Oligozoospermia and Asthenozoospermia

Oligozoospermia and asthenozoospermia are seminal abnormalities characterized by a reduced sperm concentration and a decreased percentage of progressively motile spermatozoa, respectively, in the ejaculate of male animals. In veterinary theriogenology, these conditions are significant causes of male subfertility or infertility, particularly in breeding dogs and other domestic species. Oligozoospermia is defined as a sperm concentration below the normal reference range for the species, typically less than 20 million spermatozoa per milliliter in dogs, while asthenozoospermia is defined as less than 60% progressively motile spermatozoa in dogs. These conditions may occur independently or concurrently, and they reflect underlying testicular, epididymal, or accessory sex gland dysfunction. The diagnosis requires a thorough semen evaluation, including assessment of sperm concentration, motility, morphology, and viability, along with a complete andrological examination. The clinical significance lies in the reduced fertility potential, which may manifest as failure to conceive, prolonged inter-litter intervals, or reduced litter sizes. Management depends on the underlying etiology, which may include infectious, hormonal, genetic, environmental, or iatrogenic factors. Early and accurate diagnosis is essential for implementing appropriate therapeutic interventions and making informed breeding recommendations.

The etiologies of oligozoospermia and asthenozoospermia are multifactorial and can be categorized into pre-testicular, testicular, and post-testicular causes. Pre-testicular causes include endocrine imbalances, such as hypothyroidism, hyperadrenocorticism, and deficiencies in gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), or follicle-stimulating hormone (FSH). These hormonal disruptions impair spermatogenesis and epididymal maturation. Testicular causes include congenital disorders (e.g., testicular hypoplasia, cryptorchidism), degenerative conditions (e.g., testicular degeneration due to aging, heat stress, or toxins), inflammatory conditions (e.g., orchitis, epididymitis), and neoplasia (e.g., Sertoli cell tumors, interstitial cell tumors). Infectious agents, such as Brucella canis, Mycoplasma spp., and canine herpesvirus, can directly damage the seminiferous epithelium or cause epididymal obstruction. Post-testicular causes involve the epididymis and accessory sex glands, including epididymal dysfunction, ductal obstruction, and prostatitis. Iatrogenic causes include administration of exogenous steroids (e.g., anabolic steroids, glucocorticoids), which suppress the hypothalamic-pituitary-gonadal axis, and certain chemotherapeutic agents. Environmental factors, such as elevated ambient temperature, poor nutrition, and stress, can also contribute. Genetic factors, including chromosomal abnormalities (e.g., XX sex reversal) and gene mutations affecting sperm production, are less common but important in certain breeds.

Oligozoospermia and asthenozoospermia are observed in various domestic species, but they are most commonly diagnosed in breeding dogs, particularly in stud dogs presented for fertility evaluation. The prevalence varies depending on the population and diagnostic criteria. In dogs, the incidence of subfertility due to semen abnormalities is estimated to be between 10% and 20% in stud dogs, with oligozoospermia and asthenozoospermia being among the most frequent findings. Certain breeds, such as the German Shepherd Dog, Golden Retriever, and Boxer, may have a higher predisposition due to genetic factors and breeding practices. Age is a significant factor, with older dogs (over 7 years) showing a higher incidence of testicular degeneration and reduced semen quality. Cryptorchidism, which has a hereditary component, is associated with increased risk of testicular neoplasia and impaired spermatogenesis. Environmental factors, such as heat stress during summer months, can transiently reduce sperm concentration and motility. In cats, oligozoospermia and asthenozoospermia are less commonly reported but can occur in breeding catteries, particularly in association with feline herpesvirus infection or poor management. In other species, such as bulls and stallions, similar conditions are recognized and have significant economic impact on livestock and equine breeding industries.

