Three Quarter Horses, a stillborn filly (horse No. 1), a female fetus aborted at approximately 6 months of gestation (horse No. 2), and a 1-month-old colt that had been weak at birth (horse No. 3), had
myopathy characterized histologically by large spherical or ovoid inclusions in skeletal and cardiac myofibers. Smaller inclusions were also found in brain and spinal cord and in some cells of all other tissues examined. These inclusions were basophilic, red-purple after staining with
periodic acid-Schiff (both before and after digestion with
diastase), and moderately dark blue after staining with
toluidine blue. The inclusions did not react when stained with
Congo red. Staining with
iodine ranged from pale blue to black. Their ultrastructural appearance varied from amorphous to somewhat filamentous. On the basis of staining characteristics and
diastase resistance, we concluded that these inclusions contained
amylopectin. A distinctly different kind of inclusion material was also present in skeletal muscle and tongue of horse Nos. 1 and 3. These inclusions were crystalline with a sharply defined ultrastructural periodicity. The crystals were eosinophilic and very dark blue when stained with
toluidine blue but did not
stain with
iodine. Crystals sometimes occurred freely within the myofibers but more often were encased by deposits of
amylopectin. This combination of histologic and ultrastructural features characterizes a previously unreported storage disease in fetal and neonatal Quarter Horses, with findings similar to those of
glycogen storage disease type IV. We speculate that a severe inherited loss of
glycogen brancher
enzyme activity may be responsible for these findings. The relation of
amylopectinosis to the death of the foals is unknown.