Terms oligospermia, oligozoospermia, and low sperm count refer to semen with a low concentration of sperm and is a common finding in male infertility. Often semen with a decreased sperm concentration may also show significant abnormalities in sperm morphology and motility (technically oligoasthenoteratozoospermia). There has been interest in replacing the descriptive terms used in semen analysis with more quantitative information. Diagnosis The diagnosis of oligozoospermia is based on one low count in a semen analysis performed on two occasions. For many decades sperm concentrations of less than 20 million sperm/ml were considered low or oligospermic, recently, however, the WHO reassessed sperm criteria and established a lower reference point, less than 15 million sperm/ml, consistent with the 5th percentile for fertile men. Sperm concentrations fluctuate and oligospermia may be temporary or permanent. Sources usually classify oligospermia in 3 classes: Mild: concentrations 10 million – 15 million sperm/mL Moderate: concentrations 5 million – 10 million sperm/mL Severe: concentrations less than 5 million sperm/mL The diagnosis of oligozoospermia requires a work-up via semen analysis (listed in Male infertility). Causes There are many causes for oligospermia including: Pre-testicular causes Pre-testicular factors refer to conditions that impede adequate support of the testes and include situations of poor hormonal support and poor general health including: Hypogonadism due to various causes Drugs, alcohol, smoking Strenuous riding (bicycle riding, horseback riding) Medications, including androgens. Testicular factors Testicular factors refer to conditions where the testes produces semen of poor quality despite adequate hormonal support and include: Age Genetic defects on the Y chromosome Y chromosome microdeletions Abnormal set of chromosomes Klinefelter syndrome Neoplasm, e.g. seminoma Cryptorchidism Varicocele (14% in one study) Trauma Hydrocele Mumps Malaria Defects in USP26 enzyme in some cases Mast cells releasing inflammatory mediators appear to directly suppress sperm motility in a potentially reversible manner, and may be a common pathophysiological mechanism for several of the above-mentioned factors. Post-testicular causes Post-testicular factors decrease male fertility due to conditions that affect the male genital system after testicular sperm production and include defects of the genital tract as well as problems in ejaculation: Vas deferens obstruction Lack of Vas deferens, often related to genetic markers for cystic fibrosis Infection, e.g. prostatitis Ejaculatory duct obstruction Idiopathic oligospermia (oligoasthenoteratozoospermia) In about 30% of infertile men no causative factor is found for their decrease in sperm concentration or quality by common clinical, instrumental, or laboratory means, and the condition is termed