This is also known as Azoturia, Monday Morning Sickness, Tying - Up and Set Fast and it affects the horses muscles causing them to harden and cramp up, it can vary in its severity from mild cramps to the horse being unable to move.
Causes of Equine Rhabdomyolysis
The most common cause of Azoturia is from a carbohydrate overload combined with time off. This used to be very common with draught horses who would have Sunday off and then have an episode of Azoturia on the Monday, hence the term Monday Morning Sickness. The glycogen in the hard feed builds up in the muscles and produces an excess of lactic acid to be produced when the horse resumes work, the lactic acid in turn damages the muscle tissue and subsequent blood flow to the muscles. As the Lactic Acid cannot be dispersed quickly enough it causes an episode of Azoturia to occur.
These can vary depending on the severity of the attack.
The use of pain killers and anti inflammatory drugs are used initially to make the horse more comfortable Fluids are often administered to help flush out the toxins. The horse should then have box rest followed by gentle exercise and a new feeding program that is appropriate for the level of work. If you are riding when the horse develops symptoms then dismount immediately and place rugs over the horse to keep the horse warm
If you are able put them directly into a warm stable with plenty of bedding and water. If you are away from the yard then organize transport home as quickly as possible you do not want to ride them as this will only further damage muscles and cause more pain.