History
A 20-year-old gray Lusitano stallion was examined at the Veterinary Medical Teaching Hospital of the University of California-Davis because of a 3-month history of unilateral (right-sided) scrotal swelling and 10-day history of generalized swelling of the preputial sheath. There was no history of trauma as a cause for the swellings. During the week prior to examination, the preputial sheath became progressively more swollen and the stallion did not exteriorize the penis during urination. The stallion had last been used for breeding 2 years previously, and the owners wanted semen collected from the stallion during the current breeding season.
Initial examination revealed that the horse was bright, alert, and responsive and in good body condition (body condition score, 5/9; body weight, 476 kg [1,047 lb]). All vital signs were within reference range (rectal temperature, 37.94°C [100.3°F]; heart rate, 44 beats/min; and respiratory rate, 28 beats/min).
The stallion was sedated with detomidine hydrochloride (0.01 mg/kg [0.0045 mg/lb], IV) to facilitate evaluation of the reproductive tract. Examination revealed extensive, widespread ulcerative lesions on the urethral process, glans penis, penile shaft, and prepuce (Figure 1). The right testis was grossly large and firm; palpation did not elicit signs of pain and revealed that the testis felt cool. The left testis was palpably within anticipated limits. A mass (1.5 × 2 cm in diameter) that was suspected to be a melanoma was identified on the perineum near the anus; no other masses were detected during transrectal examination. Palpation of the accessory sex glands did not elicit signs of pain and revealed that they were of the anticipated size and consistency. The inguinal lymph nodes were not palpably large.
Photograph of widespread ulcerative lesions on the penis of a 20-year-old Lusitano stallion.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1105
Ultrasonography was performed and revealed that the right testis was large (10.7 × 10.2 × 10 cm) and had a heterogeneous appearance with abnormal architecture (Figure 2). The left testis was of anticipated size (7.3 × 5.8 × 5.3 cm) and appearance with homogeneous parenchyma; both the central vein and epididymis were identified. Transrectal ultrasonography was also performed and revealed that the accessory sex glands were within anticipated limits. Biopsy specimens of lesions on the glans penis and prepuce were collected.
Ultrasonographic image of the right testis of the stallion in Figure 1. Distance between the cursors (A to A) is 10.7 cm.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1105
While we awaited results of the histologic examination, we made numerous attempts to collect semen from the stallion for cryopreservation. Because of the extensive nature of the penile lesions, there was profuse hemorrhage and hence blood contamination of the semen, despite use of an open-ended artificial vagina and separating the ejaculate by collecting individual pulses in an attempt to obtain only blood-free semen. We attempted to reduce the blood contamination by processing the semen with both a commercially available density-gradient solutiona and centrifugation, but this was unsuccessful because of the density of the blood-contaminated semen sample. Pharmacologic induction of ejaculation was attempted with imipramine hydrochloride (2.2 mg/kg [1 mg/lb], PO) administered 2 hours before the administration of xylazine hydrochloride (0.3 mg/kg [0.14 mg/lb], IV); however, ejaculation was not achieved.
Question
What are the most likely causes of the lesions on the penis and prepuce of this stallion and the unilaterally large scrotum? Please turn the page.
Answer
The most likely differential diagnoses for the penile lesions include squamous cell carcinoma, sarcoids, and habronemiasis; the combination of a unilaterally large scrotum with abnormal testicular architecture evident ultrasonographically and with no history of trauma is most consistent with testicular neoplasia.
Results
Histologic examination confirmed squamous cell carcinoma of the penis. Because of the extensive nature of the lesions, en bloc resection was recommended to prevent potential metastasis to distant sites. The owner declined this treatment option because of the perception that en bloc resection was too invasive. Instead, the owner elected for resection of the lesions on the penis and prepuce via partial penile amputation and segmental posthetomy, respectively, and removal of both testes. If possible, gamete rescue was to be attempted for spermatozoa harvested from the epididymis of the left testis.
