CHAPTER 3 Infertility and Diseases of the Reproductive Tract of Stallions
Although horse breeding has evolved considerably in the last 50 years, reproductive research has not approached the levels that already exist in other species. Much of the research effort in horses has been directed toward understanding stallion fertility. Many factors have been reported to affect a stallion’s fertility, including a variety of diseases. Clinicians should have a working knowledge of reproductive physiology, ideal stallion characteristics, management expectations, extrinsic and intrinsic influences on fertility, and methods for semen evaluation before assessing stallion fertility. This chapter deals with some of the diseases and problems that affect stallion fertility.
An evaluation should estimate the stallion’s fertility when compared with other stallions with known fertility. Stallions should be expected to achieve at least a 75% pregnancy rate when bred naturally to 40 mares or 90% pregnancy rate when bred artificially to 120 mares. These numbers coincide with a single breeding season, good management, and mares of good fertility.
Stallions can be used for performance or for breeding and are sometimes utilized for both during the breeding season. Commonly, young stallions are retired for breeding after a successful performance career. Stallions may be able to breed and perform during the same breeding season, but physically demanding athleticism adversely affects sperm production.
Stallions are usually evaluated for breeding soundness under specific circumstances: if semen is to be frozen for later use or cooled for immediate transport; if the breeding method should change; prior to purchase; due to an increase in the size of the mare book for the coming breeding season; following a significant injury or systemic disease episode; or when decreased fertility is suspected. Serial evaluations to determine daily sperm output are most helpful following disease or injury or with an increase in the mare book. Following an evaluation, the stallion should be classified as satisfactory, questionable, or unsatisfactory for the intended use.
The farm manager or owner is responsible for daily operation of the breeding farm. He or she determines the degree of veterinary involvement, the quality and quantity of mares in the stallion’s book, the methods used to select mares for breeding each day, and the level of herd health practices used. Management decides when to begin and end the breeding season and the breeding method appropriate for the farm. Management skills ultimately determine the farm’s success as measured by the number of the stallion’s mares that foal the next year. Additionally, management should keep accurate records of the breeding season events. These records are necessary for periodic and end-of-season analysis of breeding efficiency and management expertise. Analysis of the records allows future changes in the breeding operation and identification of problems.
An increased number of barren or problem mares in a stallion’s book can make the stallion appear subfertile as judged by conception rates per estrus or foaling rates. Overbooking a stallion, that is, breeding too many mares per unit of time (day, week, or month) could deplete his sperm supply. Overuse may also cause abnormal behavior, especially in immature stallions. Examples of undesirable behavior include reduced libido, aversion to serving an artificial vagina, savaging mares, and aggression.
Breeding farm managers should be knowledgeable about basic reproductive physiology. They should be aware that horses are seasonal breeders and are most fertile during the long days of summer. Practically, this knowledge could potentially reduce the number of breedings per estrus and per pregnancy. Management must utilize veterinary expertise to enhance breeding efficiency.
Inappropriate herd health measures or poor breeding hygiene may adversely affect a stallion’s fertility. Spermicidal chemical residues on equipment that contact the stallion’s ejaculate may cause failure to conceive. Veterinary clinicians asked to help find causes for infertility should carefully examine the management practices used on the breeding farm as a potential source of the stallion’s perceived subfertility.
The age at which puberty occurs can vary among breeds of horses, but stallions generally begin producing significant spermatozoa for ejaculation at an average of 83 (±2.9) weeks of age. Sexual maturity may not occur for another 1 to 3 years, depending on breed.
Extremes of body condition can affect fertility. Stallions that are maintained either too fat or too thin may experience decreased sperm production when compared with stallions in optimal body condition. Adjustments in the plane of nutrition are necessary during certain times of the year to maintain optimal body condition; stallions not actively breeding mares require lower caloric intake, as opposed to during the breeding season when they need more calories. The greater the number of breedings per week and the more active a stallion is when not breeding, the more calories he will need. Ideally, periodic nutritional adjustments should be made to maintain stallions in optimal body condition.
The greater the frequency of ejaculation, the more the stallion’s sperm numbers may be depleted. Younger stallions with smaller testes produce fewer spermatozoa, which can be depleted more quickly, than do mature stallions with larger testes. The optimal number of ejaculates per unit of time depends on information found by serial semen collections, as well as on the stallion’s age and testicular size, the season of the year, and appreciation of the stallion’s inherent fertility gained by assessing his previous ability to impregnate mares.
