Disorders of the Testes

In the absence of genetic and hormonal influences to the contrary, normal mammalian sexual differentiation will be female. In the embryo, cells from the genital ridge become bipotential, or indifferent, gonads. The single most important factor that determines the development of a testis from the indifferent gonad is located on the short arm of the Y chromosome. This has been termed the sex-determining region Y, the Sry gene. Other genes on the autosomes and the sex chromosomes are also involved in determination of testis or ovary [1]. The hormones produced by the fetal testis in turn are responsible for the differentiation into the male phenotype. Fetal Sertoli cells produce Müllerian-inhibiting substance (MIS), which, with its receptor, causes regression of the Müllerian ducts that would otherwise have developed into the oviduct, uterus, and cranial vagina. Fetal Leydig cells produce testosterone, which, with androgen receptor, causes the Wolffian ducts to stabilize and develop into the ductuli deferentes and epididymes [2,3]. Dihydrotestosterone causes the urogenital sinus, the genital tubercle, and the genital swelling to differentiate into the urethra and prostate, penis, and scrotum, respectively [3]. Testicular differentiation is observed in the canine fetus at day 36 of gestation. Müllerian duct regression is complete by day 46 [4].

Factors produced by the fetal testis also cause testicular descent from its fetal position near the caudal pole of the kidney, through the inguinal canal, into the scrotum. The transabdominal testicular descent is mediated by insulin-like 3 ligand (Insl3) and its receptor. Insl3 is produced by pre- and postnatal Leydig cells and induces growth and differentiation of the gubernaculum from the caudal suspensory ligament. The transabdominal migration is androgen-independent, whereas, the inguinoscrotal descent is mediated by testosterone. Testosterone causes regression of the cranial suspensory ligament [2,5]. During the inguinoscrotal phase of migration, shortening of the gubernaculum and eversion of the cremaster muscle occurs. Normal testicular descent is a prenatal event in cats, and is normally complete by 10 to 42 days of age in dogs [6].

Cryptorchidism

Failure of normal testicular descent is referred to as cryptorchidism, which means hidden or covered testes. The condition can be bilateral or unilateral. The undescended, or "retained" testes may be in an intraabdominal location anywhere along the line from the caudal pole of the kidney to the inguinal canal. Undescended testes can also be located in the inguinal canal or the subcutaneous tissue between the external inguinal ring and the scrotum. On rare occasions, a unilateral cryptorchid testis is found in the perineal subcutis just caudal or lateral to the scrotum.

The neonatal testes in puppies and kittens are mobile and easily move out of the scrotum back toward the inguinal area. For this reason, it is sometimes difficult to confidently diagnose cryptorchidism in young animals. By 6 to 8 weeks of age, a typical age for puppies and kittens to be acquired and presented for their first vaccinations, both testes should be palpable in the scrotum. Anecdotal reports exist in dogs of testicular descent occurring as late as 6 months of age, but this is not normal. Some authors have recommended that a diagnosis of cryptorchidism not be made until a dog is 6 months old.6 Whether these reports represent exceptionally mobile scrotal testes rather than truly delayed descent is not known.

Cryptorchid testicles continue to produce testosterone. Therefore, secondary sexual characteristics, and typical male libido and behavior are seen. Spermatogenesis is impaired by the higher temperature to which non-scrotal testes are exposed, and also by other factors such as hormone receptor defects in the cryptorchid testis. Bilaterally cryptorchid animals are expected to be sterile. Monorchidism, the condition of unilateral testicular aplasia, is rare in dogs and cats.