The horse was treated by perioperative administration of gentamicin sulfate (6.6 mg/kg [3 mg/lb], IV, q 24 h), penicillin G procaine (25,000 U/kg [11,364 U/lb], IM, q 12 h), flunixin meglumine (1.1 mg/kg [0.5 mg/lb], IV, q 24 h), and a single dose of tetanus toxoid (1 mL, IM). The horse was then sedated by IV administration of xylazine hydrochloride (1.1 mg/kg). Anesthesia was induced by IV administration of diazepam (0.2 mg/kg [0.09 mg/lb]) and ketamine hydrochloride (2.5 mg/kg [1.14 mg.lb]) and maintained by administration of isoflurane in oxygen via a semiclosed circle system. A balanced electrolyte solutionb was administered IV (rate, 5 mL/kg/h [2.23 mL/lb/h]) during the anesthetic period.
The horse was positioned in dorsal recumbency and aseptically prepared for bilateral castration. Incisions were made in the scrotum lateral to the median raphe over both the left and right testes to facilitate a closed castration technique with primary closure of the skin incisions. The ductus deferens of the unaffected left testis was ligated at the most proximal location possible to minimize loss of spermatozoa prior to attempted gamete rescue. Partial phallectomy and segmental posthetomy were performed to remove the penile and preputial masses.1 The suspected melanoma was excised from the perineal region. The testis and penile and preputial masses were submitted for histologic examination; the suspected melanoma was not submitted for histologic examination.
Gross and histologic examination of the right testis and resected portion of the distal aspect of the penis and urethra was performed. The right testis measured 17 × 11 × 9 cm. Approximately 95% of the testicular parenchyma was displaced by a 12 × 9.5 × 9-cm, multilobular mass that effaced and compressed all but a small peripheral segment of grossly evident testicular parenchyma. The mass was soft, bulging, and pale tan with red mottling in some areas, whereas it was fibrous, firm, and pale tan to yellow in other areas (Figure 3).
Photograph of the cut surface of the right testis after it was removed from the stallion of Figure 1. A large mass effaces most of the parenchyma, leaving only a small peripheral rim of grossly evident testicular parenchyma (arrows). The mass contains soft and bulging areas (asterisk) as well as areas that are firm and yellow (arrowhead). Bar = 2 cm.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1105
Histologic examination revealed that the mass was composed of sheets of neoplastic cells separated by anastomosing bands of fibrous stroma, within which cells occasionally formed packets. Neoplastic cells were round with a moderate amount of eosinophilic cytoplasm. Nuclei were hyperchromatic and round with vesicular chromatin and a prominent magenta nucleolus. Multinucleated neoplastic cells (up to 7 nuclei/cell) were seen occasionally. Cellular atypia was moderate. There were 28 mitotic figures in 10 hpfs (400×). Small- and medium-caliber vessels frequently contained groups of neoplastic cells, which is diagnostic for malignant neoplasia (Figure 4). Small numbers of lymphocytes were scattered throughout the mass. Morphologically, the mass was consistent with a seminoma, although the induction of abundant fibrous stroma was suggestive of a sustentacular (Sertoli) cell tumor. Because the mass had areas that were morphologically consistent with both a seminoma and Sertoli cell tumor, immunohistochemical stains (antibodies against inhibin, neuron-specific enolase, Oct 3/4,c and anti-Müllerian hormoned) were applied. Stains failed to label the neoplastic cells, and the mass was diagnosed as a collision tumor with a malignant seminoma and a suspected second neoplastic population (Sertoli cell tumor). The surrounding seminiferous tubules were compressed and lacked identifiable spermatozoa precursors.
Photomicrograph of a tissue section of the testicular mass. Sheets of neoplastic cells are separated by dense bands of fibrous stroma. A vessel contains numerous neoplastic cells (asterisk). Inset: Higher-magnification view of the neoplastic cells reveals moderate anisocytosis and anisokaryosis as well as a mitotic figure (arrowhead). H&E stain; bar = 80 μm and 20 μm for the photomicrograph and inset, respectively.
