Azoturia, new thoughts on an old condition

Azoturia is a problem that affects the muscles of many horses. The signs associated with the condition range from stiffness and mild cramp to the horse becoming unable to move at all.  Other terms that have been used for the problem are Monday Morning Disease, Tying-Up, Myositis and Setfast. It is unlikely that a single process can explain all the clinical types, but the term Equine Rhabdomyolysis Syndrome is probably the most accurate description. 

Azoturia can affect any horse of any age but is much more common in fillies and mares than geldings and stallions. This condition affects all breeds and sizes of horses and is traditionally seen in animals that have been exercised following a period of rest without a reduction in feed intake, hence the old name called Monday morning disease as most work horses were never exercised on a Sunday. It is particularly seen in horse on a high carbohydrate diet (in particular horses feed straights (barley and oats).  The signs develop particularly at the end of strenuous exercise and the horse may pull itself up or the rider becomes aware that the horse is reluctant to go forward.  Forcing the horse to move only exacerbates the problem as it causes further muscle damage. Horses suffering from azoturia have intense muscle cramps and pain and are reluctant to move.  In extreme cases horses can have such severe muscle tissue damage that breakdown products from the muscles result in severe (occasionally irreparable) kidney damage.  The products of muscle damage cause the urine often to have a red tinge.  The amount of muscle damage is usually assessed by a blood sample and the levels of two enzymes (CK and AST) are an indicator of the amount of damage.  The same enzymes can also be monitored to assess how the muscles are repairing. 
Following work in the USA over the last 8-10 years it has become recognised that some horses suffering with azoturia have the condition Equine Polysaccharide Storage disease Myopathy (EPSM).  EPSM has recently been reported in the UK (ref 1).  EPSM in the States has particularly been identified in draft horses however other breeds are also affected, breeds noted to get EPSM are Belgians, Percherons, Clydesdales, Shires, Haflinger, Norwegian Fjord, Quarter horses, Appaloosa, Irish Draught, Friesian, and draft crosses.  Pedigree analysis of horses confirmed with EPSM support a familiar (hereditary) basis for the condition.  A similar familiar basis has been suspected in Appaloosa and Appaloosa cross horses in the UK (ref 3). The signs of EPSM are not always associated with a lot of exercise nor after a period of inactivity, they can come on after a few minutes of mild exercise.

A diagnosis of EPSM is confirmed following histopathology of a muscle biopsy from either the semimembranosus or semitendinosus muscles to the side of the tail.  Using special stains it is possible to examine the muscle cells and horses with EPSM have very high levels of glycogen and polysaccharides in the affected cells .  A laboratory has recently been set up at the University of Edinburgh to investigate equine nerve and muscle problems like EPSM.

Providing a high fat, low carbohydrate diet is the most important part of treatment. There are now proprietary foods available with high fat concentrations.   Selenium and Vitamin E may help reduce the chance of further attacks due to their anti-oxidant affect although their effect has been over-rated in the past.  Either grass or legume (alfalfa) hay.  It is also critical that horses with EPSM get as much daily turnout and as much regular work as possible. Because the muscles are loaded with excessive carbohydrates it often take many months of the high fat diet for the signs to decrease.  
If you suspect that your horse is suffering from EPSM contact the Hospital for further advice.

References
1. McGowan C, Menzies-Gow N, McDiarmid A, & Patterson-Kane J. Vet. Record 152. 109.
2. Valberg S, Geyer C, Sorum S, & Cardinet G.. Am. J. vet. Res.  57. 286.
3. McDiarmid, A. (2005).   Vet Record. 156. 95.