Common Stallion Reproductive Disorders

A variety of conditions can impact a stallion’s fertility, from inherited to acquired. Here’s why they occur and how to prevent them.
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Reproductive disorders can be detrimental to a horse’s use as a breeding stallion. | Anne M. Eberhardt/The Horse

Reproductive disorders in stallions are any abnormality that interferes with or impacts a stallion’s fertility. For breeders hoping to collect semen or schedule their stallion for live breeding, these disorders can be detrimental if not properly diagnosed and treated.

Common Stallion Reproductive Disorders

Veterinarians consider cryptorchidism perhaps the most common and well-known developmental reproductive disorder in stallions. Cryptorchidism is a heritable disorder in which one of the testes fails to descend into the scrotum. “In normal colts, both testes migrate (or descend) into the scrotum between two weeks prior to, and two weeks after, birth,” says Leo Brito, DVM, PhD, Dipl. ACT, assistant professor of large animal reproduction at the University of Pennsylvania’s School of Veterinary Medicine, in Kennett Square. “The disorder is more common in American Quarter Horses, American Saddle Horses, and Percherons. However, it is less common in breeds like Thoroughbreds and Standardbreds.”

Cryptorchidism can affect both testes but most commonly affects one. Cryptorchid testes still produce testosterone and these horses typically display normal stallion characteristics and behavior, he added. “However, a retained testes does not produce normal sperm. We suspect cryptorchidism when only one testis is present in the scrotum or when an animal with no scrotal testes displays stallionlike behavior.”

Whether or not the owner of a cryptorchid stallion plans to breed him in the future, veterinarians suggest castrating these horses due to the increased risk for developing tumors. “Palpation and ultrasonography are performed for diagnosis of cryptorchidism,” adds Brito. Veterinarians can also evaluate the horse’s testosterone and AMH (anti-Mullerian hormone, an endocrine marker for cryptorchidism) levels to confirm the condition.

Disorders that result in abnormal sperm production are also common in stallions. “In these cases, sperm counts, the number of motile sperm, and the number of normal sperm are low and cause subfertility,” says Brito. This can be caused by increased testicular temperature from fever, local inflammation, and scrotal thermal insulation (from fluid accumulation), or systemic illness, prolonged stress, advanced age, and potential genetic abnormalities.

“Evaluation of breeding records and semen analysis are important to determine the progress and severity of the disorder,” he says. “Management strategies for this type of stallion reproductive disorder include addressing the underlying causes, if those are apparent, and adjusting breeding or semen collection frequency in attempts to improve sperm counts.”

Why Do Stallion Reproductive Disorders Occur?

Stallion reproductive disorders typically result from bacterial, viral, or nematode infection, parasitic irritation, or, as we’ve already noted, genetics. “Some of these conditions are acquired whereas others are inherited,” Daigneault explains. “In comparison to the mare though, there are relatively few disorders that significantly impact stallion fertility. Many are transient, meaning that proper treatment or time usually corrects pathological (involving disease or damage) conditions.”

Inherited Stallion Reproduction Disorders

While a horse’s breed typically plays a role in genetic disorders in general, there is little evidence to suggest inherited reproductive disorders in stallions are breed-specific (although cryptorchidism is more common in some breeds, it can occur in any), except in a few cases. “One exception may be a condition known as IAR (incomplete acrosome reaction), where the sperm fail to expose an important region on the sperm itself (the acrosome, a specialized structure on the head of the sperm) that is responsible for penetrating the egg,” says Brad Daigneault, PhD, an assistant professor at University of Florida College of Veterinary Medicine, in Gainesville. “Thoroughbred stallions may be more predisposed to this genetic condition, and this condition is thought to be associated with a gene called FKBP6, but these observations are only associative and require further investigation.” Working closely with your veterinarian to determine if there is a potential for these inherited reproductive disorders to be passed down is the best practice for preventing them.

Keep in mind genetic disorders are established prior to birth but might not be detectable until the colt is at reproductive age (full reproductive capacity occurs by age 3).

Acquired Stallion Reproduction Disorders

Some horses develop reproductive disorders due to environmental factors, such as contracting bacteria or viruses from other horses. Common infections causing reproductive disorders in stallions include orchitis, seminal vesiculitus, and epididymitis.

Then there are the disorders that are secondary, “such as an infection resulting from injury to the sheath or penis that may result from less-than-ideal housing conditions,” said Daigneault. “The anatomy of an individual stallion is rarely a consideration for predisposition to a reproductive disorder unless due to a rare congenital abnormality.”

Sometimes stallion reproductive disorders stem from injuries sustained during actual breeding process. “Traumatic scrotal, testicular, and penile injuries are common acquired disorders in stallions, most often resulting from a mare’s kick or sudden movement during breeding,” Brito says. These situations are emergencies and require immediate veterinary intervention to minimize negative effects on fertility and to preserve the stallion’s breeding potential, he added. The most common signs of trauma are obvious swelling, skin abrasions or lacerations, and heat or pain on palpation.

For most pathological reproductive disorders, breeding stallions might be placed on rest with a course of antibiotics, anti-inflammatories, and analgesics before they can return to breeding or semen collection. “Often a negative test is also preferred in the case of viral or bacterial infections to reduce disease transmission from seminal plasma or direct contact with other horses,” Daigneault explains. “A quarantine period may also be prescribed to reduce the risk of disease transmission or reinfection. Good prevention practices include cleaning sheaths of stallions on a frequent basis to minimize attraction for insects while reducing a hospitable environment for secondary bacterial infections.”

Preventing Stallion Reproductive Disorders

You can minimize the likelihood of your stallion developing a reproductive disorder through good management practices, including keeping him current on vaccinations, regular disease testing, and quarantining him when needed to prevent disease transmission, says Daigneault. “Other practices include insect control that can be further mitigated by cleaning the sheath on a frequent basis, applying insect repellents, fly masks, and removing pasture manure.” Frequently groom and observe your stallion to increase your awareness of potential abnormalities such as scrotal swelling, penile abrasions, and preputial (sheath) abnormalities, before they progress. “For breeding stallions, maintaining accurate and frequent semen collection records that describe semen characteristics will help you to detect changes that may be attributed to reproductive disorders,” he adds.

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