Equine Genetic Disorders Reviewed

Equine Genetic Disorders Reviewed

Two genes mapping to the equine X chromosome (red and green dots), as seen on fluorescence in situ hybridization (or FISH) analysis. Researchers use this technique to evaluate horses' chromosomes for abnormalities.

Photo: Teri Lear, PhD

What do sex-reversal syndrome, junctional epidermolysis bullosa, and autosomal trisomy have in common? They're all equine genetic diseases. But these conditions also have distinct differences that can help practitioners evaluate and diagnose suspect cases.

At the Equine Endocrine and Genetic Disorders Symposium, held Nov. 21 in Lexington, Ky., Teri Lear, PhD, equine genetics researcher at the University of Kentucky's Gluck Equine Research Center, delivered a presentation reviewing how practitioners can recognize genetic diseases in everyday practice.

Lear explained that there are two types of genetic disease: chromosome abnormalities and inherited disease. The former can include chromosome or DNA mutations (termed disorders of sex development, or DSD) or variations in chromosome structure or number. The latter includes diseases passed genetically from a parent to an offspring.

She reviewed how veterinarians can examine chromosomes and DNA (including the use of techniques called karoyping and fluorescence in situ hybridization, or FISH) before describing how these disorders appear in clinical practice.

Chromosome Abnormalities

Equine Turner syndrome—The first DSD Lear described was equine Turner syndrome, in which a mare is missing one X chromosome (XO instead of XX; in other words, she has a total of 63 chromosomes, not 64). Lear said this defect can occur in all breeds and is the most common chromosome abnormality. These mares typically present with a small body size, poor conformation, angular deformities, and small and inactive ovaries with an underdeveloped reproductive tract. XO mares are infertile, she added.

Sex-reversal syndrome—Next, Lear described sex-reversal syndrome, in which a horse has the outward appearance of a mare, but is genetically male (XY instead of XX). This defect has been identified sporadically in Thoroughbreds, Standardbreds, Quarter Horses, and in families of Arabian horses. These infertile horses might be bigger bodied than most mares and have small inactive ovaries and a nontubular uterus.

Sex-reversal syndrome can also go the other way: Outwardly male horses can have an XX sex chromosome instead of an XY. Lear said these horses have malelike external genitalia, but lack sperm; ovarian tissue; increased anogenital length; an enlarged clitoris or small penis; and a closed vagina. Most of these horses exhibit stallionlike behavior, Lear said, and some have high testosterone levels. XX males are also infertile.

Autosomal translocations—Another type of chromosome abnormality is autosomal translocation, which occurs when there is an interchange or transfer of chromosomal segments between two or more different chromosomes. Lear said translocations are known to cause repeated early embryonic loss (generally within the first 45 days of pregnancy), but some affected mares can produce foals. Affected horses appear normal, she said.

Autosomal trisomy—The last chromosomal abnormality Lear described was autosomal trisomy, a condition similar to Down syndrome in humans in which there are three copies of a particular chromosome, instead of the normal two. Lear said affected horses can have clinical signs including abnormal genitalia, an overbite, angular limb deformities, scoliosis, a domed skull, neurologic deficits, and metabolic disorders.

Some affected horses are less severely affected than others, she noted. She described one filly that was considered a dummy foal (or a foal that suffered from lack of oxygen during delivery), but later developed progressive neurologic and musculoskeletal deficits, inappropriate social responses, a dull mentality, a protruding tongue, abnormal joint growth, and a history of falling in her stall. After evaluation, Lear said the filly was diagnosed as having 40% normal chromosomes and 60% chromosomes with trisomy.

Inherited Disorders

Lear closed her lecture with a discussion on inherited genetic disorders. Many of these are breed-specific, she said, and practitioners should keep that in mind when faced with a possible case. She touched briefly on:

  • Junctional epidermolysis bullosa (or JEB), a condition found in American Saddlebred and Belgian foals that causes skin lesions over pressure points of the body in newborn foals and results in large areas of skin loss. The disease is a lethal condition, and affected foals typically are euthanized shortly after birth.
  • Lavender foal syndrome is a fatal, neurologic disease found in Arabian horses worldwide. Common clinical signs in affected foals include seizures; opisthotonos (severe hyperextension of the head, neck, and spinal cord); stiff, paddling leg movements; nystagmus (involuntary movement of the eyeballs); and, often, a soft lavender color to the foals' coats.
  • Dwarfism in Miniature Horses is an area recently researched by one of Lear's graduate students, John Eberth. He recently confirmed four types of dwarfism in Miniatures, caused by mutations in a specific gene important for cartilage development. Eberth's research on the topic is ongoing.

Lear also noted that her current and future research goals involve evaluating:

  • Contracted foal syndrome in Thoroughbreds;
  • Parrot mouth in Thoroughbreds and other breeds; and
  • Lordosis (sway back) and congenital cataracts in American Saddlebred horses.

Take-Home Message

While researchers like Lear have developed a good understanding of equine genetic disorders, there is still much to learn. But for now, equine practitioners can use the solid knowledge base Lear and many others have developed to evaluate and diagnose horses with chromosomal abnormalities and inherited disorders.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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