Bruising in horses occurs when blunt trauma ruptures small blood vessels beneath the skin or within tissue, allowing blood to pool without breaking the surface. The body responds to this hemorrhage with inflammation — the affected area becomes warm, swollen, and painful. External bruises on a horse are often invisible under the coat and must be found by palpation or by observing the horse’s pain response to pressure, unlike the visible discoloration that appears in fair-skinned humans.
The most clinically significant bruising in horses occurs in the hoof — the structure where sole bruising is both hardest to see and most damaging. Sole bruises result from pressure applied to the sensitive structures beneath the sole — a stone, uneven ground, or thin-soled conformation pressing on the coffin bone. The blood pooled in the sensitive sole appears as a reddish or purple stain when the farrier trims the hoof. Sole bruises produce lameness that ranges from mild toe-first landing to non-weight-bearing depending on severity; in the early stages they are commonly mistaken for a hoof abscess.
Soft-tissue bruising elsewhere in the body heals through the normal inflammatory cycle: the pooled blood is reabsorbed as macrophages clear the debris, and swelling resolves over days to a few weeks depending on the size of the hemorrhage. Cold hydrotherapy applied within the first 24 to 48 hours reduces inflammation and speeds resolution. A hematoma — a discrete, organized collection of blood in a tissue space — can form from more significant hemorrhage and may require veterinary drainage if it is large or becomes infected. The cannon bone region and the poll are common trauma sites after impact with fences or stall walls.
Further Reading: The Wikipedia article on bruising covers the general physiological mechanism of hemorrhage and tissue repair; the Merck Veterinary Manual’s section on disorders of the foot in horses addresses sole bruising and its clinical management.