Cellulitis is the inflammation of the superficial tissue layers beneath the skin, caused by a bacterial infection. It usually occurs secondary to a wound or infection of the deeper tissues. Wounds may even be superficial and small, however anything that breaks down the skin barrier provides a pathway for bacterial organisms to enter the body. Moderate to severe cases of rainrot can sometimes cause cellulitis in horses, due to secondary invasion by bacteria.

Cellulitis usually starts around the immediate area of the infection, such as the wound or joint, etc. However it will quickly spread downward and sometimes upward in the affected leg. Horses with cellulitis may also be lame. Horses that develop cellulitis as a result of a superficial wound are usually mildly lame however if it develops as a result of a deeper tissue infection, such as septic arthritis at a joint, lameness is usually moderate to severe.

Cellulitis is distinguishable from 'stocking up' through noting the temperature of the swollen area, as cellulitis will feel warm to the touch, as opposed to stocking up, which feels normal or slightly cool. Horses with stocking up also will have bilateral swelling--in that both legs will be swollen. Horses with cellulitis may sometimes have both legs swollen if they have an infection in both legs, but it is more likely that it will be usually be associated with one leg, not both.

Every time cellulitis occurs, it caused lymphatic inflammation and possibly some permanent damage. Severe or repeated episodes of cellulitis can lead to lymphedema.