OVC Pathobiology
University of Guelph
The terminology of infertility and reproductive pathology requires a reset! This is what I am doing here. Any reproductive disease that is the result of a failure of normal development is a Disorder of Sexual Development (DSD). Examples are the congenital and or hereditary diseases such as testicular hypoplasia, cryptorchidism, and infertility because of genetic rearrangements or deletions. I will cover these in the section on Disorders of Sexual Development. Many are XY SRY+ testicular DSD.
The two main concerns with disease of the testis as well as the rest of the reproductive tract are infertility and diseases that cause morbidity or mortality. Infertility is usually the domain of the theriogenologist (therí(on) is Greek for beast and gén (os) is Greek for offspring or sex) and the pathologist can contribute by identifying the underlying reason or diagnosis and the cause. Male infertility is a combination of lack of production of spermatozoa, production of viable spermatozoa, transportation and maturation of spermatozoa and being able to deposit the spermatozoa in the appropriate location mixed with seminal fluid..There are general reviews on diseases of the reproductive tract of the stallion including Edwards (2008)
Edwards JF. Pathologic conditions of the stallion reproductive tract. Anim Reprod Sci. 2008; 107: 197-207.
Aspermia is the inability to produce semen. Azoospermia is a lack of spermatozoa in semen.
These relate to semen evaluation and are not directly the domain of the pathologist. Causes of aspermia and azoospermia will be considered in the various sections below.
Testicular atrophy is when the testis becomes smaller in size. It is a macroscopic term. The corresponding microscopic change is degeneration. It requires experience to differentiate atrophy from hypoplasia histologically, and a knowledge that the testis became smaller is important.
Maintenance of the seminiferous epithelium requires a close intergration of pituitary hormones, systemic and local environments, and of the crosstalk between germ cell, testicular sustentacular (Sertoli) cell and interstitial endocrine cell. Injury to any one of these will upset the equilibrium necessary for normal spermatogenesis.
The causes of testicular atrophy - degeneration are legion. They are divided conveniently into those with a known cause and those without. This latter group is called idiopathic testicular degeneration.
Here are some of the known causes
- trauma
- high environmental temperature
- fever
- local inflamation
- scrotal dermatitis
- periorchitis
- poor health and debility
- advanced age.
- hernioplasty
Woodward et al (2023) examined the transcriptome of a small number of normal stallions and those with degeneration (age related?). This is a preliminary paper with data that is difficult to interpret. It does provide a start to evaluating the gene related changes in degeneration of the testis. There appeared to be a shift to an inflammatory type.
Woodward E, Schlingmann K, Tobias J, Turner R. Characterisation of the testicular transcriptome in stallions with age-related testicular degeneration. Equine Vet J. 2023; 55: 239-252.
Hernioplasty
Stallions subjected to hernioplasty develop mild degeneration of the the testis, presumably from slight pressure changes to venous outflow.
Gracia-Calvo LA, Ezquerra LJ, Ortega-Ferrusola C, Martín-Cuervo M, Tapio H, Argüelles D, Durán ME. Histological findings in equine testes one year after standing laparoscopic peritoneal flap hernioplasty. Vet Rec 2016; 178: 450
Oristaglio Turner RM. Pathogenesis, Diagnosis, and Management of Testicular Degeneration in Stallions. Clin Tech Equine Pract 6:278-284
Macroscopic changes
Microscopic changes
Testicular necrosis is where there is death of regions or all of the testis. Torsion of a fully descended testis is virtually unheard of in all species expect for the horse.
Some cases of testicular necrosis are only found a long time after the necrotic event. What remains is the epididymis and little else. Some of these are considered monorchidism. The general rule is that the epididymis required a testis to develop (testicular sustentacular (Sertoli) cell production of Mullerian inhibitory substance (MIS)) so that the uterus doesnt develop and the epididymis does. For example Santschi et al (1989) reported on 3 cases of monorchidism were the epididymis was present.
Santschi EM, Juzwiak JS, Slone DEMonorchidism in three colts. J Am Vet Med Assoc 1989; 194: 265-266.
The horse is one of the few species that can develop torsion in a normally descended testis. All other species get torsion of undescended/cryptorchid testes only.
