Amylopectinosis in fetal and neonatal Quarter Horses.

This research reports a previously unidentified storage disease in fetal and neonatal Quarter Horses, yielding symptoms similar to glycogen storage disease type IV. The illness is characterized by inclusions in various tissue types believed to contain amylopectin, suggesting a possible severe inherited deficiency in glycogen brancher enzyme activity.

Study Overview

• Three Quarter Horses were examined in this research each displaying a myopathy characterized by large spherical or ovoid inclusions in skeletal and cardiac muscle fibers.
• The subjects were a stillborn female horse (horse #1), a six-month-old aborted female fetus (horse #2), and a one-month-old colt that had been weak from birth (horse #3).
• Smaller inclusions were also found in the brain, spinal cord, and some cells of all other tissues examined.

Diagnostic Findings

• The tissue inclusions were basophilic, staining red-purple with periodic acid-Schiff (both before and after digestion with diastase), and a moderately dark blue after toluidine blue staining.
• The inclusions did not react when stained with Congo red.
• Staining with iodine ranged from pale blue to black, while their ultrastructural appearance varied from amorphous to somewhat filamentous.
• These staining characteristics and resistance to diastase led the researchers to conclude that the inclusions contained amylopectin, a type of complex carbohydrate.
• A distinctly different kind of inclusion material was also present in the skeletal muscle and tongue of horse Nos. 1 and 3. These inclusions were crystalline with a sharply defined ultrastructural periodicity. These crystals did not stain with iodine and were often encased by deposits of amylopectin.

Hypothesis and Related Considerations

• Based on these findings, researchers suggest that the disease may be due to a severe inherited loss of glycogen brancher enzyme activity. This enzyme is crucial for the formation of glycogen used by the body for energy storage.
• The disease’s resemblance to glycogen storage disease type IV, a known genetic disorder that prevents the body from properly utilizing glycogen, further supports this hypothesis.
• The exact relationship of amylopectinosis to the death of the foals remains unknown, and further research is required to understand any causative relationships or treatment options.