Cellulitis literally means inflammation of the subcutaneous tissue, the tissue sitting between the skin and deeper tissues of muscle, bone and tendon/ligament. Cellulitis has an appearance similar to edema but can be distinguished from simple edema (e.g. stocking up) by the presence of heat and pain.
Cellulitis may be caused by inflammation alone, or the inflammation may be secondary to infection. The most common example is cellulitis that develops around a wound. For the first one to three days after an injury, the acute inflammatory response results in local heat, swelling and pain in the surrounding area. Bacteria contaminating the wound may also penetrate into the surrounding tissues. Local inflammation that persists for longer than three days likely means bacteria have gotten into the tissues.
Chemical cellulitis is also fairly common in horses. Horses sensitive to ingredients in shampoos, liniments, fly sprays or other topicals have reactions that range from just inflammation of the upper skin levels to a deeper cellulitis. Inflammed skin is also less able to resist bacterial infection, which can penetrate and cause a bacterial cellulitis even if there are no obvious breaks in the skin.
Relatively minor skin irritations/breaks, such as those caused in insect bites/stings, irritation from plant material, or clipping, may compromise the skin barrier enough to set up a bacterial cellulitis. Pastern and lower-leg cellulitis may also arise from deep infections/abscesses in the foot.
If cellulitis develops within 72 hours after a wound or skin irritation, the first-line therapy is inflammation control. Drugs like phenylbutazone or flunixin (Banamine) may be useful, but the best treatment is cold-water hosing and icing. Wrapping over ice, or cottons soaked in alcohol and cooled in the freezer, can give additional control of swelling, but wrapping without icing/cooling measures is dangerous. The heat trapped under wraps both encourages inflammation and may promote spread of bacteria through the tissues. Swelling can also occur above and below the ends of the wrap, leading to interference with circulation. Liniments, sweats and neoprene or plastic sweat wraps should also be avoided.
If the problem is not obviously improving after 24 to 48 hours, or if there is also a wound and it is obviously looking infected, contact your vet. There’s a good chance the horse will need antibiotics.