Pus is the viscous, opaque fluid produced at a site of bacterial infection. It consists of dead and living neutrophils, the bacteria they have engulfed or failed to contain, cellular debris from lysed tissue, and the serum that leaked into the tissue during the inflammatory response. The color ranges from off-white to yellow or green, depending on the predominant bacterial species and the concentration of myeloperoxidase released by dying neutrophils; Staphylococcus tends toward cream-white, Pseudomonas toward blue-green.
In horses, pus is encountered most often at three sites: the hoof sole and coronary band in abscess drainage, subcutaneous and deeper tissue following wound contamination or injection, and the guttural pouches in severe strangles. A localized pocket is termed an abscess; when the infection spreads without walling off it is called cellulitis. The body walls off pus in a fibrous capsule to limit spread, which is why an untreated abscess eventually points and ruptures to the surface. Distinguishing pus from serous or serosanguinous exudate guides the clinical decision: serous fluid is normal early healing, purulent fluid indicates active bacterial colonization. See Discharge for the broader classification of equine exudates.
Because pus signals active infection rather than a wound in normal repair, its presence directs management toward bacterial identification and systemic or local antimicrobial treatment. That management layer, including culturing, antibiotic selection, and surgical drainage, is covered on the equine health authority.