Factors in Exertional Myopathy

Genetics determine whether an animal is predisposed and at risk of developing symptoms. Environment, management, and nutrition often determine whether the disease actually becomes clinically apparent and how severely it manifests.

In detail, multifactorial exertional myopathy (including PSSM/MIM forms) is a classic example of the interaction between genetic predisposition and management factors.

Genetic variants, for example in the GYS1 gene (PSSM1) or in MIM-associated genes such as FLNC (formerly referred to as a PSSM2 factor), determine:

• whether a horse is predisposed at all
• to what extent muscle cells are primarily structurally impaired or affected as a consequence of a metabolic dysfunction
• which pathological mechanism is involved (e.g. disturbed energy metabolism versus structural instability)

Without a genetic predisposition, true exertional myopathy generally does not occur, or only very rarely. Details on the currently known genes can be found under: Understanding DNA Test Results for PSSM1 and MIM.

It should be noted that current testing can assess only the most important known genetic predispositions. Therefore, the genetics of these testable genes alone do not yet explain when symptoms will occur or how severe the clinical course will be. 

Important factors influencing the onset of symptoms include:

• Exercise management
  Irregular training is associated with a high risk. “Weekend horses” are particularly predisposed.
• Housing conditions
  Stall confinement with limited exercise increases the risk.
  Open stabling or pasture turnout reduces the risk.
• Stress
  Transport, changes in stabling, or competitions may trigger episodes.
• Training intensity
  Sudden peaks in workload are considered critical.

Many episodes do not occur “spontaneously” but arise following management-related mistakes.

Proper feeding – that is, feeding tailored to the horse’s needs – is a key factor in maintaining the health of every horse, regardless of its predispositions.

In the case of exercise-induced myopathy, symptoms can often be significantly reduced through a tailored feeding regime, depending on the type and extent of the genetic predisposition.

Specialised, independent consultations, which also take other factors into account, would be recommended in this case.

More on: Feeding advice