MUSCLE DISORDERS

THE CLINICAL PICTURE

1. You finally finish tying the last pack while your friend has been circling on his four year old filly.  This is their first mountain trip and the nervous filly dug a big hole at the hitching rack while the packs were being tied.  The string has just left the trailhead when your friend calls out that his filly is sweating and is walking funny behind.
2. It has been an uneventful trip into the wilderness taking most of the day.  The horses and mules were turned loose in the meadow while you enjoyed the first camp meal.  At dusk you stroll out into the meadow and find your saddle horse standing away from the other stock that are quietly grazing.  As you approach, you notice he seems distressed, has flared nostrils, and is quivering.  When you try to move him, he will not walk.
3. The last day of the trip you leave the canyon floor at midday and work your way up to the trailhead.  It is warm and the creeks you crossed last year are dry.  By late afternoon you notice the third pack horse is lagging behind so you change his position and take him on lead.  At the trailhead you unsaddle the stock and offer them a drink in the packing corral.  The horse you led won't drink, is breathing fast, and seems depressed.
4. The first stop on the trip is about two hours out.  The trail is mostly downhill and the going is easy.  The horses get restless after having drank at the spring and you tell your friends to get aboard to continue.  As you leave the spring one rider falls behind.  You stop to see if there is a problem and he says his horse is acting strange and doesn't want to walk.

There are several conditions which involve muscle function in horses.  These conditions include: tying up, azoturia, Monday morning sickness and exhaustion.  Muscle inflammation (myositis) or as it is technically termed "rhabdomyolysis" occurs as a result of a number of predisposing factors.
 

DIET / EXERCISE - The typical case is one of horses that have irregular exercise while being maintained on high energy rations.  Symptoms most often occur when work is resumed after a period of rest.  Significant changes in the duration or amount of work performed may result in myositis particularly in horses not conditioned for such work.

DISPOSITION / HORMONE FACTORS - Nervous or "high strung" horses are more commonly affected.  There also seems to be a higher incidence of myositis in fillies or mares as compared to geldings.

GENETIC FACTORS - Heavily muscled horses and horses with a high percentage of "fast twitch" muscle fibers have a greater myositis risk factor than others.

PREVIOUS HISTORY - Horses that have been affected with myositis are often prone to reoccurrence of the disorder.

OTHER FACTORS - Such as environmental temperature, humidity, load, and stress all influence the horses ability to maintain hydration and normal muscle function.

Regular exercise patterns and appropriate conditioning prior to work are important.  Reduction in high energy rations during period of non-activity will reduce the risk of myositis.  In some regions, Vitamin E and trace element Selenium supplementation has been useful in reducing the incidence of muscle disorders.

Electrolytes (trace elements in body fluids) particularly sodium and potassium are involved in normal muscle function.  The mineral content of the ration may require consideration of electrolyte supplementation.

Hydration (adequate body water) is essential for normal blood circulation to muscles and is crucial factor in muscle metabolism.

Some nervous horses will benefit from a low dose of tranquilizer prior to activity.

CLINICAL SIGNS

Horses suffering from muscle dysfunction offer a wide array of symptoms.  A change in disposition i.e. expression of anxiety, excess sweating, or failure to sweat, stiffness, reluctance to walk out may progress to muscle quivering or muscle cramping, pain, increased heart and respiratory rate and elevated body temperature (1030 - 1050F).  Severely affected horses may knuckle or go down and unable to rise.  During the period of muscle damage, products from muscle damage break down may appear in the urine giving it a dark brown color.  There may be secondary kidney damage from the flood of degenerating muscle products into the blood stream.

Myositis can be confused with colic, pain from improperly fit tack (i.e. pressure or chafing injuries), and lameness from other causes.  Evaluation of the horse's attitude, and vital signs, (pulse, respiratory rate, capillary refill time, temperature and palpation of muscles) is of benefit in determining the severity of the condition.

TREATMENT

Astute judgement is necessary to determine if continued exercise is advisable.  Simple muscle spasms or cramps may respond to walking if little or no muscle damage has occurred.  However, continued exercise of the horse with myositis can lead to even more severe muscle damage.  If there is any doubt as to what the condition is - stop any further exercise.  Horses with mild cases of myositis that are well hydrated may benefit from low doses of tranquilizer to reduce anxiety.  Non steroidal anti-inflammatory drugs such as phenylbutazone, dipyrone or banamine give good symptomatic relief Horse that are dehydrated need ample water to restore body fluids.  Concentrated oral electrolytes and tranquilizer are not indicated in dehydrated horses.  Elevated body temperatures can be reduced with water sprays or showers.  Standing in running water is of benefit.  Very cold water should be avoided over the heavy muscled areas as it may increase cramping.

In severe cases, prompt medical attention is necessary.  Large volumes of special electrolyte solution may be given intravenously or by stomach tube.  Relief of pain and distress may require frequent treatments with muscle relaxants and analgesics.

Horses that recover from an episode of myositis should be allowed to rest and re hydrate for at least 24 hours prior to traveling.  Even then, careful judgement is necessary to determine the level of activity or stress the horse can endure.