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Kara A. Kolster DVM DACT
Theriogenologist, Springfield Veterinary Center, Glen Allen, Virginia, USA

Subfertility can be defined as semen parameters that are significantly below expected normal levels or as sporadic success in a male’s ability to sire litters. Subfertility may be congenital or acquired. If a male is not tested until he misses breedings, we may not know whether he was born with or developed the problem later on.

Evaluating subfertile dogs requires accurate semen analysis, including knowledge of the appropriate tools and tests to use, and evaluation of as many parameters as possible.

Motility evaluation
Total motility is the percentage of sperm in a sample that are alive and moving. Progressive motility is the percentage that are moving in a relatively straight, forward manner, not swimming backwards or spinning in circles. Subfertile dogs frequently show lower total and progressive motility compared to dogs exhibiting normal fertility.

Morphology evaluation
Morphology refers to how physically normal a sperm cells is, and it is correlated with fertility in multiple species, including dogs, bulls, and humans. Semen morphology is perhaps one of the most important and most commonly misused tests in semen analysis, as it is easy to overlook abnormalities without proper knowledge and procedure. Accurate evaluation requires appropriate stains, a quality microscope, and knowledge and experience in identifying sperm abnormalities.

There are several systems for categorizing sperm abnormalities. Abnormalities are typically separated by where in the development process they occur or by their suspected effect on fertility. The most common system classifies “primary” defects as those that occur in the testes early in sperm development. A dog with a high number of primary defects may have a congenital abnormality or acquired damage to the testes. Sperm with primary defects are typically not fertile because these defects prevent the sperm from either reaching the egg or from being able to fertilize the egg. Primary defects include abnormally shaped sperm heads, vacuoles (holes) in the head, bent or folded midpieces, proximal droplets, tightly coiled tails, detached acrosomes, and many others. “Secondary” defects occur later in sperm development or storage, and may or may not have an effect on fertility. Detached heads and distal droplets are the most common secondary abnormalities.

Total sperm count
The concentration of sperm in a sample is multiplied by volume of the sample to determine the total number of sperm present. There is no defined number of sperm necessary to achieve pregnancy in dogs, and need varies depending on type of semen (fresh, chilled, or frozen) and type of insemination (vaginal, transcervical, or surgical). Generally, 100-200 million motile sperm per breeding is considered the minimum acceptable number.

Viability evaluation
Viability, also known as membrane integrity, is a less common test that can be very important in evaluating subfertile dogs. The sperm membrane must be normal to allow the sperm cell to fertilize an egg. Recent evidence suggests that viability may correlate with fertility better than the percent motile sperm.

These tests can be performed manually by a human evaluator or using computer-aided sperm analysis (CASA) systems. There are advantages and disadvantages to each. Manual evaluation usually overestimates motility compared to CASA. Viability measurements may be more accurate with CASA than with manual tests. Certain CASA systems provide accurate concentration measurement, but many systems will count any debris or other cells in the sample as sperm, which falsely elevates the calculated total sperm number. There are no CASA systems currently available that accurately evaluate sperm morphology. This must be done by a knowledgeable human evaluator.

Accurate semen evaluation is the first step in diagnosing subfertility in dogs. Knowing the exact parameters that are abnormal can help determine the cause of subfertility and guide management to improve chances of pregnancy.

Part II will discuss potential treatments to improve semen quality and pregnancy rates.