Tying Up in Horses
Exertional rhabdomyolysis, most commonly known as ‘tying up’ or ‘Monday morning disease’, is a syndrome involving the breakdown or destruction of skeletal muscle cells associated with exercise.
Some horses tie up sporadically due to factors in the environment, such as an increase in training level, dietary in-balance or simply overeating.
On the other hand, some horses can suffer repeated or chronic bouts of tying up.
What are the symptoms?
– Increased heart rate and respiratory rate
– Increased sweating
– Muscle twitching
– A reluctance to move/lameness
– Firm, painful muscles over the back and croup
– Dark coloured urine (due to myoglobin release from damaged muscle). Only in more severe cases.
So what causes it?
Sporadic exertional rhabdomyolysis may occur due to a horse training beyond its athletic ability, causing muscle damage and electrolyte deficits. This can occur in any breed.
Recurrent exertional rhabdomyolysis is thought to be caused by abnormal calcium levels within muscle cells during contraction. This is seen primarily in Thoroughbreds and Standardbreds.
Polysaccharide storage myopathy (PSSM)
Excessive storage/accumulation of polysaccharides inside muscle cells. Muscle cells take up glucose after a meal and store it as glycogen as a back up energy source. There are two types of PSSM; type 1 and type 2.
Horses with type 1 PSSM have an excess of glycogen within the muscle tissue due to a mutation in the GYS1 gene. This mutation means that the glycogen synthase enzyme is far more active than it should be, storing more glycogen than the muscle tissue actually needs. Muscle biopsies show abnormally appearing glycogen in these cases. This primarily affects Percheron’s, Belgian’s and some Quarter horses.
PSSM type 2 horses also have abnormal muscle biopsies, however, lack the GYS1 mutation. Microscopy shows a disruption of the alignment of the contractile proteins within the muscle cells. This primarily affects Warmbloods.
Things to consider
Blood tests to evaluate the serum activities of creatine kinase (CK) and aspartate aminotransferase (AST) (muscle proteins contained within muscle cells and released into the blood during damage) will aid in the diagnosis of tying up. It is important to be aware that the levels of CK in the blood increase rapidly, with the highest values being recorded at around four to six hours after onset, therefore if you don’t get them tested early enough, you may have missed the peak. However, AST values increase and decrease much more slowly up to seven days to return to normal, therefore it may still be worth a blood test later on, even within a week of suspected episode in.
Genetic testing via blood or hair roots for PSSM1 is a good place to start in horses with chronic tying up that are among breeds known to carry the GYS1 mutation.
With PSSM2 horses, diagnosis may be a little more tricky. Clinical signs such as subtle lameness, poor performance, loss of topline, sensitivity to grooming and changes in behaviour can all be indicative of an array of different conditions, therefore it is important to rule other more common causes out before going down the route of muscle biopsy.
Management of the disease
Research has shown the management for all tying up, regardless of the cause, can significantly improve welfare for these horses.
A customised diet that tricks the muscle into burning the excess glycogen instead of storing it, and is able to utilise fat as an alternative energy source, is the right way to go. The four key nutritional goals are;
1. Offer hay that is less than 12% in nonstructural carbohydrates.
2. Offer oil or rice bran if horses require additional energy. Ensure you have both omega-3 and omega-6 fatty acids, as omega-3 has great anti-inflammatory qualities. Omega-3 can be found in flax or fish oil.
3. Offer a vitamin and mineral balancer.
4. Use a grazing muzzle when on lush pasture.
It is also important to implement a structured exercise protocol, including turnout to keep these horses moving (as long as he is not actively tying up).
If you require any more advice about tying up in horses, we will be more than happy to help via the messenger option on our Facebook page or by contacting the clinic directly: 01270 628888
E.J. Westwood. On behalf of NVG