The prevalence of cryptorchidism in the general population is not known. Of 1345 cats presented to a veterinary teaching hospital for castration, 23 (1.7%) were cryptorchid [7]. Of these, 18 were unilateral cryptorchids, with no difference in left- versus right-sided involvement. All five cases with bilaterally cryptorchid testes were intraabdominal. Of 3038 male feral cats trapped for neutering, 35 (1.2%) were cryptorchid, and 46 (1.5%) were thought to have previously been neutered based on physical examination [8]. The prevalence of cryptorchidism in canine hospital populations is reported from 1.2 to 5 % [6]. Although the mode of inheritance is not known in either species, it is well accepted that cryptorchidism is associated with familial lineage. Therefore, the prevalence is likely to vary according to breed. For example, the incidence of cryptorchidism in a study of 2929 boxer dogs was 10.7% [9]. The heritability estimates in that study indicated that the mode of inheritance was complex, but that genetic selection would be successful in changing the prevalence of cryptorchidism in the boxer breed.

Careful palpation of the scrotum is usually sufficient to establish the diagnosis of cryptorchidism in dogs and cats older than 6 to 8 weeks, keeping in mind the mobility of the testes. If doubt exists in such young animals, they should be re-examined at a later date. Occasionally, adult animals with no scrotal testes are adopted without knowledge of previous surgeries. In such cases, a question may exist as to whether the animal is cryptorchid or has been castrated. Behavior typical of intact males, or the typical odor of tom cat urine, might raise the index of suspicion of cryptorchidism. The physical examination findings of penile spines in a cat, or a well-developed prostate gland in a dog provide indirect evidence of the presence of a testis because both structures are androgen-dependent. Both involute or regress promptly after castration.

Other diagnostic tests for cryptorchidism include determination of serum concentrations of luteinizing hormone (LH) or testosterone, and ultrasonography. Following castration, serum concentrations of LH increase because negative feedback from testosterone no longer occurs. Therefore, the finding of high serum LH in a male with no scrotal testes is indicative of previous castration, whereas low serum LH indicates the presence of androgen. The nidar of the episodic secretion of testosterone in intact male dogs and cats may be below the detection threshold of some testosterone assays. Therefore, serum concentrations of testosterone should be measured before and after the administration of gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin (hCG). Castrated animals have low resting testosterone and no response to GnRH or hCG. A single, random determination of testosterone in an intact male dog or cat may also be low or undetectable, depending on the assay. High resting testosterone or an increase in serum concentrations in response to GnRH or hCG confirms the presence of a testis. Ultrasound may identify the presence and location of a cryptorchid testicle. This might be helpful in planning the surgical approach. Negative ultrasound findings do not necessarily exclude the possibility of cryptorchidism because the undescended testis can be small.

Castration is the recommended treatment because in dogs the risk of testicular neoplasia in the cryptorchid testis is significantly greater than in scrotal testes. Although testicular neoplasia is uncommon in cats, it has been reported in cryptorchid testes [10]. If, at the time of surgery, the ductus deferens, testicular vessels, and/or the epididymis are found, it is highly likely that a testis also exists on the ipsilateral side. Monorchidism (testicular aplasia) is rare. When testicular aplasia is suspected, it is important to confirm the true absence of testicular tissue with appropriate hormonal testing as described above. Medical management to cause descent of intra-abdominal testes has been unsuccessful. To date, results of treatment for testes in the inguinal subcutis have been largely anecdotal in veterinary medicine and highly variable in human and veterinary medicine. Some have argued that the reported successful cases could have descended spontaneously or that they represented highly mobile, otherwise normal testes [11].

Testicular Neoplasia

Testicular tumors are common in dogs but rare in cats. They may be incidental findings during physical examination, castration, or necropsy, or they may cause clinical signs. The most common testicular tumors are Sertoli cell tumors, Leydig (interstitial) cell tumors, and seminomas. They occur with about equal frequency except in intra-abdominal testes in which the tumor is most often of Sertoli cell origin. Sertoli and Leydig cell tumors can produce hormones and cause paraneoplastic syndromes [12].

