Monday Morning Sickness (or disease) is an old slang term used for a condition properly known as Azoturia. In medical terminology it is referred to as Exertional Myopathy. It is a potentially fatal condition that makes it very difficult and painful for the horse to move and can cause failure of the kidneys.
Azoturia is most commonly found in horses that are heavily exercised on a regular basis, and then not worked for one or two days but still fed the same amount of food. This results in glycogen (a carbohydrate) building up in the muscles. When the horse returns to work, this glycogen breaks down quickly and produces an overload of lactic acid. The lactic acid damages skeletal muscles, releasing muscle enzymes and myoglobin, which can lead to acute kidney failure.
The name Monday Morning Sickness came from farmers who recognized the condition in their plow horses when they went back to work after having the weekend off. About fifteen minutes to an hour into their work on Monday morning, the horses would become anxious, sweat profusely, and their pulse would begin to race. Then all of the major muscles would begin stiffening and the horse’s hind end would stagger and wobble. Sometimes they would completely collapse.
Tying-up is a milder form of the condition that is usually seen in race and performance horses after a hard workout. While cooling down, the muscles stiffen and tremor. The muscles of the hindquarters are particularly tense, hard and painful. The pain causes anxiety and sometimes sweating.
It is important to be aware of azoturia and tying-up because sometimes the signs of discomfort may resemble those of colic. However, while the common treatment for colic is to keep the horse up and walking, such treatment for a horse suffering from azoturia or tying-up would be fatal.
If you suspect your horse is having a case of azoturia or tying-up, immediately stop all activitie. Do not try to return the horse to its stall. Any movement will make the condition worse. Talk quietly and try to keep the horse as calm as possible. Cover him with a blanket and get veterinary assistance immediately. It is okay if the horse lies down, but it is best if he does not lie flat on his side.
Horses that get quick veterinary assistance, remain standing or lying down without stretching out on their side, and regain their normal pulse rate within 24 hours have a very good chance of a full recovery. The longer the pulse remains high, and the longer the horse remains lying on his side, the less likely he will recover. Once a horse recovers from this condition, he is prone to future attacks.
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