What causes white muscle diseases in horses

Swiss Equestrian Federation SVPS

Caution is advised when buying horses and choosing breeding animals. That is why it is better to be prepared than to look back. Research into such diseases is correspondingly important. In a multi-part series in the “Bulletin”, some of the more common diseases that have a hereditary component in horses are described in more detail. The third part of this series is about muscular diseases, some of which also have genetic components.

Cross-lay, cross-lay, Monday morning disease, tying-up syndrome or rhabdomyolysis - as we now know, there are various muscle diseases that can cause the classic symptoms of the syndrome previously known only as "cross-lay". The extent of such muscle damage can vary widely. The spectrum ranges from a stiff gait that is practically imperceptible to the rider to a life-threatening situation for the horse. In addition to various external influences, genetic factors can also play a role.

The symptoms usually appear at the beginning of work. If the disease is mild, the horse usually does not want to move or is reluctant to move, is not riding and, if the blood is tested in the laboratory, shows increased muscle values. With a more severe course, the horse suddenly begins to sweat and moves unusually stiffly.

The muscles can tremble and the hindquarters can buckle because the horse can no longer tense the muscles. If these horses lie down, it is possible that they will not be able to get up again. It is strongly recommended that work on such a horse be stopped immediately. The back, croup and thigh muscles appear swollen, hard as a board and painful. Since muscle tissue is destroyed in this disease and the muscle pigment is then excreted in the urine, it turns red to dark brown.

root cause
The regular lay is a factor illness. This means that various underlying causes and several unfavorable factors come together and lead to the development of the clinical symptoms. Possible triggering factors are, for example, physical stress or demands that do not correspond to the horse's level of training, as well as interruptions of one to several days in training or work with a full feed ration.

Genetic components also play a role: an example that has been well studied nowadays is the carbohydrate storage disease PSSM (polysaccharide storage myopathy), which has so far mainly been found in quarter horses and various cold-blooded breeds.

Disease emergence
The old theory of hyperacidity of the muscles as the cause of the cross beat has meanwhile been clearly refuted. Depending on the underlying disease, however, the cause can be an abnormal carbohydrate or fat metabolism in the cell.

In the genetic disease PSSM, for example, the storage of too many wrong sugar molecules leads to the destruction of the muscle cells. The muscle protein, the so-called myoglobin, then flows from the dead cells into the bloodstream and is excreted through the urine, which turns red to dark brown in color. This increased myoglobin content in the blood can also damage the kidneys.

The affected horse must be immobilized immediately. So you should definitely not ride any further and instead organize transport and call the vet. The horse should also be covered and calmed down. If a horse is stuck, you shouldn't chase it up. Better to provide him with enough soft bedding. It is also important to offer the horse tempered water at all times. The vet will then try to relieve the horse's pain, identify the source of the problem and, if possible, remedy it with medication. These include anti-inflammatories and, if necessary, infusions to protect the kidneys from damage.

The prognosis depends very much on the severity of the cross stroke. It ranges from very good to rather bad. It also always depends on how an affected horse responds to the treatment.

As a horse owner, you can also prevent muscle diseases that are partly genetic. The following points must be observed:

  • On standing days, cut the concentrate ration at least in half.
  • Adapt the physical load to the level of training and the age of the horse.
  • Sufficiently long warm-up phase: The horse should be walked at least 10 to 15 minutes before starting work.
  • Ensure sufficient vitamin E and selenium supply and supply of minerals.
  • Monitoring by analyzing the muscle enzymes using blood samples, for example after a major training session or competition.
  • If cross-strokes occur repeatedly: Have a genetic test for PSSM type 1 or, if necessary, a muscle biopsy performed.
  • In the case of genetically predisposed horses, special care must be taken to ensure that the horses are always moved very regularly. Standing days should be avoided completely if possible. In addition, a change in feeding, i.e. avoiding concentrated feed rich in carbohydrates and, if necessary, replacing it with a high-fat feed, can help prevent new outbreaks of disease.

Nicole Basieux, Nathalie Fouché (ISME) & Vince Gerber (ISME)