Association of neutrophil morphology with bacterial isolates in equine tracheal wash samples.
Authors: Jocelyn N A, Wylie C E, Lean M, Barrelet A, Foote A K
Journal: Equine veterinary journal
Summary
# Editorial Summary: Neutrophil Morphology and Bacterial Infection in Equine Tracheal Washes Neutrophilia on tracheal cytology is a hallmark of lower airway disease, yet the relationship between how neutrophils appear microscopically and the presence of bacterial infection has been poorly characterised until now. Researchers at Rossdales analysed 1,100 tracheal wash samples collected between 2013 and 2015, examining whether degenerate neutrophils (scored ≥2/4 on a standardised scale) correlated with positive bacterial culture of respiratory pathogens, accounting for horse age and season. The presence of potentially pathogenic bacteria was strongly associated with degenerate neutrophilia, with affected horses having 4.5 times greater odds than culture-negative cases—an association that held particular significance in young horses (1–3 years old), who showed substantially higher degenerate changes than those over 9 years. Seasonal patterns emerged too, with winter samples demonstrating more degenerate changes than spring/summer presentations. Practitioners should incorporate assessment of neutrophil morphology into routine tracheal wash interpretation: the presence of degenerative changes, especially in younger animals, justifies investment in culture and sensitivity testing to guide antimicrobial selection, whilst absence of these features may help support non-infectious differential diagnoses and avoid unnecessary antibiotic use.
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Practical Takeaways
- •When you see degenerate neutrophils on tracheal wash cytology, especially in younger horses, culture and sensitivity testing is strongly indicated to guide targeted antimicrobial therapy
- •The absence of degenerate neutrophil changes may support non-infectious causes of airway inflammation, helping narrow your differential diagnosis
- •Age and season should be considered when interpreting tracheal wash cytology results, as younger horses and winter submissions show higher baseline degenerate neutrophil rates
Key Findings
- •Degenerate neutrophils in tracheal washes were associated with 4.5-fold increased odds of bacterial culture positivity (P<0.001)
- •Horses over 9 years old had 40% lower odds of degenerate neutrophilia compared to 1-3 year-old horses (OR 0.6, P<0.02)
- •Spring/summer season reduced odds of degenerate neutrophilia by 60% compared to winter (OR 0.4, P<0.001)
- •Cytomorphological evaluation of neutrophil degeneration improves diagnostic utility of tracheal wash samples for identifying bacterial pathogens