The most common clinical sign of testicular neoplasia is enlargement of the testis. Paraneoplastic syndromes commonly occur with testicular tumors that produce hormones. These syndromes include atrophy of the contralateral testis, alopecia, and hyperpigmentation, gynecomastia, pendulous prepuce, squamous metaplasia of the prostate, and bone marrow suppression. The bone marrow toxicity caused by estrogen can result in anemia, thrombocytopenia, and/or leukopenia. Therefore, the clinical signs may be referable to anemia or bleeding. Intra-abdominal testicular tumors may cause mechanical interference with other abdominal organs. Infertility is also a possible result of testicular neoplasia, but most affected dogs are past their breeding years.

Several risk factors for testicular neoplasia have been identified in dogs. Age is probably the most important. Testicular tumors are unlikely in dogs younger than 6 years of age but are fairly common in dogs 10 or more years old. In dogs older than 6, the incidence of testicular cancer has been calculated to be 68.1/1000 dog-years at risk [13]. Cryptorchidism is also a significant risk factor. In a study of 1266 dogs, testicular cancer was diagnosed 10.9 times more commonly in cryptorchid testes than in scrotal testes [14]. Environmental carcinogens [15] and certain gene expressions [16] are also risk factors.

Neoplasia should be suspected for any testicular mass in an old dog. Gynecomastia and pendulous prepuce have been referred to as feminization. Feminization is caused by estrogen alone, or by an imbalance of estrogen and androgens [17]. These findings in an intact male, especially if alopecia is also present, should make one highly suspicious of an estrogen-producing testicular tumor. In a dog with an enlarged prostate, squamous metaplasia is suspected on the basis of finding excessive numbers of squamous cells in the ejaculate or in fine-needle aspirate samples from the gland. Estrogen is the cause of prostatic squamous metaplasia. The presence of excess estrogen can be confirmed by exfoliative cytology of the preputial mucosa, which cornifies under the influence of estrogen just as does the vaginal epithelium. A testicular tumor is the most likely source of excess estrogen in a male dog.

The diagnosis of testicular neoplasia is often made on the basis of palpation of a testicular mass. Tumors are usually more firm than the surrounding testicular tissue. If a testicular tumor is not palpable in a dog with feminization or prostatic squamous metaplasia, or other situations where testicular neoplasia is a likely diagnostic consideration, testicular ultrasound should be performed. Ultrasound is useful in identifying small tumors, especially when they are deeper in the parenchyma and difficult to feel. Although fine-needle aspiration or biopsy could be used to confirm the diagnosis of neoplasia and to differentiate testicular tumors from one another, this is rarely indicated because the treatment for all is surgical removal of the affected testis. Because of possible bone marrow toxicity from estrogen production by the tumor, preoperative evaluation should include a complete blood count. Because most affected dogs are geriatric, a preoperative biochemical panel and urinalysis are also appropriate.

If the dog still has value as a stud, unilateral castration should be performed. If not, both testes could be removed. The affected testis is then submitted for histopathologic evaluation. In contrast to the situation in men, most canine testicular tumors are benign. Therefore, complete staging of the disease could reasonably be delayed until histopathologic confirmation of malignancy. Intra-abdominal metastasis is more likely than pulmonary. Treatment options for malignant and metastatic tumors should be considered in consultation with a veterinary oncologist.

Figure 73.1 A-D. 10-year-old Golden Retriever with paraneoplastic syndrome from an estrogen-producing interstitial cell tumor. A. Alopecia and hyperpigmentation, mammary and nipple development, pendulous prepuce (white arrow), asymmetric testes (black arrow). B. Cornification of preputial mucosal epithelium. C. and D. Resolution of alopecia, hyperpigmentation and feminization following castration. (Courtesy of Dr. William Schall, Michigan State University).

