Beyond the Microscope: Why Sperm Morphology Is the Cornerstone of Fertility Assessment in Stud Dogs

Semen analysis is currently regarded as the most important clinical laboratory test for evaluating fertility in stud dogs. It reflects the functional integrity of the seminiferous tubules, epididymides, and accessory sex glands, and is routinely used to assess breeding soundness, identify potential causes of infertility, and guide clinical and management decisions¹. 

Among its various parameters, sperm morphology holds particular clinical importance. Morphology is closely associated with sperm function, and its evaluation provides insight into fertility potential. A higher proportion of abnormal spermatozoa has been consistently associated with reduced fertility across species^1,2. 

Clinical Significance in Canine Fertility 

In dogs, the percentage of morphologically normal sperm is strongly linked to reproductive outcomes. Animals with more than 60% normal sperm have been reported to achieve pregnancy rates of approximately 61%, whereas those with less than 60% normal sperm demonstrate significantly lower pregnancy rates of around 13%¹. 

Similarly, in artificial insemination settings, semen doses that resulted in pregnancy contained a higher proportion of normal sperm (approximately 65%) compared to those that did not result in pregnancy (approximately 42%)^1,3. These findings indicate that sperm morphology should be interpreted as a clinically relevant parameter when evaluating breeding suitability. 

Causes of Abnormal Sperm Morphology 

A reduced proportion of normal sperm may be associated with multiple clinical and management-related factors. These include testicular tumors, orchitis, systemic illness such as fever, increased intrascrotal temperature, and variations in breeding frequency such as prolonged sexual abstinence or excessive use¹. 

Congenital testicular dysfunction has also been suggested as a potential cause of abnormal spermatogenesis^1,3. Identification of such abnormalities may assist in clinical diagnosis and help determine reproductive prognosis. 

Challenges in Morphology Evaluation 

Despite its importance, sperm morphology evaluation is often underestimated in clinical practice. It requires appropriate methodology, technical expertise, and careful interpretation. Studies have demonstrated significant variability among veterinarians evaluating the same semen samples using different methods¹. 

This variability contributes to sperm morphology being considered one of the most inconsistently applied tests in semen analysis¹. Differences in techniques and classification systems can lead to inconsistent results and may affect clinical decision-making. 

Need for Standardization 

The lack of standardized classification systems and reference materials remains a major limitation. To address this, annotated sperm image databases and illustrated guides have been developed to support consistent classification and training¹. 

Such resources, when based on expert classification, may improve inter-observer agreement and enhance the reliability of morphology assessment in clinical practice. 

Practical Considerations for Veterinarians 

Sperm morphology can be evaluated using brightfield microscopy with stains such as eosin/nigrosin or Diff-Quik, or by phase-contrast microscopy. Phase-contrast methods allow evaluation of unstained preparations and may reduce artefacts introduced during staining and drying^1,4. 

Evaluation is typically performed at 1000× magnification using immersion oil, which allows detailed visualization of sperm structures. Consistency in technique and classification approach is recommended to improve reproducibility of results¹. 

Conclusion 

Sperm morphology is a critical component of semen analysis and provides valuable information about fertility potential. While challenges in evaluation exist, appropriate training, standardized methods, and careful interpretation may enhance its clinical utility. Incorporating morphology assessment into routine reproductive evaluation may support more informed breeding decisions and improved outcomes. 

Reference 

1. Brito LF, da Silva MC, Kolster KA. Standardisation of dog sperm morphology classification. Reproduction in Domestic Animals. 2025 Feb;60(2):e70024. https://onlinelibrary.wiley.com/doi/pdf/10.1111/rda.70024 

2. García-Vázquez, F. A., J. Gadea, C. Matás, and W. V. Holt. 2016. “Importance of Sperm Morphology During Sperm Transport and Fertilization in Mammals.” Asian Journal of Andrology 18, no. 6: 844– 850. https://journals.lww.com/ajandrology/_layouts/15/oaks.journals/downloadpdf.aspx?an=00129336-201618060-00007 

3. Tesi, M., C. Sabatini, I. Vannozzi, et  al. 2018. “Variables Affecting Semen Quality and Its Relation to Fertility in the Dog: A Retrospective Study.” Theriogenology 118: 34–39. https://doi.org/10.1016/j.theriogeno logy.2018.05.018. 
4. Soler, C., and T. G. Cooper. 2016. “Foreword to Sperm Morphometrics Today and Tomorrow Special Issue in Asian Journal of Andrology.” Asian Journal of Andrology 18, no. 6: 815–818. https://journals.lww.com/ajandrology/_layouts/15/oaks.journals/downloadpdf.aspx?an=00129336-201618060-00001