Quittor is a term used for a condition which involves death and destruction (necrosis) of the collateral cartilages of the foot, following an infection.
Whilst infection, i.e. pus in the foot, remains the most common cause of day-today lameness in the horse, quittor, a more serious complication, is now very uncommon. Quittor more commonly affects the front rather than the hind feet and the condition was more frequently seen in the heavy (draft) breeds of horses than the lighter breeds and ponies

The collateral cartilages have a poor blood supply, and so when infected they respond poorly and the infection becomes chronic and damaging.  Draft horses would commonly wear caulks or studs in their shoes which would result in injury to the coronary band if they were to tread on each other.   The condition was known as ‘treads’ for this reason, as horses would tread on their team members whilst working.
Quittor is still occasionally seen usually following external trauma to the foot, e.g. wire cuts, or interference injuries
to the pastern and coronet.

How can quittor be diagnosed?
An intermittently discharging wound develops on the inside or outside of the hoof over the collateral cartilages,
following an injury. The area is frequently warm, swollen and painful, consistent with infection. A number of small
discharging sinuses (holes) may appear in the pastern over the collateral cartilage.
Lameness may be intermittent, varying from mild to very severe, but the horse may or may not be lame at the time of examination, because lameness usually subsides after the infection discharges (‘breaks out’).
Long-term cases may result in deformity of the hoof wall.  Radiographic (x-ray) examinations of the horse’s foot may reveal necrosis of the sidebones and/or gas shadows,
confirming infection, or ossification of the collateral cartilages depending on the stage of the condition.

Quittor is a chronic, septic condition of one of the collateral cartilages of the distal phalanx characterized by necrosis of the cartilage and one or more sinus tracts extending from the diseased cartilage through the skin in the coronary band region. It is seldom encountered today but was common in working draft horses in the past. Quittor usually follows injury to the limb on the medial or lateral aspect of the lower pastern (immediately proximal to the coronary band, over the proximal extent of the cartilage), by means of which infection is introduced into the traumatized collateral cartilage. This leads to localized sepsis or abscessation of the cartilage. The cartilage may also become infected through a quarter crack, or even a complicated foot abscess that migrates to deeper structures. The first sign is an inflammatory swelling over the region of the collateral cartilage, which is followed by sinus formation and intermittent drainage. During the acute stage, lameness occurs.

Surgery to remove the diseased tissue is required, but care must be taken not to enter the distal interphalangeal joint. Local or parenteral antibiotic therapy (or both) without surgery is likely to fail. In the absence of any therapy, poor drainage, cartilage necrosis, and recurrent abscessation lead to chronic lameness and extension to deep structures. The prognosis is unfavourable if the disease progresses to involve the distal interphalangeal joint. (Merck, Belknap)
Otherwise, if treated promptly and accordingly, many of these cases will resolve.