Canker (proliferative pododermatitis) or unguilysis is an infrequently found and difficult to treat disease of the hoof. Primarily affecting the frog in advanced stages may also infect the sole and wall. Resembling cauliflower in texture and color, it is described as a progressive chronic hypertrophy of horn-producing tissues. It can range from off-white to a yellow color. The normal horn is replaced by the vegetative growth and without treatment the effected areas spread as small finger like projections along the margin of the diseased tissue. This tissue is soft and almost cheese like in consistency. It bleeds easily when scrapped. The condition may involve more than one foot.

In the past the disease has been associated with wet and dirty conditions, but has been seen where there were no problems. Canker probably represents an infection but isolating the causal agent has been difficult. Both gram positive and gram negative organisms have been implicated and intracellular organisms have been observed in some cases. Spirochetes similar to the genus Treponema have been identified histologically in canker in some horses and other species with similar disease. The data suggest that spirochetes may contribute to the pathology of equine chronic foot diseases similar to the manner in which they contribute to the pathology of bovine papillomatous digital dermatitis and contagious ovine digital dermatitis. There has even been a report of one possible auto-immune cause.