What is canker in horses?

Canker is a hoof disease that's associated with heavy draft horses, and seldom (but not never) seen in light horses. It's an infection and enlargement of horn producing tissues - and we still don't know what causes it. It's most often found at the back, and often isn't found early enough to treat easily - usually by the time it's visible on the outside of the frog it's pretty bad. The horse has to have all of the infected tissue removed and an antibiotic dressing applied daily - often for weeks or months. Many horses with canker are not lame (which also makes it hard to detect) and can even be worked lightly while the wound heals.