This condition is also called equine rhabdomyolysis, azoturia, or Monday morning disease. It is an inflammation of the muscles. The signs are due to the destruction of a large number of muscle cells. Signs can appear during or just after work. The stride shortens and the horse stiffens and refuses to work or even walk. The muscles which form the buttocks and back are the most affected which explains the strange cramped position the horse adopts to try to relieve the pain. These muscles are often very hard upon palpation. The horse has high heart and respiratory rates due to pain and signs may sometimes look like colic (horse lying down). In the most severe cases, when the horse urinates, the urine can have a brown discoloration due to the presence of myoglobin in it. Myoglobin is present in large quantities in muscle cells; when muscle cells are destroyed, myoglobin is released into the blood stream and then eliminated by the kidneys into the urine.
Confirmation of diagnosis is made by dosage of muscle enzymes: creatine kinase (CK) and aspartate amino transferase (ASAT). These two enzymes too are present in large quantities in muscle cells and are also released into the blood stream when muscle cells are destroyed. Their dosage in the blood makes it possible to confirm the diagnosis and evaluate the extent of the injury: the higher the muscular enzyme concentrations in the blood, the higher the number of muscle cells destroyed.
Treatment must be started quickly to limit the lesions, relieve the pain and help the kidneys to eliminate the toxic waste into the blood stream without being harmed themselves. For this, a veterinarian is needed. The first thing to be done is to put the horse in a large deeply-bedded stall and to let it rest. Do not try to move the horse!
A perfusion is often needed: the vet inserts an intravenous catheter into the jugular vein and administers several litres of isotonic fluid over a few hours to help the kidneys eliminate the toxic waste, particularly myoglobin. Myoglobin is a very large molecule that can injure the kidney when it passes through it to be eliminated. If the horse doesn’t urinate by itself after the intravenous drip, the vet can administer drugs to stimulate urine production.
The vet can also administer NSAI drugs to relieve the pain. However, NSAI drugs are toxic for the kidneys and kidney function must therefore be closely monitored.
In more benign cases, stall rest for a few days and NSAI drugs to relive the pain might be the only treatment needed. In more severe cases, on the other hand, lesions can be so widespread that they become life-threatening. A common complication of tying up is renal insufficiency due to the toxicity of myoglobin for the kidneys. However, if the horse is treated rapidly, the prognosis is good in most cases. Work must be resumed very progressively and subject to regular muscular enzymes dosages: it is generally recommended that the horse not be moved until CK values are back to normal.
A tying up is most frequently caused by an effort that is too intense for the horse’s training level, when the warm-up has not been sufficient, when the horse has resumed work after a resting period, or when the horse has received too much food for the type of activity it does. Some respiratory viruses including herpes viruses and equine influenza can increase the risk of tying up. More generally, a sick or lame horse is at increased risk of tying up and should not be trained. Some feeding mistakes can also promote tying up like giving food that is too rich in carbohydrates or deficient in antioxydants (particularly selenium and vitamin E) or in electrolytes.
In these cases known as sporadic, the horse should be treated, rested and the mistake identified in order not to be repeated: regular exercise, feed adapted to work intensity, progressive and sufficient warm up, adapted to the environmental conditions (particularly the temperature), steady progression in work intensity in young horses, sufficient rest after a viral infection, etc.
However, if several tying up episodes occur, possible individual predisposition should be considered.
Muscle cells stock energy as glycogen molecules. Glycogen is a large molecule made of small glucose molecules that the muscle cells uses when needed during effort. In some horses, muscle cells make an abnormal glycogen that they are unable to use when needed, which triggers tying up. This condition is called polysaccharide storage myopathy (PSSM) and it affects several breeds including Quarter Horses, draft horses, sport horses and thoroughbreds. PSSM-associated symptoms are variable depending on the breed and discipline of the horse, from back stiffness at work to severe and repeated tying up episodes.
The diagnosis is made upon identification of abnormal glycogen in muscle cells when observing a muscle sample under the microscope after a specific coloration. Recent studies have revealed that some PSSM horses presented a genetic mutation: an enzyme implicated in the glycogen synthesis process is abnormal. It is possible to diagnose these horses with a simple DNA test performed on a blood sample or some mane hair bulbs. However all PSSM horses do not have the mutation and muscle biopsy remains necessary for the diagnosis.
Another muscle disease is described in Thoroughbreds and Standardbreds: recurrent exertional rhabdomyolysis (RER). Horses suffering from RER are prone to recurrent episodes of tying up, particularly young and nervous fillies in training. In this condition, the problem originates in the intracellular calcium regulation, calcium being the triggering factor for muscle contraction. A genetic problem is strongly suspected but a precise genetic mutation has not yet been identified.
Besides the classical recommendations to prevent tying up, it is possible to reduce the risk of episodes in predisposed horses by adapting the food: it should contain little sugar (carbohydrates) and most of the energy should be brought by lipids (fat). This type of product is available in most horse feeding brands and is recommended both for PSSM horses (less energy stocked in glycogen) and RER horses (sugar increases excitability and nervousness which are contributing factors for tying up).
It is also recommended to supplement predisposed horses with vitamin E and selenium, two antioxidants that help protect the muscle from oxidative damage. Finally, regular exercise is of prime importance for these horses.
Some other (rare) causes of rhabdomyolysis exist, which are not linked to any effort. For example, in some horses with strangles, the immune system targets muscle cells by mistake. This triggers severe pain and a massive destruction of muscle cells.
A newly recognised form of rhabdomyolysis, not linked to effort, is called atypical myopathy. It is a toxic condition due to the absorption of seeds of Acer pseudoplatanus (also called Sycamore tree). Some seeds (probably the less ripe) contain large quantities of Hypoglycin A that brings on the destruction of muscle cells. Muscles implicated in standing and breathing are mostly implicated which explains why horses often lie down and have respiratory distress. The condition is fatal in 70% of cases. Most cases occur in the autumn when seeds are present on the ground and grazing horses do not have much grass left in their pastures. Young horses may be more vulnerable. Several research projects are currently trying to find a treatment.
Author : DAUVILLIER Julie, Veterinary.