Rena's owners weren't exactly sure what they had purchased. They thought that they had bought a mare and she seemed to show
some weak cyclical estrous behavior.
Other geldings at the farm seemed to be interested in her during these times and the only stallion around, at a neighboring
farm, called to her and paced the fence-line.
The only problem was that this horse wasn't exactly normal. She was a big strong-featured horse at 17.1 hands with a lot of
muscle development. She had slightly small mammary glands for her size but with normal teats.
Lifting her tail, however, brought the problem to light, so to speak, because she had a normal rectum but no evidence of a
On a closer, more detailed examination a vulva was indeed located roughly halfway between the udder and the rectum on the
midline. One leg had to be lifted up and away from her body to even see this structure. Her vulva was normal in conformation,
slightly small for her size with a larger than normal clitoris.
Rena is a 6-year-old, 17.1 hand draft-cross mare. She is heavily muscled and displays normal mare behavior and mild but present
A he or a she?Rena's owners still weren't sure. Was she a she, was she a he, or was she something in between?
Such horses are not common, but they do tend to show up in practice now and then. A knowledge of sexual developmental abnormalities
is helpful in determining who's what and in helping clients deal with potential problems generated by these conditions.
The best way to discuss abnormalities in sexual development is to follow a logical sequence of developmental events.
First, there is the genetic sex of the individual. As the genome of a horse becomes more completely mapped we know more about
equine genes and their expression.
The genetic sex of an individual is the chromosomal make-up of that individual. This naturally leads into the gonadal sex
or the way that the specific gene make-up determines whether a testis or an ovary develops. These gonads then produce fertile
gametes, or not, depending on germ cell sex-the capacity of those testis or ovaries to be productive.
Normal mammary glands are present in this mare but note that they can be mistaken visually for a smaller than normal sheath.
This horse was thought to be phenotypically a gelding by a number of people who only observed the animal and did not actually
examine the mammary area.
Hormonal sex describes the hormones that are produced by the gonads and also incorporates the development of internal genitalia
that is under hormonal control. This hormonal control also affects brain sex in that hormone influence on various parts of
the developing brain influences other aspects of anatomical development.
We need to next consider phenotypic sex as the process by which differing male and female sex hormones influence the development
of secondary sexual characteristics.
Lastly, one can discuss behavioral sex as the way that the individual functions in its herd (society). This discussion in
horses is far easier than in humans since we need not worry about social, environmental and other factors that often affect
sexual development and determination at many levels.
A complicated processObviously sexual development is a very complicated process. And, as with all such complex schemes, things can "go wrong" at
any step along the way.
The individuals resulting from abnormalities in this developmental process provide various clinical features and reproductive
challenges. Rena's condition is one of many. When a horse displays, either fully or partially, characteristics of both sexes
it is called an "intersex". If both testicles and ovaries are present that individual is called a "true hermaphrodite." These
individuals are not very common.
More routinely seen are cases where the horse only possesses one gonadal type. These animals are called "pseudo-hermaphrodites"
and labeled as either male pseudo-hermaphrodite or female pseudo-hermaphrodite depending on the nature of the gonads, not
the outward or phenotypic appearance.
Also present in the population are horses with embryonic abnormalities resulting in normal or near-normal genitalia in an
abnormal location. This was originally thought to be a possible explanation for Rena's appearance.
The perineal area of this horse reveals a normal rectum, but no visible vulva.
Normal work-upThe normal work-up of a case of sexual abnormality should involve a detailed physical examination, then a rectal palpation
and diagnostic ultrasound. These procedures may show internal development of ovaries or testicles. Many times, however, the
defects in development have occurred at a very early stage and normal gonads do not develop and are not detected by either
palpation or ultrasound.
This was the case with Rena, and it then ensured that her condition was not merely a normal individual with an embryological
"misplacement" of genitalia. While the presence of some type of ovarian tissue was suspected, based on the history of her
"cycling", that tissue was not in the form of normal ovaries and nothing was found on ultrasound exam. Blood was drawn and
examined for hormonal analysis. Levels of testosterone, estrogen and progesterone were checked.
In this case, Rena's blood assayed as normal for a gelding or a non-pregnant mare. This continues the gender confusion and
points out that the sequence of sexual development mentioned above should always be followed in order to determine the "true
sex" of an abnormal individual.
Rena appears, phenotypically, to be an abnormal female with some male-like tendencies. She is large, muscular and slightly
coarser than a female of her breed. While she has female external genitalia, it is not normal (smaller with enlarged clitoris)
and it is abnormally located. She has undetermined hormonal and gonadal sex and it is probably only at the genetic level that
we can definitely determine if "heeza sheeza" or not.
This horse's vulva is located midway between the mammary area and the rectum on the midline. To actually view the vulva the
right leg was abducted from the body (upper left of photo) and the tip of the stifle can be seen. A small vulva can be seen
with a slightly larger clitoris. The left hind leg occupies the lower right portion of the picture.
KaryotypingEquine karyotyping (a determination of the gene structure of an individual) is done only at the Molecular Cytogenetics Laboratory
in the Department of Veterinary Anatomy and Public Health at Texas A&M University in College Station, Texas. The testing requires
10 ml of blood collected in sodium heparin (vacutainer) tubes. Dr. Bhanu Chowdhary who runs the equine karyotyping services
at Texas A&M and who authored the chapter on Cytogenetics and Physical Gene Maps in The Genetics of the Horse (CABI Publishing,
eds. A.T. Bowling and A. Ruvinsky), cautions practitioners to wrap the tubes in paper towels before packing in ice as the
blood should not be frozen. The samples should then be sent overnight. Equine karyotyping costs $150 per sample.
Chowdhary reports that roughly 50 cases have been tested for suspected hermaphrodism and of these only 11 had abnormal genetic
make-up. "A wide range of numerical aberrations of the sex chromosomes are known in the horse," he writes, "and the karyotype
XO is among the most common-57 percent- of all sex chromosome abnormalities."
This condition is called Turner's syndrome in humans and the effects are similar in both species. Affected individuals are
shorter than normal with poor body conformation, small gonads lacking follicular development and an irregular or absent estrus
cycle. These individuals are usually infertile though an occasional pregnancy may occur. This is especially true in horses
if these mares are bred early in life.
Another abnormality, XXX is also present in horses, though it is rare-only 11 cases reported to date. Chowdhary feels that
there may be more of these mares however but, since genetic evaluation is usually not done, they go undetected.
XXX mares are phenotypically normal but uniformly infertile. Another condition causing problems in the horse is X/XX. This
mosaic condition represents 20 percent of all sex abnormalities in the horse. These mares are again phenotypically normal
with normal external genitalia but with "gonadal dysgenisis."
These mares vary from subfertile to sterile, according to Chowdhary.
An Arabian mare with a history of fertility problems was later diagnosed as XX/XXX with gonadal dysgenisis. It may be that
a higher than previously thought percentage of particularly difficult breeding cases may, in fact, be genetically abnormal
mares that remain barren despite the best veterinary and management efforts. More attention should probably be focused on
karyotype analysis of potential problem breeders.