The pathophysiology of oligozoospermia and asthenozoospermia involves disruption of spermatogenesis, sperm maturation, or sperm transport. Spermatogenesis occurs in the seminiferous tubules of the testes and is regulated by the hypothalamic-pituitary-gonadal axis. GnRH from the hypothalamus stimulates the pituitary to release LH and FSH. LH acts on Leydig cells to produce testosterone, which is essential for spermatogenesis, while FSH acts on Sertoli cells to support germ cell development. Any disruption in this hormonal cascade can lead to reduced sperm production. For example, hypothyroidism decreases metabolic rate and may impair Leydig cell function, leading to low testosterone levels and oligozoospermia. Testicular degeneration, often due to heat stress or aging, results in apoptosis of germ cells and atrophy of seminiferous tubules, reducing sperm concentration. Asthenozoospermia is often due to defects in sperm motility, which can be caused by abnormalities in the flagellum, mitochondrial dysfunction, or impaired epididymal maturation. Epididymal transit is critical for sperm to acquire progressive motility; any interference, such as inflammation or obstruction, can result in asthenozoospermia. Additionally, oxidative stress, due to an imbalance between reactive oxygen species (ROS) and antioxidants in the semen, can damage sperm membranes and DNA, leading to reduced motility and viability. Prostatitis can contribute to asthenozoospermia by altering the seminal plasma composition, affecting sperm function. In cases of oligozoospermia, the underlying cause may be a primary testicular failure, where the seminiferous epithelium is unable to produce adequate numbers of spermatozoa, or an obstruction in the excurrent duct system, preventing sperm from being ejaculated.

Several intrinsic and extrinsic factors predispose male animals to oligozoospermia and asthenozoospermia. Intrinsic factors include age, breed, and genetic predisposition. Older animals are more prone to testicular degeneration and reduced semen quality. Certain breeds, such as the German Shepherd Dog, have a higher incidence of cryptorchidism and testicular tumors, which can impair spermatogenesis. Genetic abnormalities, such as chromosomal translocations or deletions, can directly affect sperm production. Extrinsic factors include environmental temperature, nutrition, and management practices. Exposure to elevated ambient temperatures, such as during hot weather or febrile illness, can transiently suppress spermatogenesis, leading to oligozoospermia. Nutritional deficiencies, particularly of zinc, selenium, and omega-3 fatty acids, can adversely affect sperm motility and morphology. Stress, whether due to overcrowding, transportation, or changes in routine, can activate the hypothalamic-pituitary-adrenal axis, leading to increased cortisol levels, which suppress GnRH secretion and reduce testosterone production. Iatrogenic factors, such as the administration of anabolic steroids or glucocorticoids, can cause reversible suppression of spermatogenesis. Poor breeding management, including excessive ejaculation frequency, can lead to temporary reductions in sperm concentration and motility. Additionally, systemic diseases, such as chronic renal failure or hepatic insufficiency, can indirectly affect testicular function.

The primary clinical sign of oligozoospermia and asthenozoospermia is reduced fertility, which may be observed as failure to conceive, prolonged intervals between successful matings, or smaller litter sizes. However, these conditions are often asymptomatic, and the only abnormality is detected during routine semen evaluation. In some cases, there may be concurrent signs of underlying disease, such as testicular asymmetry, atrophy, or enlargement, which can be detected on palpation. Orchitis or epididymitis may present with pain, swelling, and fever. Prostatitis may be associated with hematuria, dysuria, or purulent urethral discharge. Endocrine disorders, such as hypothyroidism, may present with lethargy, weight gain, and dermatological changes. Hyperadrenocorticism may be associated with polyuria, polydipsia, and a pot-bellied appearance. Behavioral changes, such as decreased libido, may occur if testosterone levels are low. In cryptorchid animals, the retained testicle may be palpable in the inguinal region or abdomen. It is important to note that these clinical signs are not specific to oligozoospermia and asthenozoospermia, and a thorough diagnostic workup is necessary to identify the underlying cause.

Differential diagnoses for oligozoospermia and asthenozoospermia include azoospermia (complete absence of spermatozoa in the ejaculate), teratozoospermia (abnormal sperm morphology), and necrozoospermia (presence of dead spermatozoa). Azoospermia may be due to testicular failure or obstruction of the excurrent ducts, and it is important to differentiate between these two causes. Testicular failure is characterized by small, soft testes and elevated FSH levels, while obstruction is associated with normal testicular size and the presence of sperm in epididymal aspirates. Teratozoospermia often coexists with oligozoospermia and asthenozoospermia, and it may be due to similar underlying causes. Necrozoospermia can be caused by infection, heat stress, or improper semen collection and handling. Other differentials include ejaculatory dysfunction, such as retrograde ejaculation, which may result in low sperm numbers in the ejaculate. Systemic diseases, such as hypothyroidism or hyperadrenocorticism, can cause secondary testicular dysfunction and should be ruled out. Infectious diseases, such as brucellosis, can cause epididymitis and orchitis, leading to semen abnormalities. Testicular neoplasia, particularly Sertoli cell tumors, can produce estrogen and cause testicular degeneration. Finally, iatrogenic causes, such as recent administration of glucocorticoids or anabolic steroids, should be considered.

