The preferred method to allow close inspection of the penis (Figure 13-2 and Figure 13-3) is to stimulate penile tumescence through exposure of the stallion to a mare in estrus. This procedure also permits assessment of sexual behavior, including erection capability. Manual extraction of the penis from the prepuce for examination is difficult and usually met with resistance from the stallion. In shy stallions, the penis can often be visualized from a distance while the horse urinates. Urination can sometimes be stimulated by placing the horse in a freshly bedded stall; shaking the bedding may increase the horse’s urge to urinate. Tranquilization (acepromazine or xylazine) elicits penile prolapse, making the penis accessible; however, tranquilizers, especially those that are phenothiazine-derived (e.g., acepromazine), can cause penile paralysis or priapism and therefore should not be used indiscriminately. In addition, tranquilizers will probably render the horse ataxic which may interfere with mounting when collection of semen is attempted.

The penis may need cleansing before its inspection (Figure 13-4) because epithelial debris mixed with secretions from the preputial glands accumulates in the preputial cavity and on the exterior of the penis when it is unattended. The root and proximal body of the penis are buried in tissue, thereby limiting the examination to the exposed body and glans. The penis should be examined thoroughly, and any palpable or visual lesions should be recorded. Particular attention should be given to the fossa glandis and urethral process because they are partially concealed and smegma accumulates in these areas (Figure 13-5, Figure 13-6 and Figure 13-7). Common penile lesions include those of traumatic origin and vesicles or pustules of equine coital exanthema, Habronema granulomas, squamous cell carcinomas, and papillomas (see Chapter 16).

During penile tumescence, the prepuce also is readily available for examination. If the stallion is excitable, it is often helpful to stand beside the horse with one hand on its withers while beginning to palpate the external genitalia with the other hand (Figure 13-8). The skin of the prepuce should be thin and pliable, with no evidence of inflammatory or proliferative lesions. Developmental abnormalities of the penis and prepuce are rare.

The scrotal skin should be thin and elastic, and the scrotum should have a distinct neck. The scrotum and its contents are normally pendulous (except during cold weather because of contractions of the tunica dartos) but may be drawn toward the body during palpation because of voluntary contractions of the cremaster muscles. Both testes and attached epididymides should be relatively symmetrical and freely movable within their respective scrotal pouches. Size, texture, and position of each testis always should be determined as part of a breeding soundness examination. Testicular size correlates highly with daily sperm production and output, so this measurement helps predict a stallion’s breeding potential. Testes of mature (≥4 years of age), fertile stallions generally are approximately 4.5 to 6 cm in width, 5 to 6.5 cm in height, and 8.5 to 11 cm in length; the total scrotal width (largest measurement taken across both testes and the scrotal skin) (Figure 13-9) generally approximates 9.5 to 11.5 cm (Table 13-1). Transcrotal ultrasonographic examination, including accurate measurement of size, can provide useful information about the normality of the scrotal contents (Figure 13-10 and Figure 13-11). When length, width, and height of each testis are measured in centimeters (Figure 13-12, Figure 13-13 and Figure 13-14), testicular volume (in cc) can be estimated with the formula for an ellipsoid (volume = 0.5233 × width × height × length). Volume should be computed for each testis, and the two volumes are added together to provide total testicular volume. Testicular volumes can be compared with averages established for horses of a similar age to give the examiner an impression of whether the testes are small or not (Figure 13-15). The total testicular volume (TV) in cc can also be used to predict the range of expected daily sperm output (DSO) with the following formula:

When predicted DSO is significantly less than actual DSO, low spermatogenic efficiency from testicular dysfunction should be suspected. Low spermatogenic efficiency is associated with increased degeneration of testicular germ cells, which can contribute to ejaculation of low numbers of normal sperm.

In addition to its use in obtaining accurate testicular measurements, ultrasonography can be used to detect intratesticular masses and intrascrotal fluid accumulations and to examine the cavernous spaces of the penis (see Chapter 16). Ultrasonography is also a useful adjunct to examination per rectum (e.g., for evaluation of the accessory genital glands or abdominal/inguinal exploration for an undescended testis in a stallion suspected of being cryptorchid).

