NEWS
Veterinary Topics: A horse that is a he, a she, or an it
Posted: Saturday, October 30, 2004
Sexual abnormalities might shed light on unusual cases of clinical or behavioral problems in horses
THINGS ARE NOT always what they seem. That sometimes goes for horses. The hunter pony the Stewarts had purchased seemed to be a normal mare. For a year, she had dutifully carried their daughter around the ring and all seemed well. Then the changes started.
The mare became more aggressive and began bucking when being ridden. She would try to lie down and roll to unseat her rider and even started to bite at the child when being groomed. Physical examinations during this time were normal. Her veterinarian suspected that the mare may have been experiencing hormonal surges related to her ovaries and an ultrasound examination was done, but no abnormalities were noted. Hormonal therapy with progesterone was tried, but the problems only worsened. Nothing seemed to explain her behavior.
At a Thoroughbred farm, the manager was beside himself. A well-bred stakes winner the farm had been trying to get in foal was just not responding. She seemed to cycle normally and produce a good follicle, and she had repeatedly been bred to a proven stallion. The farm veterinarian assured him that the timing on the breedings could not have been better, but the mare was still not in foal. No infection, no fluid in her uterus, no problems, and no answers. Nothing seemed to explain her behavior.
The explanation for the problems in both these mares would come much later when--with all other forms of examination exhausted--a chromosomal determination was done and both mares were found to be, well, not mares at all. They had genetic disorders that made one of them behave more like a stallion and the other infertile despite normal reproductive behavior.
Horses such as these are not common, but they do exist. An understanding of the development of sexual abnormalities might help to identify these unique individuals and aid in dealing with the potential problems caused by their conditions.
Stages of development
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 or the gene map of the horse becomes more clearly understood, we increase our understanding of equine genes and their significance to the makeup of the individual. The genetic sex is the chromosomal makeup of that individual.
These genes determine whether testes (male gonads) or ovaries (female gonads) develop. This is the gonadal sex of the individual. These gonads then produce male or female gametes and determine the germ cell sex of the individual or the capability of the gonads to be fertile and of the individual to reproduce.
Hormonal sex refers to the hormones that are produced by the particular gonads and includes the development of internal genitalia that is under hormonal control. Hormones greatly influence brain sex because hormonal interaction on various parts of the developing brain influence other aspects of anatomical development. It has been well documented that the brains of males differ from those of females, and though the reasons for those exact differences are not completely understood, it is at this level that developmental changes occur.
The next step in sexual development is that of phenotypic sex. This is the process by which differing male and female hormones influence the development of secondary sex characteristics such as bone mass, muscle size and distribution, and body shape.
The last category is behavioral sex, which relates to the way the individual functions in its herd or society. This discussion in horses is far easier than in humans since, with animals, there is no reason to factor in social, environmental, learned, or other influences that often affect sexual development and determination at many levels in man.
Physical traits
Sexual development is a complicated process, and, as with all such complex schemes, things can go wrong at any step along the way. Horses that result from abnormalities in the developmental process present an array of clinical features and reproductive challenges.
The Stewarts' hunter pony and other horses can be found that display characteristics of both sexes. These individuals are called "intersex" animals.
If both ovaries and testicles are present, then the horse would be a true hermaphrodite. These horses can be phenotypically either male or female so they can appear normal but, as seen with the Stewarts' mare, the true nature of the individual's genetics eventually will come out.
True hermaphrodites are uncommon. Both surgeons and reproductive specialists who consulted on the hunter pony said they had seen only one other such mare. She had become so aggressive and did not respond to standard treatment that she was eventually euthanized. It was only at examination after death that the true nature of "her" problem was discovered and the reasons for her failure to respond to therapy were uncovered.
More horses with sexual abnormalities possess only one gonadal type. These animals are called "pseudo-hermaphrodites" and are labeled as either male or female pseudo-hermaphrodite depending on the nature of the gonads and not on the outward or phenotypic appearance of the individual. Many other variations occur as well, and the only way to determine accurately whether horses are males, females, or something in between is through equine karyotyping analysis.
Genetic test
Equine karyotyping is the determination of the genetic structure of an individual, and this testing is done only at the Molecular Cytogenics Laboratory in the Department of Veterinary Anatomy and Public Health at Texas A&M University. The simple test uses ten milliliters of blood and is done at a cost of $150 per sample.
"There are a wide number of numerical aberrations of sex chromosomes known to exist in the horse," said Bhanu Chowdhary, Ph.D., who runs the karyotyping services at the university. Normal males are XY while normal females are XX. These are the individual genes that control genetic sex.
Chowdhary pointed out, "[The karyotype XO]" is one of the most common abnormalities seen in the horse, and this genetic variant accounts for nearly 57% of all sex chromosome problems." This condition (XO) is called Turner's syndrome in humans and affected individuals are generally shorter than normal with poor body conformation, small gonads lacking follicular development, and an irregular or absent estrous cycle. These individuals are usually always infertile, although an occasional pregnancy can occur.
This is especially true if these horses are bred early in life. Chowdhary cautions that the presence of an earlier pregnancy in a mare's history often delays the consideration of an XO chromosomal abnormality as a reason for poor conception in a mare that cannot become pregnant despite the lack of clinical problems.
Other similar abnormalities also exist. Trisomy or XXX has been found in horses, but it is considered rare with only 11 cases recorded. Chowdhary feels there may be many more of these horses in the general population but not enough karyotyping is being done to provide accurate statistics. These XXX horses are phenotypically normal and may be big, well-conformed mares, but they are uniformly sterile.
Another group of problems concern horses that are X/XX. These individuals are called mosaics and represent 20% of all the sexual abnormalities seen in horses. These animals are outwardly or phenotypically normal with normal external genitalia, but they vary from subfertile to sterile, according to Chowdhary. Given these examples, it may be that a higher than previously thought percentage of particularly difficult breeding cases may, in fact, be genetically abnormal horses that remain barren despite the best veterinary and managerial efforts. More attention should be focused on karyotype analyses of potential problem breeders.
Sex reversal
Conditions of sex reversal also are seen in horses. These are individuals where there is a disagreement between the karyotypic or genetic sex and the outward or phenotypic sex. Chowdhary reports that more than 120 such cases have been documented in the horse and, given the relatively low number of equines screened for such problems, this is a significant number.
XY individuals are expected to be males, but in sex reversal they appear more like females. This is the second-most common abnormality in infertile mares. XX sex reversal is far less common, and these females actually appear more like females, but there are degrees of external male-like characteristics and there may be internal ovaries, testes, or both.
All these types of abnormalities are of interest and importance in that they may help to shed light on some unusual cases of clinical or behavioral problems in horses that seem to defy explanation. More karyotyping needs to be done to identify these individuals and to further understand the scope of these conditions. Many horses may seem to be what they are not and only a deeper look will reveal the true makeup of that individual and explain why he, she, or it behaves that way.
Kenneth L. Marcella, D.V.M., is a practicing veterinarian in Canton, Georgia.
