Odds On Racing - Harness Racing

Why The Muscle Enzyme Test is Important in Standardbreds
By Kimberly A. Rinker

Standardbred trainers will routinely have veterinarians pull blood on their horses in order to monitor blood counts, and other chemical levels within an equine athletes system. Muscle enzyme levels are a part of this simple blood test/chemistry profile, and are used to help veterinarians with their diagnosis and treatment of a patient.

Typically, there are two muscle enzymes whose levels can be read by a testing laboratory; aspartate aminotransferase (AST-also known as SGOT or GOT) and creatine kinase (CK). Because of the differences in laboratories, the machines used in the testing and the people who operate the machines, levels vary slightly from lab to lab, although the reference ranges will generally be provided along with the results of the test.

Veterinarians who want to study a horse's AST and CK levels, are in most cases trying to find out if a horse has suffered from tying-up, or in technical terms, Equine Rhabdomyolysis Syndrome.

"By studying these enzymes, veterinarians are basically trying to assess whether or not the horse has tied up," Dr. Duane J. Wilcox, D.V.M., who practices on the Chicago harness racing circuit, said. "The AST and CK levels are very close, however the rate at which they rise and fall is usually dramatically different, and this is how we're able to diagnose how critical the situation is for the horse."

The scenario works like this: the veterinarian will pull a vial of blood from the horse and send it out to the laboratory. Most veterinarians prefer to pull blood on the horse while the horse is in a resting state, for instance, early in the morning, after breakfast but before exercise.

At the nearest laboratory the blood is tested and the results are presented to the trainer the next day. The results will show the chemistry that was tested (i.e., usually 15 or more separate entities, including: red and white blood count, AST, CK, Glucose, Phosphorous, etc.), the results and units (amounts), the reference ranges (giving the highs and lows of where the levels should fall), and any remarks about the test (if the range was higher or lower than the normal levels).

The typical ranges for the AST levels are from 500 and lower, while CK levels can vary from 200 to 500.

"These are just basic parameters that we, as veterinarians, use to monitor a horse's system," Wilcox, a 1986 Purdue University graduate, said. "By monitoring the times of elevation of the AST and CK levels, a veterinarian can determine if the case of tying-up is the early or late stages of the syndrome.

"For instance, if both the CK and AST are elevated and the CK is higher, then the horse is in the acute phase of tying-up," Wilcox noted. "If the reverse is true, the horse is in the recovery stage of the tying-up syndrome."

When tying-up, a horse's AST levels peak after about 24 hours and have a half-life of seven to eight days, while in contrast CK levels peak within two to six hours and have a half-life of around two hours. These differences are what veterinarians use to help evaluate the stage of muscular damage and thus, treatment.

"Obviously, if the condition is in the acute stage, we treat it more aggressively," Wilcox noted. "The horse probably shows outward signs of stiffness and is quite uncomfortable. I would recommend an immediate treatment of an anti-inflammatory, such as Banamine and a muscle-relaxant such as Robaxin. I would also treat that horse with IV electrolyte fluids and some DMSO."

Wilcox said he recommends that trainers give the above as a daily treatment prior to exercise.

"You are trying to flush the lactic acid out of their muscle mass," he said. "By treating the horse with fluids and giving them their comfort level back, you allow them to flush their systems clean."

In healthy horses a number of factors can influence the extent of an increase in CK and AST levels directly following exercise. These factors include the intensity and duration of physical activity, as well as fitness, gender, age and diet of the horse. Clinically, lactic acid is the culprit. When the lactic acid fails to be removed from the bloodstream at a normal rate, this causes the muscle fibers to swell, which in turn causes poor mobility; the outward appearance of a stiff and jolted movement. Why is the lactic acid failing to be removed at a normal rate? Because there has been an accumulation of glycogen in the muscles during rest, (usually, but not always due to a rich carbohydrate diet). This gylcogen then gets rapidly used up during exercise, causing a marked increase in the production of the lactic acid.

"One thing that trainers always look for in a horse with elevated AST and CK levels is the darkness in the urine." Wilcox said. "In the past, this discoloration was though to be blood by some, but actually it is the myoglobin, which is a by-product of muscle breakdown, which gives the urine its dark color. Myoglobin in the muscles is akin to hemoglobin in the blood. Flushing the system with electrolytes will eventually flush out this discoloration."

A diet rich in carbohydrates and low in exercise can contribute to elevated AST and CK levels, Wilcox stressed.

"Besides the carbohydrate overload, another typical cause of tying-up occurs in cycling fillies," he said. "I don't know really if it is a hormonal thing or not but for whatever reason fillies seem more predisposed to a bout with tying-up when they're cycling."

Other factors which can contribute to tying-up include vitamin E and selenium deficiency, hormonal disturbances and thyroid imbalances, electrolyte imbalances and temperament.

Harness horses that have chronic tying-up bouts typically have their AST and CK levels tested every seven days, Wilcox said.