Page 74 - October 2016

(hematoxylin and eosin), which is the most widely used stain in medical diagnoses when looking at a biopsy or tissue sample. Only a certain kind of stain (called PAS, or periodic acid stain) will detect the abnormal glycogen in the muscle tissue samples. This means that in the past, GBED was probably not suspected when foals died suddenly. Thus, this condition has probably existed in certain Quarter Horse bloodlines for many years but was undetected.
Today, however, there is a DNA test
that can be used to find out if a horse
carries this gene or to find out if a foal
has GBED. At least 10 mane or tail hairs pulled out with the roots from dams or
sires is preferred to test for carriers. Liver
or muscle necropsy samples are preferred
to determine the status of a foal. There are several labs licensed to conduct the tests, including Animal Genetics, Inc., Progressive Molecular Diagnostics, Inc., the University of California Davis, and VetGen, Inc.
Liver or muscle necropsy samples can be used to detect the condition in any foal that dies, or a muscle biopsy can be used to check a living foal. When stained with PAS, the muscle tissue from a GBED foal has a characteristic appearance with purple globules of abnormal polysaccharide on a pale background instead
of the normal pink staining. Normal muscle glycogen produces a consistent rich pink color when stained with PAS.
Dr. Nena Winand, a veterinary molecular geneticist at Cornell University (now retired) who has worked on tracking down several genetic mutations including HERDA, a skin disorder in some family lines of Quarter Horses, owns Quarter Horses and has a personal interest in Quarter Horse genetics. “The biggest thing that strikes me about
the genetic diseases is the lack of awareness among breeders, even though there are now some genetic tests available. Breeders need to become more aware of GBED because it’s not rare. This disease is so misunderstood, however, that people often don’t have a clue when they lose foals. They may lose several foals and don’t even think to test their mares
A Periodic Acid Stain (PAS) can detect the abnormal glycogen in muscle tissue samples. When stained with PAS, the muscle tissue of a GBED foal will have purple globules of abnormal polysaccharide on a pale background (right) instead of the normal pink staining (left).
A foal suffering from GBED will not be as active as normal due to a weakness of the skeletal muscles.
or stallions for GBED. Breeders should be testing for this if they experience foal loss, especially if it’s from repeated matings of the same animals. It’s not a very expensive test considering the cost of producing a foal,” says Winand.
If horse breeders test their animals and choose not to mate carriers to carriers, there would be no more GBED foals produced.
“This disease today is a lot more prevalent than people think, and is especially prevalent in reiners, reined cowhorses, and other performance horses, depending on their bloodlines. A recent study looked at the carrier (allele) frequency for GBED in various disciplines, including pleasure horses and running horses,” she says.
GBED was found most commonly in pleasure horses, cutting horses, and a few other subgroups of performance horses, and less frequently in racehorses and barrel horses. The researchers were not sure why these racing groups had a lower frequency of these alleles, but theorized that it might be because there have been less intensive inbreeding strategies
in these disciplines compared with other subgroups, and more use of different sire
lines. In Quarter Horse racing, for instance, Thoroughbred bloodlines continue to be added through the use of appendix AQHA individu- als. The racing Quarter Horses typically have more Thoroughbred genes and fewer doubled up AQHA genes that might carry GBED. It still pays, however, to check bloodlines.
“It continues to surprise me to see how many people do not pay attention to genetic diseases and then they experience a heart-breaking situ- ation. These are diseases that cause animals to suffer, and it’s not ethical to breed individuals that will create these problems.” Winand is cur- rently serving as an expert witness in a legal case in Texas, on genetic issues.
She says testing can be done at the UC-Davis Veterinary Genetics Laboratory (for details, see www.vgl.ucdavis.edu or www.vet- gen.com). Muscle biopsies of foals can be sent to the University of Minnesota’s laboratory if a breeder or veterinarian is unsure about whether a foal has GBED or some other problem.
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72 SPEEDHORSE, October 2016
EQUINE HEALTH