Citation: Journal of the American Veterinary Medical Association 247, 10; 10.2460/javma.247.10.1105
Grossly, the distal portion of the penis and urethra had multifocal depressions in the glabrous skin that were surrounded by smooth, raised, cutaneous margins. Histologically, the depressions corresponded to areas of proliferative and dysplastic epidermis with random keratinization, anisocytosis, anisokaryosis, and up to 15 mitotic figures in 10 hpfs (400×), which was consistent with the diagnosis of carcinoma in situ. In some areas, neoplastic cells breached the basement membrane (which was consistent with the diagnosis of squamous cell carcinoma). Additionally, superficial blood vessels had cell-poor vasculitis and mild elastosis, which were consistent with a solar-induced condition. Dysplastic changes were multifocal and widespread, which suggested that glabrous skin from this area that remained on the horse had a high potential for neoplastic transformation.
Thus, the right testicular mass was diagnosed as a seminoma with intravascular tumor emboli and a suspected Sertoli cell tumor. Glabrous skin of the penis and prepuce was identified as having a squamous cell carcinoma with moderate to severe multifocal epithelial dysplasia and hyperplasia.
Discussion
Testicular neoplasia in stallions is considered to be a rare condition; however, the actual incidence of disease is difficult to determine because of the high number of male horses gelded at an early age and the lack of submission of testicular masses for histologic examination.2
Testicular neoplasia can be categorized into tumors of germ cell origin, tumors of interstitial and stromal cell origin, and tumors of other cell origin. The most common tumors of germ cell origin in horses are seminomas and teratomas. Other germ cell tumors in horses are rare conditions and highly aggressive.3
Testicular neoplasia must be differentiated from other causes of scrotal enlargement, such as orchitis, epididymitis, hydrocele, varicocele, hematoma, torsion of the spermatic cord, and inguinal or scrotal hernia.4 In the horse described here, there was no history of trauma or evidence of an inflammatory condition. The affected testis was firm, and palpation of the testis, epididymis, and spermatic cord did not reveal evidence of heat or edema or elicit signs of pain. The stallion was assessed as generally healthy with no abnormalities detected on a CBC and no signs of abdominal pain. Other differential diagnoses were ruled out on the basis of results of the testicular ultrasonographic examination. There was no evidence of intestine within the scrotum, such as in the case of herniation, and no abnormal accumulation of fluid within the tunica vaginalis, such as in the case of a hydrocele, hematocele, or varicocele. In the horse of the present report, firm, unilateral testicular enlargement and the ultrasonographic appearance of a mass obliterating the testicular parenchyma increased the index of suspicion for neoplasia. On the basis of the diffuse nature of these changes, it was believed that results for examination of a biopsy specimen or fine-needle aspirate would not have altered the need for surgical resection and that histologic examination of the entire testis would be superior for determining a definitive diagnosis.
In cases in which the testicular tumor type is associated with an increased risk of metastasis, such as for seminomas, careful palpation and examination of the spermatic cord and contralateral testis are essential. In addition, transrectal palpation may reveal large medial iliac or lumbar aortic lymph nodes in animals in which there has been metastasis. No gross evidence of metastasis was detected in the horse of the present report.
Treatment for testicular tumors is surgical removal of the affected testis or testes. Hemicastration with removal of an affected testis can be performed, which would leave the unaffected testis to preserve future breeding potential. However, for the horse of this report, the extensive nature of the penile lesions and the need for partial phallectomy to reduce the risk of neoplastic transformation made it unlikely that it would be possible to collect semen from this stallion following surgery. Therefore, it was decided that bilateral castration should be performed with the intention that it would be possible to retrieve and cryopreserve epididymal spermatozoa from the left (unaffected) testis. Although the unaffected testis appeared ultrasonographically normal before surgery, examination of a testicular biopsy specimen was not performed. Thus, it was not known at the time of surgery whether there had been any adverse effects on spermatogenesis as a result of local metastasis of the seminoma or because of thermal degeneration. Gamete rescue provides the ability to harvest and store epididymal spermatozoa from a stallion following castration, euthanasia, or death.5 In the horse described here, it was intended to preserve genetics that would otherwise have been lost.
