A lesion is any localized region of tissue that has undergone pathological change — in structure, composition, or function — as a result of disease, injury, or a developmental abnormality. The term is used broadly across veterinary and medical disciplines to describe virtually any discrete abnormality found during physical examination, diagnostic imaging, biopsy, or post-mortem evaluation. In equine medicine, identifying the location, character, and extent of a lesion is the central task of clinical examination, because it determines the differential diagnosis list and guides the choice of further investigation or treatment.
Lesions are classified by their physical characteristics: macules (flat discolored areas), papules (raised solid elevations), vesicles (fluid-filled blisters), pustules (pus-filled elevations), ulcers (breaks in epithelial continuity), erosions (superficial loss without full-thickness involvement), and nodules (solid masses within tissue). In the context of equine anatomy, a skin lesion at the fetlock or pastern might indicate pastern dermatitis (scratches/mud fever), while a lesion within the navicular bone identified on radiograph indicates degenerative joint disease. Internal lesions — such as those in the intestinal mucosa associated with strongyle migration, or in the gastric mucosa associated with equine gastric ulcer syndrome — require endoscopy or necropsy to characterize fully.
The term does not imply a specific cause or severity; a lesion may be acute, chronic, primary (the direct result of the causative agent), or secondary (resulting from a complication or host response). Clinicians describe lesions in terms of size, shape, depth, distribution, and any associated discharge or odor. Accurate lesion description is the foundation of veterinary communication, ensuring that a second examiner or specialist reviewing notes can reconstruct what was observed. For owners, recognizing when new skin lesions appear and reporting them promptly allows earlier veterinary intervention and better outcomes across most dermatological and musculoskeletal conditions.
Further Reading
- lesion (Wikipedia)
- For a clinical overview of equine skin lesion types and management: dermatitis and dermatologic problems of horses — Merck Veterinary Manual