Relative to cryptorchidism and neoplasia, other testicular disorders are much less commonly seen in general small animal practice. Testicular masses other than neoplasia include spermatocele, granuloma, and cysts. Although abscesses can form in the testis, with testicular infection it is more common to have diffuse involvement than a focal mass. Owners may assume that an enlarged scrotum must be due to testicular enlargement. Other causes of scrotal enlargement include epididymal or spermatic cord enlargement (including scirrhous cord), scrotal herniation of omentum or small bowel, hydrocele, or edema of the scrotal skin. Hydrocele is the accumulation of fluid between the visceral and parietal vaginal tunics in the scrotum. The fluid may be a transudate in the case of lymphatic obstruction of the spermatic cord, or it may be blood or exudate in the case of trauma or infection. In addition to careful palpation, ultrasound of the scrotal contents is extremely helpful in differentiating among the causes of scrotal enlargement.

Spermatocele/Sperm Granuloma

Spermatocele is a cystic dilation of the ductal system that is caused by stasis and accumulation of sperm [18]. It is much more likely to occur in the ductus deferens or epididymis than in the testis. Spermatoceles may cause a local inflammatory reaction with infiltration of monocytes and macrophages and the development of a sperm granuloma. Conversely, preexisting testicular inflammation, orchitis, may cause the obstruction and formation of a spermatocele. Spermatoceles also occur as a result of trauma, including that induced by fine-needle aspiration or biopsy. Some spermatoceles are congenital in origin. Spermatocele may be an incidental finding causing no clinical signs, or it may cause swelling and/or a palpable mass. If it is associated with an inflammatory condition, the animal may also experience discomfort.

Cysts occasionally arise from the seminiferous tubules or rete testis. These may be incidental findings during testicular ultrasonography, or the cysts may be large enough to alter the size or shape of the testis. Cysts involving the rete testis may completely obstruct the movement of sperm into the epididymis.

Whenever a lesion is found in one testis, both should be thoroughly evaluated to determine if the disorder is unilateral or bilateral. Differentiation of focal testicular lesions such as tumors, granulomas, or cysts can be accomplished by ultrasound and fine-needle aspiration. Samples should be submitted for cytologic evaluation and culture, if appropriate. When cysts or spermatoceles are incidental findings causing no clinical signs, further evaluation is not necessary. Biopsies could also be obtained but fine-needle aspiration is usually sufficient for focal lesions. Surgical removal of the affected testis is often a more cost-effective approach than biopsy because it is potentially therapeutic as well as obtaining samples for histopathologic and microbiologic evaluation.

Orchitis

Infectious or inflammatory conditions of the testes are relatively uncommon in dogs, and rare in cats. When they occur, the epididymis is often also affected. Infections may be of hematogenous origin or from penetrating wounds. Of the infectious agents, Brucella canis is of particular concern in dogs because of its insidious nature and devastating effect on reproduction [19]. Brucella canis infection usually does not cause signs of systemic illness in dogs. The transient fever and testicular swelling following infection often go unnoticed. Although B. canis can cause epididymal and testicular enlargement, it is often not of a magnitude to be found by casual observation. Scrotal dermatitis associated with the epididymitis has been reported, but this is certainly not a consistent finding. Only after dogs have become infertile do owners become aware of a problem. Occasionally B. canis infects non reproductive organs including the eyes and intervertebral discs. When it does, there are associated clinical signs. With chronic infection the testes atrophy. Other bacteria and mycotic agents such as Blastocymes can also cause orchitis. Lymphocytic orchitis has been reported and is suspected to have an immune-mediated basis [20]. Testicular trauma can cause hemorrhage, edema, and inflammation.

Orchitis is suspected when swelling, heat, and pain are found during examination of the testis. If other signs of illness are present, these should be pursued as well. Brucella canis serology should be performed in every dog with orchitis or epididymitis. The rapid slide agglutination test (RSAT) is an excellent screening test. It is highly sensitive; therefore, a negative result is good evidence against the diagnosis, assuming that the infection has been present for at least 8 to 12 weeks and the animal has not been receiving antibiotic therapy. The RSAT is not specific. False-positive results are common. If the RAST for B. canis result is positive, antibiotic therapy must be withheld until confirmatory tests are complete. If the RSAT for B. canis is negative, additional diagnostic tests for orchitis can be pursued. These could include ultrasound, seminal fluid cytology and culture, and/or fine-needle aspiration to obtain samples for cytologic evaluation and culture. Empiric treatment with antibiotics may be initiated pending the results. If trauma is the suspected cause, cold compresses may be considered to reduce swelling because pressure will have a negative effect on spermatogenesis. In the case of unilateral orchitis in a valuable stud, the fate of the contralateral testis must be considered. Spermatogenesis can be adversely affected by the heat, pressure, and/or direct extension of infection. To save the unaffected side, hemicastration might be consider if response to treatment is not prompt.