Testicular volume is a relatively accurate predictor of daily sperm output (DSO). Based on a formula for determining the volume of an ellipsoid, each testicle can be measured (mm) by ultrasonography1 (see Chapter 2). This estimate may be used to compare with actual numbers produced by routine semen collection or with breeding suitability examination results. It may also serve as a basis from which a stallion’s book may be predicted. A negative deviation from the expected DSO warrants further investigation for potential causes.
Several breeding methods or schemes are recognized for stallions: natural service either in a pasture setting or controlled by in-hand breeding; artificial insemination; cooled semen shipment to mares located at a farm other than where the stallion resides; and collection of a stallion’s ejaculate for cryopreservation and storage for later insemination. The choice of breeding method may be dictated by the breed registry or association, governed by specific rules from such, or left entirely to the choice of the farm management. There are advantages and disadvantages to each of the breeding methods.
NONINFECTIOUS CAUSES OF SUBFERTILITY
Diseases that may cause infertility in a stallion can be arbitrarily divided into noninfectious and infectious sources. Either can affect the stallion’s ability to produce or deliver sufficient normal spermatozoa to effectively produce pregnancy in mares. These disease problems may affect only the reproductive tract or may be systemic. Infectious problems may be caused by bacteria or viruses that can be transmitted venereally, by direct contact between horses, or by fomites. Noninfectious problems are not transmissible but can influence normal spermatozoa production and pregnancy rates of the afflicted stallion.
Stallions that must perform natural service will be at risk of trauma caused by the mare or from events associated with breeding. Poor footing in the breeding area may lead to musculoskeletal injuries that can limit the stallion’s mounting ability or result in psychological trauma or both. When a mare kicks a stallion during breeding, there may be damage to his external genitalia, resulting in swelling, edema, abrasions, or lacerations. Blunt trauma to the penis and sheath may cause enough swelling to prevent retraction of the penis into the sheath (paraphimosis). With the penis swinging free outside the sheath, normal blood circulation to the glans is inhibited, leading to further swelling and edema. In addition, abrasion of the penis will occur as the stallion moves about. Abrasions can rapidly progress to ulceration, and without aggressive therapy the stallion’s penis may require amputation because of irreversible damage.
The initial treatment consideration in the case of trauma should be stall confinement to prevent further damage. Second, the penis should be replaced into the stallion’s sheath. With minimal swelling, the penis may be easy to replace. When edema is extensive, the swelling should be reduced before replacement within the sheath is attempted. Application of a compression bandage beginning at the glans penis may decrease swelling sufficiently for replacement. With the penis inside the sheath, nonabsorbable sutures should be placed across the sheath opening. Retaining the penis in the sheath will allow return of normal blood circulation, reduce edema, and prevent further damage. The stallion can urinate without extending the penis. Urine scalding of the internal sheath may result, but its effect may be lessened with daily application of emollients.
If penile swelling cannot be reduced or the penis prolapses after the sheath has been sutured, then a sling should be fashioned to support the penis.2 A sling can be fashioned from a bag used for laundering delicate fabrics. These bags are made from soft cotton material and provide support without damage to the penile tissues. The bag should be cut into a rectangle of the proper length and width to accommodate the penis. The penis is suspended in the soft cloth bag close to the ventral abdomen. The bag is held in place with 3-inch gauze straps attached to the corners and tied over the stallion’s croup. Several thicknesses of cotton beneath the gauze straps are necessary to prevent skin injury.
An alternative technique has been described using a plastic bottle that has been modified to be used as a preputial “splint.” A 1-liter saline bottle has the bottom cut out and the sharp edges padded with tape to prevent abrasions. The bottle is lubricated with petrolatum jelly and the opened bottom is inserted over the end of the penis, once it is reduced to a size that will fit inside the bottle. The neck of the bottle is used to anchor stint supports of long 2- or 3-inch gauze. Four stints are made, two pair to go laterally over the flank to the croup, and two to go caudally on either side of the prepuce and testicles and up between the legs skirting the tail-head to meet the lateral stints over the back, and are tied. The bottle top is left open for urine to be passed through. The splint is reset daily following hydrotherapy as needed and the penis monitored closely for any sign of abrasion caused by the retention device.
With the penis either supported as described or placed within the sheath, cold hydrotherapy should be provided for 20- to 30-minute intervals, two to four times daily. Such therapy may enhance local blood circulation, which decreases swelling and edema. Hydrotherapy should be continued as long as swelling and edema persist. Systemic administration of diuretics, glucocorticoids, and nonsteroidal anti-inflammatory drugs may be considered for reduction of edema but will not supplant the need for hydrotherapy and penile support. With full-thickness abrasion of the penile skin, systemic antibiotics are indicated to reduce bacterial contamination because secondary infections delay healing and promote swelling and edema.
