Inguinal (Scrotal) Hernias in Horses: Causes, Risk Factors & Treatment

Inguinal hernias, also referred to as scrotal hernias, occur when a horse’s intestine protrudes through the supporting muscles around the reproductive tract, making a noticeable bulge.

An inguinal rupture occurs where a horse’s intestine slips completely through a tear in the muscle wall, settling on the other side of the musculature that normally protects it. Both conditions can lead to severe complications if left untreated.

Hernias can be present at birth or acquired, often occurring after castration. While relatively rare, inguinal hernias are a significant concern in horses due to the risk of strangulation, where blood flow to the affected area is compromised, causing extreme pain and tissue death.

Recognizing the symptoms, such as scrotal swelling and signs of colic, increases the likelihood of early detection and treatment. Understanding the risk factors, including breed predisposition and recent physical activities, can help in preventing and managing inguinal hernias.

Inguinal Hernias in Horses

Herniation describes any instance where the intestines are able to shift out of the abdominal cavity through the protective muscles around them. Different hernias are categorized according to which part of the body they arise in.

Inguinal or scrotal hernias involve herniation through the ring of tissue that separates the abdomen from the scrotum. ^{[1][2][3][4][5][6][7]} In cases of inguinal hernia, the intestine settles either inside or outside the membrane that surrounds the testicles. ^[1][2]

This condition can be acquired, often after castration of male horses. Some foals are born with a congenital form of the condition which tends to be less severe. ^[1][2][8]

Whether in adult horses or foals, the condition is most serious when the blood flow to the intestine and the testicles is interrupted, leading to extreme pain and eventually tissue death. ^[3][7]

Testicular Anatomy in Stallions

To understand what happens during inguinal herniation, it is useful to know the stallion’s key anatomical structures. ^[1][9]

• Inguinal Canal: A passage in the horse’s lower abdominal wall that allows the spermatic cord to pass from the abdomen into the scrotum. This structure is made up of the vaginal (also known as internal) and external inguinal rings.
• External Inguinal Ring: The outer opening of the inguinal canal located in the groin.
• Vaginal (or Internal Inguinal) Ring: The inner entrance to the inguinal located in the abdominal cavity. The intestine passes through this structure during inguinal herniation. In the case of inguinal rupture, the intestine passes through a tear in the vaginal ring or vaginal tunic.
• Vaginal Tunic: A thin, double layered membrane that surrounds the testicles and spermatic cord. It provides a covering for these structures as they pass through the inguinal canal. In the case of inguinal hernia, the intestine settles inside this membrane. In the case of inguinal rupture, the intestine passes through a tear in this structure or the vaginal ring and settles outside of the vaginal tunic.
• Spermatic Cord: This tube contains the blood vessels, nerves, the vas deferens, and lymphatics necessary to support and direct the actions of the testicles. It passes through the inguinal canal to the testicles. During herniation, pressure put on this structure can interfere with the delivery of blood and oxygen to the testicles, requiring partial or complete castration.

Illustration: Dr. Ana Mesa, PhD

Direct and Indirect Hernias

The terminology used to describe inguinal hernias in humans includes direct and indirect hernias. Many researchers also use these terms to describe hernias in horses. ^[2][4][5][7]

Researchers describe the difference between the two types of hernias in horses as: ^[1]

• Inguinal hernia: Used instead of indirect hernia. The most common type of hernia in which the intestine drops through the vaginal ring and settles inside the vaginal tunic.
• Inguinal rupture: Used instead of direct hernia. A hernia where the intestine drops through a hole in the vaginal tunic or the vaginal ring and ends up in the space outside the vaginal tunic.

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Symptoms

The main symptom of inguinal hernia in horses is moderate to severe colic. ^[1][2][7]

Common signs of colic include: ^{[1][10][11][12][13][14]}

• Lethargy
• Reduced appetite
• Biting or staring at the flanks
• Restlessness
• Teeth grinding
• Stretching, dog-sitting, crouching, or lying on the back
• Curling the upper lip (Flehmen Response)
• Tensed abdomen
• Kicking at the abdomen
• Lying down
• Rolling
• Decreased stomach noises
• Rapid breathing
• Sweating
• Bloating
• Changes in the color of the gums
• Rapid heartbeat

Physical Signs

Horses with an inguinal hernia may have a swollen prepuce (the skin surrounding the penis). ^[2]

The scrotum on the affected side is often: ^[1][2][7]

• Firm
• Cool
• Enlarged
• Painful

In the case of congenital inguinal hernia, scrotal swelling is typically the only symptom. ^[5]

In the rare case that a foal’s intestine becomes strangulated or experiences congenital inguinal rupture (direct inguinal hernia), additional symptoms may develop, including:^[1][2][5]

• Lethargy
• Cold, moist scrotal skin
• Swelling of the prepuce and scrotum

Causes

The causes of inguinal hernia in horses are not completely understood at this time. ^[7]

Possible explanations for acquired inguinal herniation include: ^[1][2][7]

• Movement of the intestines in the body during mating or exercise
• Morphological abnormalities in the vaginal ring
• Changes in intra-abdominal pressure during exercise

Some research suggests that genetics may play a role in this condition. ^[7] This is because some breeds have a higher risk of this condition, in some cases due to larger than average inguinal canals. ^[2]

