Biomarkers of equine asthma: A review of the current literature.

Equine asthma (EA) is a common respiratory disease in horses that is diagnosed primarily through examining fluid from the lungs, a process that can be invasive and impractical in some situations. This review analyzed current research on potential biological markers (biomarkers) found in blood, lung fluid, and other samples that could help diagnose and monitor EA more easily and accurately.

Introduction to Equine Asthma and Diagnostic Challenges

• Equine asthma is a frequent lower airway inflammatory disease affecting horses.
• Current diagnostic methods largely rely on bronchoalveolar lavage fluid (BALF) cytology, which involves collecting cells from deep in the lungs.
• This procedure is moderately invasive and may not be feasible under field conditions or in some veterinary settings.
• There is a need for alternative, less invasive diagnostic tools that can accurately detect and monitor the disease.

Aim and Scope of the Review

• The review aimed to synthesize existing evidence about candidate biomarkers associated with equine asthma.
• Primary focus was placed on biomarkers measured in blood-based matrices and BALF.
• Additionally, data from tracheal wash (TW), saliva, and exhaled breath condensate (EBC) were included where available.
• The literature search covered databases including PubMed, Scopus, and Google Scholar.
• Only original studies involving horses with mild-to-moderate or severe EA that reported potential biomarkers were included.

Summary of Studies and Biomarkers Evaluated

• A total of 57 unique studies were analyzed, covering a wide variety of candidate biomarkers.
• Biomarkers from different sample types were assessed for their diagnostic potential and association with disease severity.

Blood-Derived Biomarkers

• Investigated biomarkers included surfactant protein D (SP-D), secretoglobin (SCGB), and haptoglobin.
• These markers showed variable results; generally, their specificity when used alone was limited.
• However, some combinations of blood markers enhanced diagnostic accuracy.
• Blood markers are advantageous due to their less invasive collection compared to lung fluid samples.

Bronchoalveolar Lavage Fluid (BALF) Biomarkers

• Biomarkers derived from BALF more consistently reflected local airway inflammation and the severity of EA.
• Particularly promising BALF biomarkers included:
• Neutrophil extracellular traps (NETs) related markers, which indicate neutrophil-driven inflammation.
• Neutrophil gelatinase-associated lipocalin (NGAL), a protein associated with neutrophil activation and inflammation.
• Allergen-specific Immunoglobulin E (IgE) profiles, indicating allergic hypersensitivity relevant to EA phenotypes.
• Use of multimarker panels combining these biomarkers and stratifying by EA phenotype improved diagnostic performance.

Other Sample Types

• Where available, studies also evaluated biomarkers from tracheal wash (TW), saliva, and exhaled breath condensate (EBC).
• However, these matrices had fewer studies, and the evidence was less robust compared to BALF and blood-derived markers.

Conclusions and Future Directions

• Current evidence supports the use of integrated biomarker panels, especially those based on BALF, combined with blood-based markers.
• Such multimarker panels hold promise to improve non-invasive diagnostic accuracy and disease monitoring in equine asthma.
• However, robust validation in larger and well-characterized horse populations is needed before these biomarker panels can be implemented routinely in clinical practice.
• Future research should focus on standardizing biomarker assays, evaluating their performance in diverse settings, and exploring practical sampling methods.