Pascoe et al (1981) reported on a case of unilateral testicular torsion that may have been predisposed by a greater distance between the testis and the tail of the epididymis. The affected testis was swollen, but other details were not reported.
Raisi and Davoodi (2022) reviewed testicular torsion in animals and the stallion is one of 2 groups that get testicular torsion most commonly.
Pascoe JR, Ellenburg TV, Culbertson MR Jr, Meagher DM. Torsion of the spermatic cord in a horse. J Am Vet Med Assoc 1981; 178: 242-245.
Raisi A, Davoodi F. Testicular torsions in veterinary medicine. Vet Res Commun. 2022; 46: 303-313.
Inflammation of the testis is called orchitis.In stallions, mild interstitial orchitis is common and incidental. Similar lesions may be part of generalized vascular involvement in equine viral arteritis. Infarcts may also occur in equine infectious anemia. and as an acute, sometimes abscess-forming orchitis in infection with Salmonella abortus-equi, Streptococcus equi, and Streptococcus zooepidemicus. A pseudocyst is reported in a horse as a sequel to fibrinonecrotic orchitis. The lesion was associated with trauma, and secondary infection with Streptococcus zooepidemicus occurred.
Testicular rupture, hemorrhage and orchitis can occur with trauma to the testis, such as when a stallion is kicked while mating a mare. Spermatic granulomas in the testis occur secondarily.
Periorchitis in horses occurs as part of generalized septic disease, peritonitis and as a complication of trauma, penetrating injury, or surgery.
Actinobacillus spp
Family Pasteurellaceae; Actinobacillus equuli subsp equuli and subsp haemolyticus
Bacteria of the genus Actinobacillus are normal inhabitants of the alimentary and reproductive tract of horses. They can cause septicemia and reproductive disease (endometritis and failure of pregnancy). They are often recovered with Streptococcus equi subsp zooepidemicus.
Orchitis caused by Actinobacullus equuli subsp equuli was reported by Layman et al (2018) but no details were provided.
Layman QD, Rezabek GB, Ramachandran A, Love BC, Confer AW. A retrospective study of equine actinobacillosis cases: 1999-2011. J Vet Diagn Invest 2014; 26: 365-375.
Burkholdaria mallei
Orchitis from the bacterium of glanders is reported.
Nematodes
Halicephalobus gingivalis
Halicephalobus gingivalis can cause granulomatous orchitis as a consequence of systemic spread. Kinde et al (2000) reported on a miniature stallion that had an enlarged left testis - in addition to systemic signs. The testicular lesions were diffuse granulomatous orchitis with multinucleated giant cells, macrophages, lymphocytes, and fibroblasts. Adult nematode females, larvae, and occasional embryonated eggs were in large numbers.
Kinde H, Mathews M, Ash L, St Leger J. Halicephalobus gingivalis (H. deletrix) infection in two horses in southern California. J Vet Diagn Invest 2000; 12: 162-165
Strongyles
The larvae of Strongylus vulgaris and strongylus edentatus are reported in horses.
Smith (1973) found 11 of 120 cases of cryptorchid testes had hemorrhagic tracts and live Strongyle larvae in at least one of the testes. Histologically the lesions were intensely eosinophilic. Larvae were also present in the spermatic cord including the pampiniform plexus. They were speciated as Strongylus edentatus.
Smith JA. The occurrence of larvae of Strongylus edentatus in the testicles of stallions. Vet Rec 1973; 93: 604-606.
Strongyles
Focal lesions attributed to the migrating larvae of Strongylus edentatus is reported in the testis, tunics, and epididymis, especially of young horses. Cryptorchid testes are more commonly affected. Hemorrhagic 2 mm wide tracts containing the migrating larvae are seen. The histological lesions are initially hemorrhagic and then become eosinophilic.
Macroscopic orchitis
Microscopic (subclinical) orchitis
Degeneration of retained testes
Testes retained in the abdomen, despite their hypoplastic nature, also undergo a process of degeneration and they will atrophy. The process is a degenerative one, the histological appearance is that of degeneration, but the macroscopic change is atrophy - this is the convention used to define these particular terms as they apply to the testis. For further information about testicular degeneration, see the section below.