Torsion of the Spermatic Cord

Other causes of testicular enlargement include spermatic cord torsion and spermatic cord occlusion. Torsion of the spermatic cord has often been referred to as testicular torsion, but torsion of the spermatic cord is more accurate. Torsion of the spermatic cord occurs more often in intra-abdominal testes than in scrotal testes [21]. Often the intra-abdominal testis also has a tumor. The most prominent clinical sign of spermatic cord torsion is the acute onset of severe pain. The clinical signs may be typical of an acute abdomen when an intra-abdominal testis is involved. The pain with intrascrotal spermatic cord torsion may be manifest by reluctance to walk [22]. Torsion of the spermatic cord causes swelling and firmness of the cord, the epididymis, and the testis. The spiral nature and dilation of the testicular vessels within the torsed spermatic cord may be recognized by ultrasound.

Treatment is castration of the affected intra-abdominal testis. Castration is also usually recommended for intrascrotal spermatic cord torsion because irreparable damage to spermatogenesis may occur within hours. A scrotal hernia can also cause occlusion of the spermatic cord [23], in which case, the clinical signs are identical to torsion of the cord. More commonly, however, scrotal hernias do not occlude the spermatic cord or affect the testes. The usual signs of scrotal hernia are unilateral scrotal enlargement with minimal pain.

Testicular Hypoplasia

A variety of congenital disorders results in testicular hypoplasia or hypogonadism. Often abnormal genitalia are an associated finding. Hypoplasia and hypogonadism are recognized in young animals as they reach puberty. As discussed earlier, genetic and hormonal factors are necessary for normal determination of the indifferent gonad into a testis and subsequent sexual differentiation into a phenotypic male. Therefore, abnormalities in expression of genes that determine the differentiation of the testis, in the testicular production of hormones, and/or in hormone receptors may be involved [24-26]. Determining the specific cause of testicular hypoplasia in an individual animal usually requires extensive evaluation, particularly if the external genitalia are a normal male phenotype. An exception is the calico or tortoiseshell tom cat.

Because both the orange and the black coat color require an X chromosome to be expressed, male cats, having only one X, should not be able to show both colors. The calico or tortoiseshell coat color is therefore a marker for XXY or a chimeric state. At least 7 different chromosomal compliments have been described for male calico/tortoiseshell cats [6]. Some of these cats are fertile, phenotypically normal males with normal spermatogenesis found on testicular histopathology. Presumably these are chimeras with the normal feline 38, XY karyotype in the gonadal tissue. Some calico/tortoiseshell toms have testicular hypoplasia. Testicular histopathology may be normal in some areas; some or all seminiferous tubules without spermatogonia; or, some or all tubules with Sertoli cells only.

In other animals with testicular hypoplasia, abnormalities anywhere along the hypothalamic-pituitary-gonadal axis, as well as disorders of sexual differentiation, must also be considered. The finding of concomitant abnormalities in the external or internal genitalia, such as ambiguous or female external genitalia, hypospadia, or persistent Müllerian ducts, helps define the factors that might be involved [27,28]. Sometimes, straightforward karyotyping provides the answer.

