Is Your Horse a Weekend Warrior?

by R. Paul Schwab DVM

Since this is the recreational riding issue as well as the beginning of the “nice” time of year around here, I thought I would remind everyone about “tying up” in horses. Just like us, horses don’t do well with a sudden return to heavy exercise without some sort of pre-conditioning. Exercise, as well as nutrition, is extremely important.

Tying up is a term that is frequently used to describe horses that, after some exercise or exertion, tend to get severe muscle cramping and/or stiffness. Horses that are tied up may show anything from slight lameness (usually stiff or shortened stride) to an inability to move or even get up. It is generally most noticeable in the hind limbs and back. The muscles may appear swollen and are often hard and painful to the touch. Other clinical signs such as sweating, elevated heart and respiratory rates, and looking back at the flank are also seen and can make it difficult to differentiate between tying up and colic, although both may occur together.

Medically Speaking
Tying up is the traditional layman’s term for Equine Rhabdomyolysis Syndrome (ERS). Other terms for this condition include azoturia, set fast, Monday morning disease, and exertional rhabdomyolysis. We will refer to it as ERS for the remainder of the article.

Diagnosis of ERS is mostly based on clinical signs and results of treatment, although your veterinarian can measure muscle enzyme levels from a blood sample to confirm the diagnosis. Also, muscle biopsies may be needed to help determine why the horse is tying up.

Treatment consists of stall rest (exercise may cause more muscle damage), Banamine or Phenylbutazone, and fluids. Grains should be avoided during the treatment period. In horses that are reluctant to move, a veterinarian should be called. Severe muscle damage releases myoglobin into the bloodstream, which at high levels is toxic to the kidneys. Horses with this condition may have dark brown urine or no urine at all. Intravenous fluids will help clear the myoglobin and other toxins produced by the muscle damage from the bloodstream.

Blood work can also be an important indicator of prognosis and guide for determining when to start exercising again. Your veterinarian may want to repeat a blood sample after the horse is better to see if muscle damage is still occurring.

Causes and Types of ERS
There are numerous theories about why some horses are more prone to this condition than others. Nutrition and genetics play a key role. The two heritable forms of ERS are Polysaccharide Storage Myopathy (PSSM) and Recurrent Exertional Rhabdomyolysis (RER).

· Horses with PSSM have a problem with sugar storage in their skeletal muscle fibers. Breeds predisposed to PSSM include Quarter Horses, Paints, Appaloosas, and draft breeds. The episodes are usually seen in unfit but good body-conditioned horses at the beginning of training or after a long or hard ride.
· Horses with RER tend to be Thoroughbreds, Standardbreds, or Arabians. Occurrences may be the result of a stress-related disorder of muscle calcium regulation. The classic RER horse is a fit young filly with a nervous temperament. In contrast to PSSM horses, RER episodes tend to increase as fitness increases.

The two factors that tie both of these forms of ERS together are lack of routine exercise and too much starch in the diet.

Feeds with high starch and sugar levels include most grains (corn, oats, and barley) and molasses, and to some extent lush spring grass. High starch and sugar diets result in rapid rises in blood glucose and insulin that seem to be responsible for exacerbating ERS in horses with PSSM. (As a side note, this same type of diet may be responsible for some Developmental Orthopedic Diseases as well.) Rapid rises in blood sugar can also encourage aberrant behavior in horses.

ERS Management
Control of ERS includes diet and exercise management. Reducing starch and sugar content in the feed is of utmost importance. Good quality forage (hay or pasture) should be fed at the rate of at least 1.5% of body weight per day. If extra weight or energy is needed, feeds with low soluble carbohydrates should be used (beet pulp, soy hulls, alfalfa pellets, rice bran, or canola oil). Current research is showing that feeding fats or oils with grains may stabilize the glucose and insulin rises that occur with grain diets. Feeding small meals more often will also level out the glycemic response.

Your veterinarian can help recommend a diet based on your horse’s activity level and body condition. Vitamin E and Selenium supplementation may be important as well.

Exercise is just as--if not more--important for management of ERS. Daily exercise is recommended and stall confinement should not exceed 12 hours per day. Pasture turnout is best. Thoroughbreds and other breeds with exertional rhabdomyolysis are usually quite fit already, but will respond to exercise and stress management such as pasture turnout, companions, etc.

Resources:
Harris, Pat, “Equine Rhabdomyolysis Syndrome,” Current Therapy in Equine Medicine 4, 1997 pp. 115-121
McKenzie, Erica, et.al, “A Review of Dietary Fat Supplementation in Horses With Exertional Rhabdomyolysis,” AAEP Proceedings 2002, pp. 381-386