from a normal mare in estrus should have very few or no neutrophils; however, a rare neutrophil may be noted in association with blood contamination that occurred during the collection process. Neutrophils may be present in the uterine lumen following breeding, after uterine lavage or infusion, during the postpartum period, or in cases of endometritis.
Other WBCs such as macrophages, lymphocytes, or eosinophils are not commonly found on equine uterine cytology preparations.
Lymphocytes are approximately 7 μm in diameter (the same size as RBCs), are round to oval, and have only a small amount of cytoplasm (Figure 17.4).
Macrophages are approximately 20 μm in diameter with abundant blue‐staining cytoplasm filled with various‐sized vacuoles (Figure 17.5). Lymphocytes and macrophages are found in in the postpartum mare and in chronic uterine infections.
Eosinophils are 12–15 μm in diameter, with blue‐staining cytoplasm that contains multiple pink or red granules (Figure 17.6). Eosinophils are found in cases of pneumovagina, fungal infections, or reflux of urine into the uterus.
The average number of WBCs per high power field (hpf) is determined after evaluating at least 10 hpf in multiple areas of the slide, and is used to categorize the degree of inflammation (Table 17.1).
Another method used to categorize the inflammatory response is the number of WBCs per UEC. A ratio of 1 WBC to 20–40 epithelial cells has been used as a gauge of the degree of inflammation.Figure 17.3 Inflammatory uterine cytology with quiescent macrophage (large arrow) and many neutrophils (small arrow).Figure 17.4 A lymphocyte (arrow) in a uterine cytology sample.Figure 17.5 A pair of macrophages (solid arrows) in a uterine cytology sample along with neutrophils (open arrow) and red blood cells (arrow head).Figure 17.6 An eosinophil (arrow) in a uterine cytology sample along with neutrophils and a red blood cell.Table 17.1 Description of the number of neutrophils per hpf with corresponding inflammation classification for samples collected from a brush/swab.Number of Neutrophils per hpfClassification0 to rareNormal1–2Mild inflammation3–5Moderate inflammation>5Severe inflammation
Categorizing the degree of inflammation represented in a cytologic sample from a low volume lavage can be difficult. Centrifugation of the uterine effluent concentrates UECs, WBCs, microbial organisms, and debris into a pellet. The pellet is subsequently smeared onto a glass slide, stained and evaluated. A normal mare should have very few or no WBCs noted in the cytology from a low volume lavage. Mares with mild uterine inflammation often have >5–10 neutrophils per hpf, whereas mares with more severe inflammation usually have >10 neutrophils per hpf. The presence of microbial organisms should be interpreted with caution, as there is a higher risk potential for contamination during sample collection with a low volume lavage procedure versus the use of a double‐guarded uterine swab or brush.
Red Blood Cells
Red blood cells are 6 μm in diameter with a central pallor. These cells are commonly found in low numbers (i.e., <4/hpf) on routine cytologic evaluation.
Excessive numbers of RBCs may indicate irritation to the endometrium from infection, infused compounds, or aggressive sampling technique.
Bacteria
Bacteria can be visualized on a uterine cytology sample but often require 1,000× (10× eye piece and 100× oil immersion objective) magnification to differentiate them from debris. The presence of bacteria engulfed within WBCs can help determine if the bacteria are due to infectious endometritis or contamination.
The four most common bacterial pathogens of the equine uterus are Streptococcus equi subspecies zooepidemicus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa.
Streptococcus equi subspecies zooepidemicus are Gram‐positive cocci approximately 1.0 μm in diameter that form chains of various length (Figure 17.7).
Escherichia coli, K. pneumoniae, and P. aeruginosa are Gram‐negative rods varying in size from 3 to 6 μm in length and cannot be differentiated accurately based on cytologic evaluation (Figure 17.8).
Fungal Organisms
Yeast are typically 5–8 μm in size with a very distinct cell wall (100–200 nm) that forms a clear “halo” around the organism; this may be especially evident if the organism is surrounded by debris or other cells (Figure 17.9).
Hyphate fungi are typically 3–5 μm in width and 8 μm in length. These individual cells are commonly linked together in long branching chains (Figures 17.10 and 17.11). In many instances, detection of a fungal organism on cytologic examination may be the only evidence of fungal endometritis, as organisms do not always grow on culture.
Figure 17.7 Chains of cocci (arrow) in a uterine cytology sample collected from a mare with a Streptococcus equi subspecies zooepidemicus infection.
Figure 17.8 Gram‐negative rods (arrow) in a uterine cytology sample collected from a mare with endometritis.
Figure 17.9 Yeast organisms (Candida albicans) (arrow) in a uterine cytology sample.
Figure 17.10 Aspergillus fumigatus organisms in a mare with fungal endometritis.