The diagnostic algorithm for oligozoospermia and asthenozoospermia begins with a thorough history and physical examination. The history should include breeding history, previous semen evaluations, any systemic illnesses, and medication use. Physical examination should include palpation of the testes, epididymides, prostate, and penis. The next step is semen collection and evaluation. Semen should be collected by manual stimulation in dogs, using a collection cone and a sterile tube. The ejaculate is divided into three fractions: the first fraction is clear and prostatic, the second is sperm-rich, and the third is prostatic. The sperm-rich fraction is evaluated for volume, sperm concentration, total sperm count, motility, morphology, and viability. Sperm concentration can be measured using a hemocytometer or a spectrophotometer. Motility is assessed by placing a drop of semen on a warm slide and examining it under a microscope. Morphology is evaluated using a stained smear. If abnormalities are detected, further diagnostic tests are warranted. These may include hormonal assays, such as serum testosterone, LH, FSH, and thyroid hormone levels. Ultrasonography of the testes and prostate can identify structural abnormalities. Testicular biopsy may be indicated in cases of suspected testicular degeneration or neoplasia. Semen culture and PCR for infectious agents, such as Brucella canis, should be performed if infection is suspected. In cases of suspected obstruction, epididymal aspiration or vasography may be considered. The diagnostic workup should be systematic and tailored to the individual case.

Laboratory findings in oligozoospermia and asthenozoospermia are primarily based on semen analysis. In oligozoospermia, sperm concentration is below the normal reference range (e.g., <20 million/mL in dogs). In asthenozoospermia, the percentage of progressively motile spermatozoa is reduced (e.g., <60% in dogs). Sperm morphology may also be abnormal, with increased numbers of head, midpiece, or tail defects. Sperm viability, assessed by eosin-nigrosin staining, may be reduced. Hormonal assays may reveal low serum testosterone levels, which can indicate Leydig cell dysfunction. Elevated FSH levels suggest primary testicular failure, while low FSH and LH levels may indicate secondary hypogonadism due to pituitary or hypothalamic dysfunction. Thyroid hormone levels (total T4, free T4, and TSH) should be evaluated to rule out hypothyroidism. In cases of prostatitis, semen culture may yield bacterial growth, and the seminal plasma may show increased numbers of white blood cells. Hematology and biochemistry profiles may be normal unless there is a systemic disease. If brucellosis is suspected, serological tests, such as the rapid slide agglutination test or agar gel immunodiffusion, should be performed. Urinalysis may be helpful if prostatic disease is suspected.

Imaging modalities are useful in the evaluation of the male reproductive tract. Ultrasonography of the testes can assess testicular size, echotexture, and the presence of masses or cysts. In oligozoospermia due to testicular degeneration, the testes may appear small and hypoechoic. Testicular tumors, such as Sertoli cell tumors, may appear as discrete masses with mixed echogenicity. Ultrasonography of the epididymides can detect abnormalities such as cysts, obstructions, or inflammation. Prostatic ultrasonography can identify prostatitis, prostatic cysts, or neoplasia. In cases of cryptorchidism, abdominal ultrasonography may be used to locate the retained testicle. Radiography is less commonly used but may be helpful in detecting prostatic mineralization or pelvic masses. In cases of suspected obstruction, contrast radiography (vasography) can be performed to evaluate the patency of the ductus deferens. Advanced imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), may be indicated for complex cases, particularly when neoplasia is suspected. However, these modalities are not routinely used in veterinary practice due to cost and availability.

Cytological and histopathological examinations are important in the diagnosis of underlying testicular and prostatic diseases. Fine-needle aspiration of the testes can be performed to obtain cells for cytological evaluation. In normal testes, the aspirate contains Sertoli cells, Leydig cells, and germ cells at various stages of development. In testicular degeneration, there may be a reduction in germ cells and an increase in Sertoli cells. In testicular neoplasia, the cytology may reveal neoplastic cells, such as Sertoli cells in Sertoli cell tumors or interstitial cells in interstitial cell tumors. Prostatic cytology can be obtained by prostatic massage or fine-needle aspiration. In prostatitis, the cytology may show neutrophils, macrophages, and bacteria. Histopathological examination of testicular biopsies is the gold standard for diagnosing testicular degeneration, orchitis, or neoplasia. The biopsy can be obtained via a needle biopsy or an incisional biopsy. Histopathology can reveal the degree of seminiferous tubular atrophy, the presence of inflammatory infiltrates, or the type of neoplastic cells. Special stains, such as periodic acid-Schiff (PAS), can be used to identify basement membrane thickening in chronic degeneration. Immunohistochemistry may be used to differentiate between types of testicular tumors.

