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 horn-producing tissue of the foot), writes Dr. Tracy Turner in the "AAEP AnswerLine" column in the November edition of The Horse. Veterinarians believe the invading organism is part of the bacteroides species, which is similar to what causes "footrot" in sheep. Cases usually are found in the southeastern United States, but the problem has been diagnosed in horses all over the country.

The bacteria associated with canker causes abnormal keratin production, or overgrowth of the horn. This excess proliferation occurs underneath the horn, as the infection spreads throughout the epithelium. The horse's owner will notice the presence of a white or gray matter that is moist and spongy. This matter commonly appears in the sulci region of the hoof. If there is enough infection, heat might be felt in the hoof, but only in extreme situations.

The mystery surrounding canker is its cause. It involves a strict anaerobic process and seems to have a multi-factorial pathogenesis. The theory at this point is that canker is caused by some sort of trauma, writes Turner, who is a professor of large animal surgery at the University of Minnesota. An infection gets inside the hoof capsule and allows the horn to proliferate. It can be tough to get it out once it gets established.

Most veterinary textbooks suggest housing a horse in unsanitary conditions causes the disease. However, farms with the best of stable management and those with the worst can have horses with canker. Development of the problem might have something to do with how a horse is used. Stalled horses with little exercise seem more predisposed to the disease than horses that are active and outdoors.

To eliminate canker, veterinarians have tried many different medications. The suggested treatment is twofold:

1. Superficial Debridement--First remove horn over the affected area. Make sure that you do this superficially, because if you draw blood, infection will be driven deeper into the tissue.

2. Be sure to open the infected site to the air, and keep it clean and dry. Metronidazole applied topically or given systemically is the medication that seems to work the best. Systemically, there is no medicine that will cure canker if superficial debridement is not performed.

After applying the topical medication, apply a clean, dry bandage that is waterproof. Canker prefers moist conditions, so you must keep the affected area dry. Horses have variable response to treatment. Some cases heal within a week or 10 days, and some cases last for months. Given good, aggressive treatment, a week to 10 days of intensive treatment should have the problem under control.

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