Glycogen branching enzyme deficiency (GBED)
A fatal glycogen storage disorder distinct from polysaccharide storage myopathy (PSSM) has recently been identified in Quarter Horse foals. GBED is inherited as a recessive trait (two copies of the mutation are necessary to produce disease).
The disorder is due to a deficiency in the glycogen branching enzyme (GBE) responsible for producing a normally configured glycogen molecule in numerous tissues. Clinical signs appear to be due to a lack of intracellular glucose stores for normal tissue metabolism. Foals may be stillborn in the last third of gestation, weak after birth, or live to up to 18 weeks of age. Death may be sudden when exercised on pasture, associated with weak respiratory muscles, or foals may be euthanized due to an inability to stand. Treatable flexural deformities of all limbs and recurrent hypoglycemia and seizures occur in some affected foals. Persistent decrease in white blood cells, decreased glucose, and elevated muscle enzyme levels in the blood are common laboratory findings. Approximately 9% of Quarter Horses and Paint Horses are carriers of the mutation. GBED is a significant cause of second and third trimester abortion (up to 5% of undiagnosed abortions in quarter horses) and foal mortality in the Quarter Horse breed. GBE deficiency should be suspected in sporadic abortions or foals that present with weakness, contracture of all limbs at birth and have a combination of persistent hypoglycemia, decreased white blood cells, and elevated muscle enzymes.
Genetic testing is available using mane or tail hairs from live animals, or liver, muscle or heart samples of aborted feti at the University of California, Davis.