Understanding Uterine Lesions in Canine Subfertility: What Really Matters Clinically?

Subfertility in the bitch is a multifactorial problem, often frustrating for both clinicians and breeders. While uterine pathology is frequently implicated, distinguishing clinically relevant lesions from incidental findings remains a key challenge. Histopathological evaluation through uterine biopsy offers insight, but interpreting these findings correctly is critical to avoid misdiagnosis and unnecessary interventions¹. 

Commonly Reported Lesions 

Several uterine abnormalities have been described in subfertile bitches, including: 

• Endometritis 

• Endometrial cysts 

• Fibrosis 

These findings are widely reported across studies, yet their direct relationship with fertility outcomes has not always been clear. The lack of comparative data with fertile animals has historically limited clinical interpretation^1,2. 

Fibrosis: Not Always a Fertility Barrier 

Fibrosis has long been considered detrimental to fertility, particularly in species like the mare, where even mild changes can impair pregnancy maintenance. However, evidence in dogs suggests a different scenario¹. 

Fibrosis was observed in both fertile (14%) and subfertile (18%) bitches without a statistically significant difference. This indicates that mild fibrosis may not be a primary cause of subfertility in the canine uterus¹. 

One possible explanation lies in the unique nature of canine placentation, where glandular reorganization may compensate for mild structural changes¹. For clinicians, this means fibrosis should be interpreted cautiously and not automatically linked to poor reproductive prognosis. 

Endometrial Cysts and CEH 

Cystic endometrial hyperplasia is commonly encountered in practice and is often associated with hormonal influences, particularly prolonged exposure to progesterone and estrogen¹. It is also linked to pyometra, raising concerns when identified on biopsy. 

However, similar prevalence rates of cysts in fertile (15%) and subfertile (19%) bitches suggest that their presence alone does not necessarily indicate infertility. Additionally, age-related increases in cyst incidence were observed in both groups, reinforcing the idea that cyst formation may be a physiological or age-associated change rather than a pathological determinant of fertility. 

The True Role of Inflammation 

Among all lesions, inflammation stands out as the most clinically significant factor. A clear statistical difference in the presence of inflammation between fertile and subfertile groups (p=0.004) underscores its importance. 

Further characterization reveals that not all inflammatory types are equally relevant. Neutrophilic and histiocytic inflammation did not differ significantly between groups, likely reflecting normal physiological responses during certain stages of the estrous cycle¹. 

In contrast, lymphoplasmacytic inflammation showed a strong association with subfertility (p<0.001). This type of inflammation is often indicative of chronic pathology and may interfere with the uterine environment necessary for successful implantation and pregnancy maintenance¹. 

Age and Lesion Distribution¹ 

Age plays a subtle but important role in uterine pathology. Subfertile bitches in the study were slightly older on average, which may explain the increased prevalence of certain lesions such as cysts. However, fibrosis did not show a statistically significant age-related difference, suggesting that its development may not be solely age-dependent. 

For practitioners, this highlights the importance of considering age alongside histological findings, rather than interpreting lesions in isolation. 

Clinical Interpretation: Avoiding Overdiagnosis 

One of the key risks in using uterine biopsy is overdiagnosis, particularly when incidental findings are mistaken for clinically significant pathology. 

For example: 

• Mild fibrosis may not impair fertility 

• Isolated cysts may be age-related 

• Certain inflammatory patterns may be physiological 

Without proper context, these findings could lead to unnecessary treatment or incorrect breeding recommendations. 

Integrating Biopsy into Clinical Practice 

To maximize the value of uterine biopsy, clinicians should^1,3: 

• Correlate histopathology with reproductive history 

• Consider timing within the estrous cycle 

• Use imaging modalities like ultrasonography alongside biopsy when appropriate   

• Focus on inflammation patterns rather than isolated structural changes 

Conclusion 

Understanding uterine lesions in the bitch requires a shift from descriptive pathology to clinically meaningful interpretation. While fibrosis and cysts are commonly observed, they may not be reliable indicators of subfertility. 

Instead, inflammation, particularly lymphoplasmacytic, emerges as the most relevant lesion. By focusing on clinically significant findings and avoiding overinterpretation, veterinarians can make more informed decisions and improve reproductive outcomes in canine patients. 

References: 

1. McRae GR, Coutinho da Silva MA, Runcan EE, Stephens JA, Premanandan C. Histological Studies in the Endometrium of Fertile and Subfertile Bitches. Reproduction in Domestic Animals. 2025 Apr;60(4):e70055. https://onlinelibrary.wiley.com/doi/pdf/10.1111/rda.70055 

2. Woźna-Wysocka M, Rybska M, Błaszak B, Jaśkowski BM, Kulus M, Jaśkowski JM. Morphological changes in bitches endometrium affected by cystic endometrial hyperplasia-pyometra complex–the value of histopathological examination. BMC veterinary research. 2021 Apr 26;17(1):174. https://link.springer.com/content/pdf/10.1186/s12917-021-02875-0.pdf 
3. Batista PR, Gobello C, Rube A, Barrena JP, Arioni S, Blanco PG. Doppler ultrasonographic evaluation of medically treated female dogs with cystic endometrial hyperplasia–pyometra complex. Veterinary Radiology & Ultrasound. 2022 Jul;63(4):490-7. https://doi.org/10.1111/vru.13079.