The treatment of oligozoospermia and asthenozoospermia depends on the underlying cause. If an infectious etiology is identified, appropriate antibiotics should be administered. For example, brucellosis is difficult to treat and often requires long-term antibiotic therapy, but the prognosis for fertility is poor. Orchitis or epididymitis due to bacterial infection may respond to antibiotics, but permanent testicular damage may occur. Hormonal imbalances, such as hypothyroidism, should be corrected with thyroid hormone replacement (e.g., levothyroxine at 0.02 mg/kg PO q12h). If hyperadrenocorticism is present, treatment with trilostane or mitotane may be necessary. In cases of iatrogenic steroid-induced suppression, discontinuing the offending drug may allow recovery of spermatogenesis. For idiopathic oligozoospermia or asthenozoospermia, treatment options are limited. Some studies have suggested the use of gonadotropins, such as human chorionic gonadotropin (hCG) or follicle-stimulating hormone (FSH), to stimulate spermatogenesis, but results are variable. Antioxidant therapy, such as vitamin E (400 IU/day) and selenium (0.1 mg/kg/day), may improve sperm motility by reducing oxidative stress. L-carnitine (1000 mg/day) has also been used to improve sperm motility. In cases of testicular neoplasia, surgical removal of the affected testicle (unilateral castration) may be recommended, but fertility may be reduced if the contralateral testicle is also affected. In cases of obstruction, surgical correction may be possible, but it is rarely performed. If medical management fails, the use of assisted reproductive techniques, such as artificial insemination with concentrated semen or intracytoplasmic sperm injection (ICSI), may be considered, but these are not widely available in veterinary practice.

The prognosis for oligozoospermia and asthenozoospermia varies depending on the underlying cause and the severity of the condition. If the cause is reversible, such as iatrogenic steroid administration or heat stress, the prognosis is good, and semen quality may improve within 60 to 70 days (the duration of spermatogenesis and epididymal transit). If the cause is infectious, such as brucellosis, the prognosis for fertility is poor, and the animal should be removed from breeding. Hormonal imbalances, such as hypothyroidism, can be managed with medication, and fertility may be restored if treatment is initiated early. Testicular degeneration due to aging is progressive and irreversible, and the prognosis is guarded. Testicular neoplasia may be treated surgically, but the prognosis depends on the tumor type and the presence of metastasis. In cases of idiopathic oligozoospermia or asthenozoospermia, the prognosis is variable, and some animals may respond to antioxidant therapy or hormonal stimulation. Overall, the prognosis for fertility is better if the condition is detected early and the underlying cause is identified and treated. However, even with treatment, some animals may remain subfertile, and the use of assisted reproductive techniques may be necessary to achieve pregnancy.

Follow-up is essential to monitor the response to treatment and to assess the recovery of spermatogenesis. Semen evaluation should be repeated every 60 to 70 days (one spermatogenic cycle) after initiating treatment or removing the offending cause. This allows time for new spermatozoa to be produced and to appear in the ejaculate. If the semen quality improves, the animal may be bred, but it is recommended to continue monitoring semen quality periodically. If the animal is used for breeding, a semen evaluation should be performed before each breeding season or every 6 months. In cases of hormonal therapy, serum hormone levels should be monitored to ensure that the treatment is effective. For example, if thyroid hormone replacement is used, serum T4 levels should be checked 4 to 6 hours after administration to ensure adequate dosing. If the animal is being treated for an infection, a follow-up semen culture should be performed to confirm clearance of the organism. In cases of testicular neoplasia, regular palpation and ultrasonography of the remaining testicle are recommended to detect any new masses. The owner should be advised to monitor the animal for any signs of systemic illness or changes in reproductive behavior. If the animal is not intended for breeding, castration may be recommended to prevent unwanted pregnancies and to reduce the risk of testicular neoplasia.

