Canker is an unusual condition of the horse’s foot that affects the frog, bars, and sole. The name comes from the early belief that the condition was of a cancerous nature. However, to the best of our knowledge, canker is an anaerobic (grows in the absence of oxygen) infection in the superficial epithelium of the hoof (the outermost tissues, which produce the horn). The causative bacterium is unknown, but some researchers have suggested the organism is a part of the Bacteroides species, which is similar to what causes "foot rot" in sheep. In a more recent study researchers found spirochete (spiral-shaped) bacteria in the epithelium, which was similar to findings in cows and sheep with digital dermatitis. Equine canker cases usually are found in the southeastern United States, but they have been diagnosed all over the country.

The microorganism associated with canker causes abnormal keratin production, or overgrowth of the horn. This excessive proliferation occurs underneath the horn, as the infection spreads throughout the epithelial layer. Commonly, an affected horse will have white or gray matter that is moist and spongy appearing in the sulci region (grooves on either side and in the center of the frog) of the hoof. This characteristic growth’s appearance has been described as similar to wet cauliflower with cottage cheeselike exudates. If there is extensive infection, heat might be felt in the hoof, but usually only in extreme situations.

Canker is fairly rare and only briefly discussed in veterinary textbooks. Most texts suggest that housing a horse in unsanitary conditions causes the disease; however, farms with both the best of stable management and the worst can have horses with canker. In attempts to recreate the disease, I and others have packed the frog with manure after injecting what we believe are the causative bacteria, but we still have been unable to prove that unsanitary conditions contribute to c