For foals born with congenital inguinal hernia, the cause is presumed to be genetic. ^[5]

Risk Factors

Inguinal herniation occurs mostly in male horses. The condition is extremely rare in females. ^[1][6] Inguinal hernias are relatively rare in intact male horses. ^[1]

Scrotal hernias are more common in geldings just after castration. ^[1][4][8] Most post-castration cases occur within 96 hours following surgery but incidents have been reported up to two weeks post-operation. ^[1][4]

Inguinal hernias are less common in geldings that have recovered from castration. ^[2] This is because the vaginal ring reduces in size once the horse is castrated. ^[1]

Horses with a history inguinal herniation at birth are at greater risk of developing an inguinal herniation later in life. ^[1]

Breeds reported to have a higher incidence of inguinal hernia include: ^[1][2][6][7]

• Standardbred
• Tennessee Walking Horse
• European Warmbloods
• Andalusian
• American Saddlebreds

Horses of all ages are at equal risk of acquired inguinal hernias. ^[1]

Other risk factors for this condition are still the topic of debate. Some circumstances that are under consideration include: ^[1][2][7]

• Recent history of jumping
• Recent history of mating
• Deformity in the vaginal ring
• Trauma
• Changes to intra-abdominal pressure

Severity

Inguinal hernias are most severe when the blood flow to the trapped intestinal structures is disrupted. ^[1][7] This is referred to as strangulation and is a serious complication of inguinal herniation. ^[4]

Horses with suspected inguinal hernias must always be evaluated by a veterinarian even when strangulation is not suspected. This complication is likely to develop over time and is a serious threat to the horse’s wellbeing.

Usually, an inguinal hernia only affects one side of the horse, but in rare cases both sides can be involved. ^[1][5][7]

It’s possible for a horse to develop both inguinal herniation (indirect inguinal hernia) and inguinal rupture (direct inguinal hernia) at the same time. ^[1]

In some cases, the herniation affects the blood flow to the testicle on the affected side. If this is severe enough, the testicle needs to be removed. ^[2][3]

In some cases of inguinal rupture (direct inguinal herniation), both the intestine and the testicle move through a hole in the vaginal ring or vaginal tunic and come to rest in the space around the vaginal tunic. ^[5]

Typically, inguinal herniation involves part of the small intestine, but in rarer cases, the large intestine is involved. ^[3][5]

Foals born with congenital inguinal herniation usually do not develop strangulation of the involved structures, and can be largely left to resolve on their own without surgery. ^[1][2] If rupture does occur, it usually does so within 96 hours of the birth. ^[1][3]

Diagnosis and Treatment

Diagnosis of inguinal hernia in horses is based on: ^[1][15]

• Physical examination
• Rectal palpation
• Ultrasound

Many cases of inguinal herniation can be treated by manipulating the disordered structures from outside the horse’s body. ^[1][7] In other cases, administering anesthesia and then placing the horse on its back and/or manually manipulating the structures can help them return to their proper position. ^[16]

Some horses require surgery to remove the intestine from the vaginal tunic. ^[1][4][16] In some cases, the blood flow to the testicles is compromised, requiring castration. ^[7]

Foals born with congenital inguinal herniation can typically be left untreated, especially in the case where the intestine has stayed inside the vaginal tunic. ^[2] These cases tend to resolve within 3 to 6 months. ^[1] During this time, daily manual replacement of the intestine or colon into the abdomen along with bandaging to reduce the hernia is recommended. ^[1]

If the foal experiences a inguinal rupture or strangulation of the herniated structures, surgery is necessary to manually remove the intestine and place it back in the abdominal cavity. ^[1][2][5]

Prognosis and Prevention

If the hernia is resolved manually without the need for surgery, the prognosis for horses is good. ^[16] The prognosis for affected horses that require surgical intervention to resolve an inguinal hernia ranges from fair to good. ^[1][2]

For foals that required surgery to resolve a strangulated hernia, the prognosis ranges from fair to good. ^[1] Horses whose inguinal herniation has been resolved without the need for complete castration are usually still fit for breeding, even in the case where one testicle has been removed. ^[2]

Since the causes of inguinal hernias are not fully understood, prevention is a challenge. In castrated horses, prevention strategies focus on how the castration is performed. The vaginal tunic must be cut as close to the external inguinal rings as possible to lower the risk of inguinal herniation. ^[1]

Summary

Inguinal or scrotal hernias in horses occur when the intestine pushes on the internal structures protecting the reproductive tract, creating a bulge. Inguinal ruptures occur when the horse’s intestine pushes all the way through these structures and settles on the other side.

• Both forms of hernias can be present at birth or acquired as the result of castration or through the movement or morphology of the horse.
• The causes of inguinal hernias are not fully understood, but a genetic component is suspected.
• The condition is life-threatening if blood flow to the intestines or testicles is interrupted. For this reason, the condition always requires veterinary attention.
• Symptoms include moderate to severe colic, a swollen prepuce, and a cold, moist, swollen, and painful scrotum.
• Diagnosis is based on physical and rectal examination and ultrasound. Treatments include manually replacing the intestine into the abdomen from outside the horse, or surgery.
• The prognosis for affected horses is fair to good depending on the required treatment.