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 a bacterial 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. Dr. Woerner found two types of bacteria in this case (Morganella morganii and E. coli).
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. The odor is unmistakable!
Canker is is believed to be caused by unsanitary conditions; however, some horses are more predisposed than others regardless of their environment. In this particular case, neglect was definitely a factor. It is also more common in draft horses. Development might instead be influenced by how the horse is used. Stalled horses with little exercise seem more predisposed to the disease than horses that are active and ¬kept outdoors.
In contrast to thrush, which is a necrotic or tissue-destroying process, canker creates abnormal tissue growth and is described as a hypertrophic pododermatitis. Dr. Woerner did a biopsy of the abnormal tissue in this horse and the pathology was described as "severe, diffuse, parakeratosis" or dyskeratosis. What makes canker difficult to treat is that it spreads into healthy tissue with a healthy blood supply.
Treating canker can be challenging. Veterinarians have suggested many medications and treatments, but successful therapy is based on the following:
1. Superficial debridement (cutting away abnormal tissue) over the entire affected area.
2. Canker prefers moist conditions, so keep the treated area very clean and dry.
3. Topical treatments. Veterinarians have suggested several, but the two most effective topical therapies are the antimicrobial drug metronidazole and 10% benzyl peroxide solution. Metronidazole is usually ground into a powder and spread over the affected area. Benzyl peroxide is a potent astringent commonly found in acne medicine. It is soaked into gauze sponges and applied as a wound dressing. Systemically, no medicine will cure canker if superficial debridement is not performed.
After applying the topical medication, a clean, dry, waterproof bandage must be kept on the foot or feet. Many veterinarians recommend using shoes with treatment plates, which are more convenient than bandaging the hoof.
Horses have variable responses to treatment. Some cases heal within a week or 10 days, but more commonly cases last for months. Once the tissue has healed, it is very rare for the disease to recur. However, before healing is complete, canker might return--a trait that led many to believe it was a cancerous disease.
This is picture of one of the front feet (all four feet were affected in this horse). This picture was taken in August 2015.
After five months of treatment (debridement, metronidazole, copper sulfate and hydrogen peroxide) the foot now looks like this.