Recommandations de Philippe Benoit
« Il faut être attentif à un suivi régulier des paramètres CK et AST, à un suivi nutritionnel drastique et à une bonne hydratation ... »
Myositis: prevention and nutritions
Myositis is a pathology that affects the muscles causing sudden stiffness and making it impossible to walk. The stiffness usually occurs after work whilst the horse is cooling down. Horses are more susceptible to this pathology in winter and in cold weather.
Animals in poor condition, with overweight or working beyond their level of fitness are also more at risk.
In terms of symptoms, the horse develops sudden, severe stiffness and signs of discomfort similar to colic, or abdominal pain. In sport horses, palpation will often reveal induration and spasms of the dorsal and gluteal muscles (croup). In most cases, the horse develops a tremor, moves more slowly and, when urinating, the flow may be irregular and very dark. This particular sign is known as myoglobinuria and is caused by muscle pigment (myoglobin) entering the urine.
Major causes
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The main known causes in sport horses include::
- Severe nutritional imbalance caused by a diet too heavy in starch (a complex sugar found in cereals) for a moderately active horse. Any form of dietary excess (e.g. proteins) is also a risk factor.
- A mismatch between the physical fitness of the horse and the work required, or insufficient warming up.
Riders sometimes refer to “Monday morning disease” because horses that undergo strenuous exercise during the weekend can suddenly find themselves at rest on the Monday and develop these signs.
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Physiological explanation
During muscle contractions, there is a build-up of lactic acid (a bit like the exhaust fumes of metabolism) which, when it stagnates in the muscle cells, can destroy some of them causing the release of this internal pigment known as myoglobin.
Horses suffering from myositis suffer a period of dehydration and need a “detox”. The aim is to drain the lactic acid and “rinse out” the muscles and kidneys.
Beyond the scientific terms, this requires rapid muscle perfusion to restore normal hydration and electrolyte balance. It is important to monitor two key blood parameters, which are markers of muscle and liver function:
- Aspartate aminotransferase, or AST or ASAT
- Creatine kinase, or CK
There is a significant increase in these substances after myositis. The normal laboratory range for CK is 150–250 IU. However, following myositis it can reach several thousand IU.
This level can rise very quickly (in the space of a few hours), but with appropriate treatment it returns to normal in a few days. AST takes longer to rise but can remain at an elevated level for several weeks in cases of severe or complicated liver disease. It is also the parameter used to determine whether the horse can resume a more intense level of work.
In practical terms, how to react?
Given the above, horses at risk or those who have already shown signs of myositis must be monitored carefully for:
In practical terms, for horses prone to muscle stiffness and myositis, I recommend:
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- giving a measured diet, calculating the exact energy requirement based on the horse’s work and weight in order to prevent excess weight gain
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| 1. For fodder, give good quality meadow hay not too rich in proteins, and limit lucerne hay (try to keep protein content below 12%). |
| 2. For concentrates, give a fatty feed (fat content up to 6% for short, strenuous events (show jumping, dressage) and 9% for eventing, endurance and racing) in order to provide an alternative energy source. Weight for weight, fats contain twice as much energy as sugars (starch) and generate fewer toxins. |
| 3. They can be given in the form of special feed, or by adding 100ml vegetable oil to the basic ration each morning and evening (sunflower or rapeseed oil, or a mixture). |
| 4. Ensure the protein is easily digestible, with a good lysine content (essential amino acid), not exceeding 11–12% of the total ration. |
| 5. Check that the horse is drinking normally, especially in very hot or cold weather, and do not hesitate to use an equine electrolyte solution (see the excellent article by Dr Julie Dauvillier). |
| 6. Consider a supplement with vitamin E, carnitine and lipoic acid. |
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Reintroduce exercise very gradually but steadily, as the CK level and then the AST level begin to normalise. The return to work may take several weeks in the most serious cases, or several months before the horse can once again compete at the highest levels. A good tip is to keep the horse active, with or without a saddle, to assess its general condition and desire to move. Good grooming and back massages can encourage superficial circulation to the subcutaneous muscles. Once the horse has resumed work, enforce rest breaks and wind down gradually at the end of the day. In the case of doubt or recurring episodes, remember that science has progressed hugely and it is now possible to conduct stress tests and screen for other conditions that could exacerbate myositis such as digestive ulcers and respiratory problems, even mild.
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