Infertility in the stallion

Stallion infertility refers to a stallion's reduced ability to cause pregnancy in a fertile mare.
Nowadays, the selection of commercial breeding stallions is often based on theirs athletic performance and the equine industry selection focus is usually not fertility. Not focusing on fertility in the equine industry has resulted in per cycle pregnancy rates of 45 - 55% in a general population of mares.
Seasonal pregnancy rates in horses should be above 85% in well - managed situation. Stallions are generally considered problematic and not suitable for commercial use if their per cycle pregnancy rate falls below 30%. The reason for this is that mares are rebred many more times and most mare owners are reluctant to breed their mare more than twice, due to later foaling dates.

Infertility in the stallion is a cause of economic loss for both the stallion and the mare owners, and for this reason veterinarian working in the equine breeding industry must be familiar at least with the most common reproductive problems of stallions.

Many are the causes of infertility in the stallion; here below the most common ones will be listed:

Poor Intrinsic Fertility
These stallions are inefficient breeders and have no history of illness or trauma. Characteristics include small soft testis, low testicular volume, poor spermatozoa morphology, and low motility.

Oligospermia
This term means low sperm numbers. Many causes such as genetic, incomplete ejaculation, immaturity, small testis size, hypoplasia may explain low sperm numbers.

Testicular Degeneration
The cause of testicular degeneration may be traumatic, nutritional, toxic, genetic, parasitic, thermal, immune, or idiopathic. The most common type of testicular degeneration in the stallion is the idiopathic form. Older stallions are usually affected, however in some family lines an early senescence occurs. The process of degeneration has a variable time frame, where some stallions become sub-fertile (per cycle pregnancy rate less than 30%), and others rapidly become infertile.

Partial Ejaculates
One of the main problems identified in older stallions include a loss of breeding energy, due for example to be overweight or out of shape or to suffer from chronic arthritis, chronic obstructive pulmonary disease, cardiac disease or hypothalamic-pituitary dysfunction (adenomas).

Sperm Accumulation
This condition is generally an acquired dysfunction, but may be found at the first breeding soundness examination in a young stallion. This is a condition in horses where the stallions experience intermittent or complete obstruction of the ampullae because of spermiostasis. This phenomenon is usually noted as a transient condition. The stallion may have irregular spermiograms, such that when one of the occluded sides unblocks enormous numbers of senescent spermatozoa are released. The senescent sperm are dead sperm cells with detached head. Alkaline phosphatase is elevated in the semen of stallions that are ejaculating.

Infectious Diseases
Infectious causes of infertility include seminal vesiculitis, orchitis, and epididymitis. These conditions have a very low incidence and usually cause pyospermia. Seminal vesiculitis is an infection of the glands typically from ascending bacterial infection; orchitis and epididymitis may be caused by ascending infections, or result from external puncture wounds. Rarely parasite migration may cause problems in these tissues.

Causes of sexually transmitted diseases (STD's) can be Bacterial (Pseudomonas Aeruginosa, Klebsiella Pneumoniae Types 1, 2 or 5, Streptococcus Equi, Taylorella Equigenitalis (CEM)), Viral (Equine Viral Arteritis (EVA) and Coital Exanthema (EHV III)) and Protozoal (Trypanosoma Equiperdum (Dourine)).

Hemospermia
This is a condition where pure blood appears in the ejaculate.

Urospermia
May be a sporadic or persistent problem; urine crystals and sediments are present in the semen.

Trauma
One of the more common presentations to veterinarians is trauma associated with breeding, where mares injure stallions, often in the external preputial lamina. They often present with dramatic swelling to the point where the skin of the penis, prepuce and scrotum may crack, bleed and peel away. In examining these stallions it is important to determine the extent of the physical injury, swelling, trauma, hematoma formation, bruising etc. Trauma of the testis can lead to temporary or persistent infertility of the stallion.

Behavioural Problems
Inappropriate negative reinforcement of stallion behavior, harsh handling and punishment of displays of sexual behavior may lead to psychogenic ejaculatory failure. Stallions that had intense negative reinforcement (management errors) of their sexual behavior are often afraid to show libido. Low libido can also appear in some genetic lines. Older stallions may have had a bad breeding accident, or they may be worn out by the breeding season. Also, Some stallions develop pain, such as from sore hocks, and learn to associate it with the breeding act, so they may refuse to breed mares.

Anabolic Steroid or Glucocorticoid Use
Stallions under these treatments for long periods of time show high percentage of abnormal spermatozoa and low motility. This may be confused with poor intrinsic fertility, poor sperm quality or with testicular degeneration. Nevertheless, serial examinations will show improvement if steroid use is discontinued and at least 3 months has elapsed. Full recovery may take up to 1 year..
A breeding soundness examination (BSE) is the most important tool a veterinarian should use when diagnosing infertility in a stallion.  A BSE is performed also as part of a pre-purchase for the sale of a stallion, for young stallions entering their first breeding season, when deciding whether to retain a stallion as a breeding individual, or when there is a problem suspected. However, a BSE should be considered at the start of every breeding season to identify any changes in physical condition or semen quality, particularly as he ages.

A BSE usually includes the following procedures:

• Complete Medical and Reproductive History
• General Physical Examination
• Evaluation of Libido and Mating Behavior
• Examination of the External and sometimes Internal Genitalia
• Cultures for Bacteriology
• Semen Collection and Evaluation of Motility and Morphology
• Additional Diagnostics as indicated (e.g. Sperm Testing, Endocrine Profiling)

Over the last 25 years, artificial insemination with cooled and frozen semen has been widely accepted within the equine breeding industry. Stallions that are used on the market by means of cooled or frozen semen should be also evaluated for their ability to perform with these two reproductive techniques. In fact, a stallion could have a very good fertility when breeding naturally or with fresh semen, but could show infertility when his cooled or frozen semen are utilized.