The neutrophil-to-lymphocyte ratio is associated with disease severity and infectious etiologies of colitis in horses presenting with acute colitis.

NLR alone is a poor standalone prognostic marker but improves risk stratification when combined with other clinical findings in equine acute colitis.

This retrospective multicenter study evaluated the neutrophil-to-lymphocyte ratio (NLR) as a prognostic and diagnostic biomarker in 444 horses presenting with acute colitis (diarrhea duration <48 hours) across 5 equine referral hospitals, compared against 100 healthy controls. The NLR did not differ significantly between healthy horses (median 2.06) and colitis horses (median 1.93), and alone demonstrated poor prognostic performance for nonsurvival (AUC 0.51). However, horses with NLR <1.43 showed higher odds of nonsurvival. Horses meeting ≥2 systemic inflammatory response syndrome (SIRS) criteria had significantly lower NLR values compared to those meeting 0–1 criteria, suggesting NLR reflects severity of systemic inflammation. Horses with an identified infectious etiology had lower NLR than those without an identified pathogen, with equine coronavirus-infected horses demonstrating the lowest NLR (median 0.73), potentially indicating more pronounced lymphopenia and immune dysregulation in viral colitis. When incorporated into multivariable logistic regression models alongside other clinical and laboratory variables, discriminatory ability improved substantially (AUC 0.84). The authors conclude that NLR alone is insufficient as a standalone prognostic marker but serves as an inexpensive, readily available adjunct biomarker. It may help clinicians identify horses at risk for severe systemic inflammation and infectious colitis, particularly equine coronavirus, when interpreted alongside other clinical and diagnostic findings.

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