What is Turner Syndrome in Horses?

Turner syndrome in horses is a genetic condition where a mare is missing one of her two X chromosomes, resulting in an XO genotype instead of the typical XX. This means affected mares have a total of 63 chromosomes, rather than the usual 64.

Genetic Basis of Equine Turner Syndrome

In horses, a normal mare possesses two X chromosomes (XX) and a total of 64 chromosomes. However, mares with Turner syndrome are characterized by a monosomy of the X chromosome, meaning they have only one X chromosome (XO) and a reduced total chromosome count of 63. This specific chromosomal abnormality is recognized as the most common chromosome abnormality found in horses.

Prevalence and Occurrence

Equine Turner syndrome can affect mares of all breeds, making it a widespread concern in the equine world. The first detailed description of this condition in horses was provided by Lear, who identified it as equine Turner syndrome.

Clinical Manifestations

While mares with Turner syndrome may appear outwardly normal in many respects, the condition primarily affects their reproductive capabilities. Common clinical signs include:

• Infertility: This is the most significant and consistent finding. Affected mares are typically unable to conceive or carry a foal to term.
• Underdeveloped Reproductive Tract: The internal reproductive organs, particularly the ovaries, are often small, inactive, or dysgenetic (abnormally developed). The uterus and cervix may also be smaller than normal for their age.
• Irregular or Absent Estrous Cycles: Due to the dysfunctional ovaries, mares may not exhibit regular heat cycles, or their cycles may be very subtle or absent altogether.
• Variable Physical Development: Some mares may exhibit subtle physical differences, such as being slightly smaller than average, but many appear phenotypically normal apart from their reproductive issues.

Diagnosis

The definitive diagnosis of Turner syndrome in horses relies on karyotyping, a laboratory test that examines the number and structure of an animal's chromosomes.

• Sample Collection: A blood sample is typically collected from the mare.
• Chromosomal Analysis: Cells from the blood are cultured, and their chromosomes are analyzed under a microscope to identify the XO configuration and the total chromosome count of 63.
• Clinical Examination: A veterinarian will also perform a thorough physical and reproductive examination, which may reveal the underdeveloped reproductive tract and lack of ovarian activity.

Prognosis and Management

Unfortunately, there is no cure for Turner syndrome, as it is a genetic condition. Mares diagnosed with Turner syndrome are generally considered infertile and are typically not candidates for breeding programs due to their compromised reproductive systems. Management focuses on recognizing the condition to prevent repeated unsuccessful breeding attempts.

Key Differences at a Glance

For a clearer understanding, here's a comparison between a normal mare and a mare affected by Turner syndrome:

Understanding this genetic anomaly is crucial for equine veterinarians and breeders in diagnosing reproductive issues and making informed decisions regarding a mare's future. For more information on equine genetic health, resources like the University of Kentucky's Equine Programs offer valuable insights.