Both testes should be oval, with a smooth regular outline and a slightly turgid, resilient texture. The orientation of each testis within the scrotum can be determined accurately via palpation of the attached epididymis. The epididymis normally is palpable in its entirety as it courses over the dorsolateral surface of the testis; however, identification of its borders is sometimes difficult (Figure 13-16). The caudal ligament of the epididymis, a remnant of the gubernaculum, remains palpable during adult life as a small (∼1 cm) fibrous nodule adjacent to the epididymal tail, which, itself, is attached to the caudal pole of the testis. This remnant serves as a landmark for determination of testicular orientation within the scrotum.

Exploration of the spermatic cord is possible via palpation through the neck of the scrotum, although its specific contents often are not definable. Transcrotal ultrasound examination can also be used to visualize these structures. Spermatic cords should be of equal size and uniform diameter (2 to 3 cm). Acute pain in this area usually is associated with inguinal herniation or torsion of the spermatic cord (see Figures 16-21 and 16-23). Inguinal herniation can usually be confirmed with ultrasonography, which reveals some degree of hydrocele (anechoic fluid accumulation between parietal and vaginal tunics) with hyperechoic gut wall patterns, in which fluid within the bowel lumen is sometimes visible (see Figure 16-20). Transrectal palpation or ultrasonography of internal inguinal rings confirms the herniation. Torsion of the spermatic cord can result in congestion of the affected testis (palpably turgid and slightly hyperechoic) with thickening and engorgement of the spermatic cord from vascular obstruction (see Chapter 16).

The internal genital organs (Figure 13-17) can be examined via palpation per rectum. Although adequate restraint is of paramount importance to this procedure, minimal but effective restraint is the key to a safe examination and varies from stallion to stallion. The disposition of the stallion should be determined at the onset, and the examination should be canceled if the risk factor is high. Such precautions protect both the stallion and the operator from severe injury. Before the examination, the stallion should be placed in a stock, if available. Ideally, the stock should be equipped with a solid rear door to help prevent leg extension if the stallion decides to kick. The height of the door should be level with the mid-gaskin region of the stallion’s hindquarters.

The hand should be well lubricated, and all manure in the rectum and distal colon should be removed before evaluation of pelvic and abdominal structures is attempted. The two vaginal rings (abdominal orifices of the inguinal canals) are palpable as slit-like openings approximately a hand’s breadth ventrolateral to the pelvic brim (Figure 13-18). The deferent duct and pulse of the testicular artery usually can be detected at the opening. The site is evaluated for size and for evidence of adhesions or herniation of viscera. The diameter of the opening is normally 2 to 3 cm. A larger opening may predispose the stallion to inguinal herniation or scrotal hydrocele. Some of the accessory genital organs (i.e., the ampullae and bilobed prostate gland) are also readily detected via palpation per rectum and transrectal ultrasonography (Figure 13-19, Figure 13-20 and Figure 13-21). Lesions of the accessory genital glands, however, are uncommon in stallions.

Superficial bacterial colonization of the equine prepuce, penis, and distal urethra results in unavoidable contamination of the mare’s reproductive tract during coitus. A variety of environmental bacteria can be isolated from these sites, many of which contribute to the normal nonpathogenic bacterial flora of healthy stallions. These commensal bacteria tend to prevent overpopulation of the external genitalia with potentially harmful organisms (Klebsiella pneumoniae or Pseudomonas aeruginosa). Mismanagement of breeding stallions (through repeated penile washings with soaps or disinfectants or poor selection and upkeep of bedding) may convert the bacterial population on the surface of the penis from a mixed group of harmless bacteria to a population teeming with potential pathogens. The external genitalia of some stallions harbor large numbers of opportunistic bacteria. However, most bacteria are not considered pathogens unless they are recovered serially in heavy growth or unless postbreeding endometritis with the organism results in mares that have been bred to the stallion. The organism that causes contagious equine metritis, Taylorella equigenitalis, represents the only known bacterium capable of consistently producing venereal disease in horses. The stallion serves as a lesionless carrier of this disease, harboring the bacteria on its external genitalia, with subsequent horizontal transmission to the mare’s reproductive tract at breeding.