "which theoretically allows excretion of
red blood cells into the urine, along with significant protein leakage."
3. 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,
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, causing bruising and leading to hematuria (also known as
Hematuria has 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.
Fortunately, most cases of exercise-induced hematuria resolve within days of cessation of exercise and do not develop into
more severe conditions.
4. Bladder infections, kidney and bladder stones, various cancers and other serious conditions can cause red urine, so persistent
cases of red urine must be treated seriously and those horses should be subjected to a complete diagnostic work-up, 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 helps reduce bladder-wall trauma and keep renal blood flow as nearly normal as possible. Better
fitness and conditioning can reduce the severity of hematuria because an efficient cardiovascular system and non-fatigued
muscles require less blood flow, protecting kidney function.
5. The other significant cause of urine discoloration in the performance horse is myoglobin which 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, because 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, always should be treated seriously. Myoglobin is damaging to the
kidneys 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.
Tough call for practitioners
Veterinarians working horse shows, three-day events, endurance rides and other equine competitions often are 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 owners, trainers, 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
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 Terris.
When in doubt, it is better in these situations to err on the side of caution. Clinicians need only remember those lessons
learned early on: Red means stop.