The process of degeneration of a retained testis is thought to mirror the degenerative changes of a scrotal testis, although the lack of spermatogenesis or spermatic arrest already exists. Tubules shrink in size, the basement membranes become wavy, and with time, the testicular sustentacular (Sertoli) cells disappear so that all that remains is an outline of the previous tubule. The epididymis remains as a hypoplastic tube, but over time, it too may 'fade'and only a vestige remains. This usually occurs after the testis becomes completely degenerate. On occasion, the testis suffers a severe process where it dies, and the necrosis is assumed to be a vascular event. Torsion of the retained testis can occur and both testis and epididymis will be affected.
This is clinically called monorchidism.
Clements et al (2020) reported on 10 cases of monorchidism in cryptorchids. Each had a vaginal process, 8 had an epididymis and deferent duct, 5 had a ligament of the tail of the epididymis and testicular vessels. Histology was done on 3 cases and they had "aplastic testicular tissue", "a small nodule of dense fibrous tissue with pigment laden macrophages" and a "fibrotic nodule".
Clements PE, Coomer RPC, McKane SA, Gorvy D, Bladon B, Mair T. Clinical findings in 10 horses diagnosed with monorchidism following exploratory laparotomy or standing laparoscopic surgery. 2020; 32: 431-436
Testicular hypoplasia of horses is not well studied and prevalence is not available, partly because most male horses are gelded. It occurs with sex chromosome DSD, and as a sporadic condition. Many cases are mild and unilateral – one testis is smaller that the other.
Testicular hypoplasia in horses is discussed under Disorders of Sexual Development
Cryptorchidism (Retained testes)
Equine cryptorchidism is discussed under Disorders of Sexual Development
Testicular hypoplasia
Testicular dissymmetry is a common clinical presentation. The natural tendency for clinicians is to assume that the larger testis is abnormal and potentially contains a neoplasm. The larger testis may be presented for examination. Consideration for testicular dissymmetry should be given for another possibility - unilateral testicular atrophy. A combination of changes may be operative, and should be considered when the larger testis is grossly 'normal'. One may be smaller, and the other larger. Neoplasia is a common cause of testicular enlargement, and some testicular neoplasms cause testicular atrophy of the contralateral testis. Testicular hypertrophy is seldom considered though. The list of conditions causing testicular enlargement therefore include
- neoplasia (primary)
- neoplasia (secondary)
- orchitis
- cystic rete testis
- rete cysts
- spermatocele
The testis is capable of hypertrophy. This enlargement is not dramatic. The basis is believed to be a greater stimulation of interstitial endocrine cells and testicular sustentacular (Sertoli) cells by the hormones of pituitary origin (LH and FSH) that are not inhibited in a negative feedback system. Enlargement of an otherwise normal testis is rarely a primary disorder. Secondary hypertrophy is a unilateral condition that indicates an underlying disease in the contralateral testis.
Hyperplasia of testicular elements is an unusual finding. Despite a greater number of particular elements, hyperplasia does not cause enlargement of the testis. It is a microscopic finding usually. There is no known link with neoplasia.
Testicular sustentacular (Sertoli) cell hyperplasia
Sex cord - stromal
Neoplasms with a phenotype resembling the cells that originate from stroma or sex cords of the primitive gonad include the interstitial cell and testicular sustentacular (Sertoli) cell tumour.
Interstitial cell tumour
Interstitial cell tumors grow slowly and expansively with surrounding compression atrophy. They are not notably invasive. In stallions with interstitial cell tumors, semen production and fertility may be reduced.
Gelberg and McEntee (1987) reported on 9 stallions with interstitial cell tumors. There was no followup to determine prognosis.
Gelberg HB, McEntee K. Equine testicular interstitial cell tumors. Vet Pathol 1987; 24: 231-234.
Testicular sustentacular (Sertoli) cell tumour
Pratt et al (2003) reported on a cryptorchid horse that had a testicular sustentacular (Sertoli) cell tumor that had metastasised along the pampiniform plexus and to the spleen, liver lung and lymph nodes.
