Understanding Cytological Endometritis in Dairy Cows: Diagnosis, Pathophysiology, and Clinical Challenges

Cytological endometritis is a postpartum uterine disorder that presents a unique diagnostic and clinical challenge. Unlike metritis or purulent vaginal discharge (PVD), affected cows often show no obvious external clinical signs, making diagnosis dependent on evaluation of the endometrium itself. Despite its subclinical nature, persistent uterine inflammation can delay uterine recovery and compromise reproductive efficiency. For practicing veterinarians, understanding when and how to diagnose cytological endometritis is essential for informed reproductive management. 

Defining Cytological Endometritis 

Cytological endometritis is characterized by persistent inflammation of the endometrium, identified by an increased proportion of polymorphonuclear neutrophils (PMNs) in endometrial cytology samples. Diagnosis relies on microscopic evaluation rather than clinical observation. 

The accepted PMN threshold varies with the interval after calving because uterine inflammation normally decreases as involution progresses. Consequently, diagnostic cut-offs should always be interpreted in relation to days in milk rather than using a single value throughout the postpartum period¹. 

Since affected cows may appear clinically normal, cytological evaluation remains the most reliable method for identifying this condition. 

Choosing the Appropriate Diagnostic Method 

Several techniques have been used to evaluate postpartum uterine health, including uterine biopsy, cytobrush sampling, low-volume uterine lavage, and vaginoscopy. Among these, cytobrush sampling has become the preferred diagnostic approach because it provides representative endometrial cells while remaining practical for routine clinical use¹. 

Histopathology remains the reference method for evaluating endometrial inflammation, but it is invasive and less suitable for routine field application. Comparisons between histopathology and cytology demonstrate good agreement, supporting the clinical usefulness of cytological examination in dairy practice². 

Accurate sample collection is equally important. Proper technique minimizes contamination from the caudal reproductive tract and improves the reliability of cytological interpretation. 

Understanding the Inflammatory Process 

The pathogenesis of cytological endometritis remains incompletely understood. Current evidence suggests that persistent inflammation, rather than ongoing bacterial infection alone, plays a central role in many affected cows. 

Interestingly, comparisons of the uterine microbiome at 10, 21, and 35 days in milk found no consistent differences between healthy cows and those diagnosed with cytological endometritis at 35 days postpartum³. This observation indicates that persistent inflammation may continue even after major microbial differences have resolved. 

One proposed explanation is a failure in the normal resolution of inflammation. During postpartum uterine involution, immune responses must eliminate contamination while simultaneously supporting tissue repair. If inflammatory pathways remain active beyond this normal period, endometrial recovery may be delayed despite the absence of substantial bacterial challenge. 

The interaction between pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) is increasingly recognized as an important regulator of uterine immune responses^4,5,6. Achieving an appropriate balance between pathogen recognition and tissue repair appears critical for restoring normal uterine function. 

Ongoing Clinical Challenges 

Although cytological endometritis is well recognized, several practical questions remain unanswered. Diagnostic thresholds differ according to postpartum stage, and no single cytological cut-off is universally applicable. In addition, inflammation may persist despite minimal detectable bacterial involvement, making treatment decisions less straightforward. 

These challenges reinforce the importance of interpreting cytological findings alongside the cow's postpartum stage and overall reproductive status rather than relying solely on laboratory results. 

Practical Clinical Insights 

For veterinarians managing postpartum reproductive health, cytological endometritis should be considered a disorder of persistent endometrial inflammation that often requires targeted diagnosis rather than clinical suspicion alone. Incorporating cytobrush sampling into reproductive examinations can improve detection of subclinical uterine disease, particularly in cows that fail to establish pregnancy despite the absence of obvious reproductive tract abnormalities. Understanding that persistent inflammation may not always reflect active infection also supports more thoughtful interpretation of diagnostic findings and highlights the need for individualized clinical decision-making. 

References  

1. LeBlanc SJ. Postpartum reproductive disease and fertility in dairy cows. animal. 2023 May 1;17:100781. https://www.sciencedirect.com/science/article/pii/S1751731123000770 

2. Pascottini OB, Hostens M, Dini P, Vandepitte J, Ducatelle R, Opsomer G. Comparison between cytology and histopathology to evaluate subclinical endometritis in dairy cows. Theriogenology. 2016 Oct 1;86(6):1550-6. https://doi.org/10.1016/j.theriogenology.2016.05.014 

3. Pascottini OB, Van Schyndel SJ, Spricigo JW, Rousseau J, Weese JS, LeBlanc SJ. Dynamics of uterine microbiota in postpartum dairy cows with clinical or subclinical endometritis. Scientific reports. 2020 Jul 23;10(1):12353. https://www.nature.com/articles/s41598-020-69317-z.pdf 

4. Sheldon IM, Cronin JG, Bromfield JJ. Tolerance and innate immunity shape the development of postpartum uterine disease and the impact of endometritis in dairy cattle. Annual review of animal biosciences. 2019 Feb 15;7(1):361-84. https://pmc.ncbi.nlm.nih.gov/articles/PMC6450715/pdf/nihms-1020046.pdf 

5. Sheldon IM, Molinari PC, Ormsby TJ, Bromfield JJ. Preventing postpartum uterine disease in dairy cattle depends on avoiding, tolerating and resisting pathogenic bacteria. Theriogenology. 2020 Jul 1;150:158-65. https://pmc.ncbi.nlm.nih.gov/articles/PMC7234917/pdf/nihms-1550548.pdf 

6. Zhivaki D, Kagan JC. Innate immune detection of lipid oxidation as a threat assessment strategy. Nature Reviews Immunology. 2022 May;22(5):322-30. https://pmc.ncbi.nlm.nih.gov/articles/PMC8454293/pdf/41577_2021_Article_618.pdf