Priapism is defined as penile erection in the absence of sexual stimulation that does not recede within a short period of time. The corpus cavernosum penis (CCP) is engorged with blood that soon develops fibrin and undergoes spontaneous clotting. The etiology is unclear but has been associated with the administration of phenothiazine tranquilizers, general anesthesia, neuromuscular dysfunction, equine herpesvirus infection, cachexia, dourine, and idiopathic. Early medical management consists of diuretics, nonsteroidal anti-inflammatory agents, manual massage, cold hydrotherapy, and suspension and support of the penis. General anesthesia and massage of the penis to decrease venous engorgement, application of a compressive elastic bandage, and lavage of the CCP using 10 to 12 gauge ingress and egress cannulas along with heparinized saline have all been described as useful when medical management is unrewarding or the value of the stallion warrants more aggressive therapy. When lavage is performed it is recommended to continue until arterial blood is flushed from the egress cannula. Several attempts to lavage or irrigate the CCP may be required for resolution. More aggressive surgical intervention may be considered by creation of a shunt between the CCP and the corpus spongiosum penis (CSP). Failure to aggressively manage and resolve this condition will lead to loss of the stallion as a breeding animal and may necessitate castration and phallopexy (Bolz technique).3
Trauma to the scrotum and testes may occur along with injuries of the penis and sheath. Scrotal and testicular injuries or edema require aggressive therapy to reduce the likelihood of permanent damage to the stallion’s fertility due to compromise of the thermoregulatory mechanism. With swelling, edema, or inflammation of the testes and scrotum, abnormal spermatozoa will be produced until normal heat exchange can be restored.
When the scrotum or testes are injured, ultrasonographic examination of the involved organs is beneficial to help determine the extent of damage. Testicular injuries could permit exposure of spermatozoa to the systemic circulation and result in antisperm antibody production. Antisperm antibodies can cause subfertility or sterility.
Stallions that experience genital injuries may develop psychological or behavioral abnormalities, scarring within the reproductive system, and immunologic changes as described. Such residual damage may not become apparent until the stallion is reintroduced to the breeding shed routine. Management should be cautioned about the likelihood of psychological or behavioral problems, or both, that may occur in injured stallions and should be encouraged to slowly and gradually introduce the stallion back into his former routine.
Hemospermia causes infertility that may vary with the quantity of red blood cells in the ejaculate. The mechanism by which blood causes infertility has not been determined. Hemospermia may result from either external or internal lesions in the stallion’s reproductive organs. External lesions include those from traumatic injury such as lacerations, hematomas, abrasions, ulcerations, and dermatologic conditions. Lacerations and abrasions may occur during breeding, as when the mare’s tail hairs cut the glans or urethral process during coitus. Hematomas may rupture externally or into the urethra causing spontaneous bleeding or hemorrhage during urination or ejaculation. Ulcerations may arise from infectious disease lesions either externally or internally.
Internal urethral ulcerations lead to contamination of the ejaculate with blood because of rapid expansion and contraction of the tissues during urethral pulsations. The ulcers may be located anywhere from the urethral process to the pelvic urethra near the ejaculatory ducts. Spontaneous hemospermia can occur in stallions that are bred heavily, as a sequela to bacterial urethritis, from scar tissue that results from a stallion ring, or from infection of the accessory sex glands.
Diagnosis of hemospermia may begin by observation of accumulated dried or fresh blood on the stallion’s rear legs, sheath, or ventral abdomen. Fresh blood may be noted on the stallion’s penis following coitus or may be visible grossly in the collection container after semen collection. If fresh blood appears on the stallion’s penis following coitus, the source of hemorrhage could be from the mare’s genitalia (vaginal or hymen rupture, or trauma of a varicocele within the vestibule) or from the stallion. To determine the origin of blood, the mare must first be ruled out as a source, then the stallion’s penis and sheath should be examined with a small-diameter flexible endoscope, with care taken to prevent iatrogenic urethral trauma. The examiner should be cognizant that ulcers may occur anywhere along the urethra. The most frequent location for urethral ulcers may be in its pelvic portion as the urethra bends distally over the ischial arch, or near the ejaculatory ducts at the cranial termination of the urethra.
Treatment for hemospermia consists of sexual rest, local medications placed in the pelvic urethra via a perineal urethrostomy, systemic antimicrobial administration, acidification of the urine, or surgery (perineal urethrostomy). Laser surgery may be useful for cautery of the ulcer(s).