Pratt SM, Stacy BA, Whitcomb MB, Vidal JD, De Cock HE, Wilson WD. Malignant Sertoli cell tumor in the retained abdominal testis of a unilaterally cryptorchid horse. J Am Vet Med Assoc 2003; 222: 486-490.
Germ cell tumors
Neoplasms of the testis categorized as germ cell tumours have a phenotype indicating they originated from the primordial germ cells that originated in the yolk sac and migrated to the gonad. They become spermatogonia. These cells have an amazing degree of pluripotency, being able to differentiate to any tissue in the body. When they form tissues of all different embryonic types (ectoderm (including neuroectoderm, mesoderm, and endoderm) they are called teratomas.
Seminoma
Seminoma is the most common neoplasm of the adult male horse. It is more common in retained testes.
Batista et al (2024) reported on a diffuse seminoma and performed immunohistochemistry for germ cell neoplasia - OCT4 and C-KIT were positive. AE1AE3, Placental Alkaline Protein (PAP) and Inhibin were negative.
Batista LAS, de Alencar Santos Júnior D, Rodrigues AS, Menezes AA, do Nascimento MJR, de Galiza GJN, de Medeiros Dantas AF, Frade MTS. Morphological and immunohistochemical characteristics of diffuse seminoma in horses: A case report. Reprod Domest Anim. 2024; 59: e14706.
Govaere et al (2010) reported a case of bilateral seminoma.
De Lange et al (2015). reported on 2 stallions with retained testes and seminoma. They were both metastatic throughout the abdomen.
Batista et al (2024) reported on a case. They performed immunohistochemistry - OCT4, cKIT, AE1AE3, Placental Alkaline Protein and Inhibin. The cells were OCT4 and cKIT positive. The rest were negative.
Batista LAS, de Alencar Santos Júnior D, Rodrigues AS, Menezes AA, do Nascimento MJR, de Galiza GJN, de Medeiros Dantas AF, Frade MTS. Morphological and immunohistochemical characteristics of diffuse seminoma in horses: A case report. Reprod Domest Anim. 2024; 59: e14706.
De Lange V, Chiers K, Lefre L, Cools M, Ververs C, Govaere J. Malignant seminoma in two unilaterally cryptorchid stallions. Reprod Domest Anim. 2015; 50: 510-513.
Govaere J, Ducatelle R, Hoogewijs M, De Schauwer C, de Kruif A. Case of bilateral seminoma in a trotter stallion. Reprod Domest Anim. 2010; 45: 537-539.
Seminoma with neuroectodermal differentiation
Teratoma
Teratomas are the most common neoplasm of the testis of the young horse. There are many reports including
Cribb and Boure (2010) reported on the surgical removal of a teratoma from a young horse.
Cribb NC, Bouré LP. Laparoscopic removal of a large abdominal testicular teratoma in a standing horse. Vet Surg 2010; 39: 131-135.
Embryonal carcinomas
Mixed germ cell - sex cord stromal (gonadoblastoma
Zanghi et al (2004) diagnosed a stallion with a testicular tumor as having a malignant mixed sex cord-stroma tumor.
Zanghi A, Catone G, Marino G, De Vico G, Nicòtina PA. Malignant mixed sex cord-stromal tumour in a stallion. Reprod Domest Anim. 2004; 39: 376-379.
Neoplasms of the collecting system (including the rete)
This section is restricted to neoplasms only. If we choose to use the term 'tumour' as a swelling, then tumours can also be cysts! Cysts of the testis are described below.Rete neoplasms
The collecting ducts of the testis include the rete testis and efferent ductules. Neoplasms of these ductules are epithelial in type, so epithelial tumors of the testis are believed to be derived from these. Neoplasms with an phenotype resembling epithelial tissue can also be part of a teratoma (or teratocarcinoma) and occasionally, cystic testicular sustentacular (Sertoli) cell tumours can have an epithelial appearance. It is important to perform multiple sections to exclude teratoma and to do immunohistochemistry for various cytokeratins to positively identify the neoplastic cells as epithelial.