Squamous cell carcinoma is the most common neoplasm of the external genitalia of male equids. It typically affects older horses and is most frequently located on the glans penis, such as in the horse of the present report. Furthermore, for a horse with a history of regular anthelmintic treatments and a nonpigmented prepuce and penis, such as the horse described here, the index of suspicion would be increased that the penile lesions were likely caused by a squamous cell carcinoma, rather than as a result of a sarcoid or habronemiasis.
Squamous cell carcinoma can be induced by exposure to UV light through induction of DNA damage that overwhelms the DNA repair machinery and leads to neoplasia. Because the glabrous skin of the glans penis of the stallion described here had other evidence of solar-induced damage and the squamous cell carcinoma was multifocal, it is likely that these lesions were caused by exposure to UV light (direct exposure or indirect exposure through reflection from substrates such as sand). An overall success rate of 55.7% for the treatment of squamous cell carcinomas has been reported.6
In the stallion described here, partial phallectomy and segmental posthetomy were performed to remove penile and preputial masses. On the basis of results of the histologic examination, there was concern that if glabrous skin were allowed to remain, it would likely have a high potential for neoplastic transformation; therefore, en bloc resection was indicated. The owner declined the option for en bloc resection because of the highly invasive nature of the surgery and the potential for serious complications; however, the owner was warned that the prognosis for this horse was guarded because of the incomplete tumor resection.
It is likely that a genetic mutation is the cause of melanomas. A mutation in the Syntax 17 gene has been identified as the causative agent for a gray coat and melanoma development.7
If the testicular tumor contained 2 distinct neoplastic cell populations, it likely was a collision tumor. It also can be speculated that the testicular tumor represented induction of 1 neoplastic population by the other neoplastic population through secretion of cytokines, hormones, or growth factors or mutations in tumor-suppressor genes.
Outcome
During assisted recovery from anesthesia, the stallion fell, which resulted in fracture of the left tuber coxa. The horse was confined to a box stall and provided analgesics for 14 days. The horse then was discharged to the owner with instructions for an additional 8 weeks of confinement in a box stall to allow for healing of the fractured tuber coxa.
Gamete rescue was performed for spermatozoa in the left epididymis by use of techniques described elsewhere.5 The semen was flushed from the epididymis, centrifuged, and reconstituted with 10% seminal plasma obtained from a fertile donor stallion. Extendere was added to the semen, and it was cryopreserved. Post-thaw analysis of a sample confirmed that a total of 15 breeding doses (250 million progressively motile spermatozoa/dose) were obtained.
EquiPure, Nidacon International AB, Gothenburg, Sweden.
Veterinary Lactated Ringer's Irrigation, Abbott Laboratories, North Chicago, Ill.
Provided by Dr. Kevin Woolard, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California-Davis, Davis, Calif.
Provided by Dr. Alan Conley, Department of Population Health and Reproduction, School of Veterinary Medicine, University of California-Davis, Davis, Calif.
EZ-Freeze-LE extender, Animal Reproduction Systems, Chino, Calif.
References
1. Brinsko SP, Blanchard TL, Varner DD, et al. Chapter 16: surgery of the stallion reproductive tract. In: Manual of equine reproduction. 3rd ed. St Louis: Mosby, 2011;242–275.
2. Schumacher J. Testicular neoplasia of horses: an underreported condition. Equine Vet J 1999;31:270–272.
3. Valentine BA. Equine testicular tumors. Equine Vet Educ 2009;21:177–178.
4. Christensen BW, Ernst NS, Powe JR, et al. Theriogenology question of the month. J Am Vet Med Assoc 2007;231:531–534.
5. Bruemmer JE. Collection and freezing of epididymal stallion sperm. Vet Clin North Am Equine Pract 2006;22:677–682.
6. van den Top JGB, de Heer N, Klein WR, et al. Penile and preputial squamous cell carcinoma in the horse: a retrospective study of treatment of 77 affected horses. Equine Vet J 2008;40:533–537.
7. Seltenhammer MH, Sundström E, Meisslitzer-Ruppitsch C, et al. Establishment and characterization of a primary and a metastatic melanoma cell line from grey horses. In Vitro Cell Dev Biol Anim 2014;50:56–65.