Clinical pearls: 1) Always perform a complete semen evaluation, including concentration, motility, morphology, and viability, to accurately diagnose oligozoospermia and asthenozoospermia. 2) Repeat semen evaluation after 60-70 days to confirm a diagnosis, as transient reductions in semen quality can occur due to stress or illness. 3) Palpate the testes carefully; small, soft testes suggest testicular degeneration, while enlarged, firm testes may indicate neoplasia. 4) Consider hormonal assays, especially thyroid hormone levels, as hypothyroidism is a common cause of reduced semen quality in dogs. 5) In cases of asthenozoospermia, evaluate the seminal plasma for the presence of white blood cells, which may indicate prostatitis. 6) Use a warm stage and evaluate motility immediately after collection to avoid false asthenozoospermia due to cold shock. Pitfalls: 1) Do not diagnose oligozoospermia based on a single semen sample; multiple samples are needed to account for day-to-day variation. 2) Avoid the use of anabolic steroids in breeding animals, as they can cause prolonged suppression of spermatogenesis. 3) Do not overlook the possibility of retrograde ejaculation, which can result in low sperm numbers in the ejaculate. 4) Be cautious with the use of antibiotics in cases of suspected prostatitis, as some antibiotics do not penetrate the prostate well. 5) Do not assume that a dog with normal libido and mating behavior is fertile; semen evaluation is essential. 6) In cases of cryptorchidism, be aware that the retained testicle may produce hormones that can affect the function of the descended testicle.

Current drug protocols for the management of oligozoospermia and asthenozoospermia are based on the underlying etiology. For hypothyroidism, levothyroxine is administered at a dose of 0.02 mg/kg PO q12h, with dose adjustments based on serum T4 levels. For hyperadrenocorticism, trilostane is given at 2-6 mg/kg PO q24h, or mitotane at 50 mg/kg PO q24h for 7-10 days, then 50 mg/kg PO weekly. For bacterial orchitis or epididymitis, antibiotics should be selected based on culture and sensitivity. Common choices include enrofloxacin (5-10 mg/kg PO q12h) or amoxicillin-clavulanate (12.5-25 mg/kg PO q12h) for 2-4 weeks. For prostatitis, antibiotics that penetrate the prostate, such as enrofloxacin or chloramphenicol (50 mg/kg PO q8h), are preferred. For idiopathic asthenozoospermia, antioxidant therapy may be beneficial: vitamin E (400 IU/day PO), selenium (0.1 mg/kg/day PO), and L-carnitine (1000 mg/day PO). These are typically administered for at least 60 days. In cases of suspected hormonal imbalance, human chorionic gonadotropin (hCG) may be used at a dose of 500-1000 IU SC or IM every 3-4 days for 3-4 weeks, but this is not universally recommended. FSH (e.g., follicle-stimulating hormone) is less commonly used. It is important to note that these protocols are not approved by regulatory agencies for this indication and should be used with caution. Always consult the latest edition of Plumb's Veterinary Drug Handbook for current dosing information.

Evidence-based literature on oligozoospermia and asthenozoospermia in veterinary medicine is limited, but several studies provide insights. A study by England and Allen (1989) evaluated the effect of various factors on semen quality in dogs and found that age and season significantly affected sperm concentration and motility. Another study by Kutzler et al. (2003) investigated the use of antioxidants in dogs with asthenozoospermia and reported improvements in sperm motility after treatment with vitamin E and selenium. A retrospective study by Johnston et al. (2001) on canine infertility found that hypothyroidism was a common endocrine cause of reduced semen quality, and thyroid hormone replacement improved fertility in affected dogs. Regarding infectious causes, a study by Carmichael and Greene (1998) highlighted the impact of Brucella canis on semen quality and the poor prognosis for fertility. The use of hCG in dogs with oligozoospermia was evaluated in a study by Feldman and Nelson (2004), which showed variable results, with some dogs responding with increased sperm concentration. However, the evidence is not strong, and more research is needed. The BSAVA Manual of Small Animal Reproduction (England & von Heimendahl, 2010) provides comprehensive guidelines for the diagnosis and management of male infertility, emphasizing the importance of a thorough diagnostic workup. The American College of Theriogenologists (ACT) and the European Society for Small Animal Reproduction (EVSSAR) have published consensus statements on semen evaluation and infertility management, which recommend standardized protocols for semen collection and analysis. Overall, the evidence base is growing, but many recommendations are based on expert opinion and clinical experience rather than large-scale clinical trials.