Miscellaneous primary neoplasms
The presence of cells from a wide range of tissue types means that neoplasia of those cells can result in an almost infinite range of neoplasms. This reported in case reports or other sites are indicated below.Hemangioma/hemangiosarcoma
Smooth muscle tumour (Leiomyoma or leiomyosarcoma)
Allison and Moeller (1999) reported the diagnosis of bilateral leiomyosarcomas in the testis of an 11 month old horse.
Stoll et al (2015) reported on a 19 year old pony that had bilateral 'leiomyomas'.
Allison N, Moeller RB Jr. Bilateral testicular leiomyosarcoma in a stallion. J Vet Diagn Invest. 1999; 11: 179-182.
Stoll AL, Tucker R, Witte TH, Smith KC. Bilateral leiomyoma in the testes of a horse. Equ Vet Educ 2015; 27: 113-117
Tumours metastatic to the testis
Testes are not routinely examined in animals with metastatic neoplasia, and, if a metastatic disease is present and the testis is found to have a metastasis, mention is made in the report, but an electronic literature search is unlikely to find it. One might expect the most likely candidate would be lymphoma.Lymphoma
Testicular spermatocele and spermatic granuloma
A spermatocele is a cavity filled with spermatozoa. Small spermatoceles are dilated tubules such as the straight seminiferous tubule (also called the tubuli recti), or rete testis. Those cavities that are not lined by epithelium are where spermatozoa leak into the interstitium. These cavities quickly incite an inflammatory response and become spermatic granulomas. Spermatoceles and spermatic granulomas are unusual in the testis as a primary lesion as most are secondary to orchitis, needle biopsy or trauma.
Cysts of the testis
Cysts within the testis, lined by epithelium and with a central cavity filled with fluid, can be derived from the rete testis, a network of ducts or from remnants of mesonephric tubules called the efferent ductules. Mesonephric tubules have ciliated epithelium and this is a major differentiating factor.
Cystic rete testis
A cystic rete testis is different to other cysts in that it is a cystic dilation of the rete tubules secondary to obstruction of fluid outflow. It can be secondary to segmental aplasia of the mesonephric duct or spermatic granuloma of the epididymal head. Most cases would be missed or not deemed significant, especially if the primary lesion was identified. Typically, the mediastinum of the testis is affected so the testis has a large central cavity.
Schumacher et al (1994) reported on a case of cystic rete testis in a cryptorchid horse.
Rijkenhuizen et al (2020) reported on a cystic intra-abdominal testis with a large single cyst suspected to be a rete cyst.
Rijkenhuizen ABM, Lichtenberg D, Weitkamp K. Cystic intra‐abdominal testicles: Standing laparoscopic removal in two colts. Equine Vet Educ 2020; 32: e130-e135Schumacher J, Lenz SD, Walker W. Cystic rete testis associated with cryptorchidism in a horse. Vet Pathol 1994; 31: 115-117.
Efferent ductule cyst
Cysts of the efferent ductules have an epithelial lining and a central cavity filled with fluid. These are different to cystic rete testis, a condition where there is cystic dilation of all of the rete testis from obstruction to outflow as would occur in segmental aplasia of the epididymis or spermatic granuloma of the epididymal head (see above). They are lined by ciliated epithelium, a distinguishing feature from a 'rete cyst'.
Rete testis cyst
Epidermoid cyst
Cystic interstitial cell tumour
One of the differential diagnoses of cystic rete testis is a rete cyst. Cystic interstitial cell tumour can, on rare occasions, be cystic and confused with a true cyst. Somewhere in the cystic cavity will be neoplastic cells. There will be no epithelium so if a cystic structure is found and histologically there is no epithelial wall, examination of the formalin fixed tissue will be necessary to identify the origin of the 'cyst' or, more correctly, thepseudocyst.
Interstitial cell tumors in stallions occur most often in undescended testes. They contain 2 cell types; the first is essentially a hypertrophic interstitial endocrine cell but the other is a pleomorphic fusiform cell with fibrillar, vacuolated cytoplasm and indistinct borders. Increased hormone concentration is reported in horses with these tumors, and viciousness corrected by castration, was observed.
The efferent ductules are the tubules that link the rete testis to the epididymal duct. Diseases, such as spermatic granuloma of the epididymal head, and cysts of the ducts are to be found under disease of the epididymis (non inflammatory) below.