Because animals with testicular hypoplasia are unlikely to have normal semen quality and usually are otherwise not desirable for breeding, extensive workup may not be warranted for the benefit of the individual. However, if more than one animal in the lineage is similarly affected, workup may be worthwhile for the breed. Physical examination will confirm the abnormally small testes. If need be, the functional ability of hypoplastic testes could be evaluated by measuring testosterone before and after the administration of GnRH or hCG, and by evaluating semen. The results are unlikely to be normal, but if they were, some owners might elect not to have the animal castrated.

Testicular Atrophy

In contrast to hypoplasia, testicular degeneration and atrophy are acquired conditions. Testicular atrophy is common in geriatric dogs, especially those over 10 years of age. The American Kennel Club will not register puppies sired by dogs older than 12 years without documentation of normal semen. The testis is easily damaged and atrophy often results. The spermatogonia are usually more sensitive to damage than are Sertoli and interstitial cells. Therefore, testosterone production may continue even when spermatogenesis is lost. Heat, pressure, infection, certain toxins and drugs (including glucocorticoids), radiation, nutritional deficiencies, and trauma (including biopsy) are a few of the many causes. By the time a male is presented with atrophic or degenerate testes, it may be difficult to determine the initiating cause. If a dog still has value as a stud, testicular function should be evaluated over longer than a 2-month period because the canine spermatogenic cycle is 62 days [6]. The prognosis for atrophic or degenerate testes to recover fertility is guarded, even if the cause can be found and corrected. If the condition is unilateral, submitting the affected side for culture and histopathologic evaluation would be reasonable.

Azoospermia

Male animals may be presented for evaluation of functional, rather than physical, abnormalities of the testes. A thorough history and physical examination should be performed. Brucella canis serology should be done in any dog presented for infertility. The first step in evaluating the endocrine function of the gonad is to determine serum concentrations of testosterone before and after GnRH or hCG. The first step in evaluating spermatogenesis is to perform a semen evaluation. Readers are referred to the textbook Canine and Feline Theriogenology [6] for a complete discussion of the diagnostic approach to infertility in the stud. Our discussion will be limited to the absence of sperm, azoospermia, in the ejaculate because this is a situation that often warrants testicular biopsy. In animals that still have sperm in the ejaculate, biopsy is often recommended as the last diagnostic test because damage to the testicular parenchyma will be done in the process. Although in the absence of complications, testicular biopsy of normal testes apparently does not have an adverse effect on semen quality in dogs [29], the same may not be true for testes wherein spermatogenesis is already marginal.

The absence of sperm in the ejaculate may reflect retrograde ejaculation, incomplete ejaculation, obstruction to sperm outflow, or lack of spermatogenesis. Retrograde ejaculation of a few spermatozoa into the urinary bladder is normal in dogs and cats. The magnitude is greater when electroejaculation is the technique used for semen collection [30]. Although it has been considered by some to be a potential cause of azoospermia, the author has not observed retrograde ejaculation of such magnitude to explain a complete lack of sperm in the seminal fluid. To evaluate the possibility, the number of spermatozoa in the seminal fluid collected is compared with the number obtained from the urinary bladder after ejaculation.

The canine epididymis produces alkaline phosphatase. Therefore, the concentration of alkaline phosphatase in seminal fluid can be used as a marker for the presence of epididymal fluid in the ejaculate. Fluid originating from the epididymis should also contain motile spermatozoa. When it does not, but alkaline phosphatase concentrations are high, either bilateral obstruction to outflow of sperm from the testes to the epididymes exists, or the testes are not producing sperm. If an azoospermic sample has low concentrations of alkaline phosphatase, two possibilities exist. One is that only the first fraction (pre-sperm) of the ejaculate, which originates from the prostate, was collected. The other possibility is bilateral obstruction to outflow of sperm from the epididymes.

In dogs and cats, samples obtained by fine-needle aspiration of the testis may be used to assess the presence or absence of spermatozoa. However, biopsy specimens will be necessary to fully assess the spermatogenic cycle [31]. Meticulous surgical technique and the postoperative application of cold compresses to the scrotum will help minimize post-biopsy swelling [32].