Canker: What Is It?

Q: My veterinarian mentioned that she was treating a horse on a nearby farm for canker. What is it? Where does it come from? And how do you treat it? Is it different from thrush?

A: 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 tissues of the foot). Veterinarians believe the invading organism is a part of the bacteroides species, which is similar to what causes "footrot" in sheep. Cases usually are found in the southeastern United States, but it has been diagnosed 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 and 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 very strict anaerobic process, and seems to have a multi-factorial pathogenesis. For research we've tried to recreate canker in the horse, but have never been successful. Our theory at this point is that canker is caused by some sort of trauma. 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.

Canker is fairly rare, and appears only briefly in veterinary textbooks. Most texts suggest that housing a horse in unsanitary conditions causes the disease, but I've found that canker is not prejudicial. Farms with the best of stable management and those with the worst can have horses with canker. In our attempts to recreate the disease, we have packed the frog with manure after injecting what we believe is the invading bacteria, but still have been unable to prove that unsanitary conditions contribute to the onset of canker.

Development of canker might have something to do with how the horse is used. Stalled horses with little exercise seem more predisposed to the disease than horses which are active and outdoors.

While thrush is a necrotic process, canker is a hypertrophic pododermatitis. Both thrush and canker are found in the same region of the foot, but instead of the tar-like substance evident in horses with thrush, canker resembles rotten cauliflower. Some people say hooves with canker have a distinct odor, but I believe that thrush is much more odiferous than canker. Thrush is an aerobic process caused by another mysterious bacteria, thought to be fusobacterium necrophorus spp., which works superficially and does not undermine tissue. When thrush eats at tissue, the inner tissues of the digit are protected until bacteria gets deep enough to deteriorate more sensitive structures. Canker spreads in live tissue, without the help of oxygen.

To eliminate canker, veterinarians have tried many different medications. Suggested treatment is two-fold:

1. Superficial Debridement--First remove horn over the affected area. Make sure that you do this very 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 very clean and dry. We've found that Metronidazole--an antibiotic/microbial-- applied topically or given systemically has worked 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. Years ago, owners lost some horses to canker. That now is the exception rather than the rule because veterinarians are able to recognize and treat the disease early in its process.

Given good, aggressive treatment, a week to 10 days of intensive therapy should have the canker under control. Once your horse is completely rid of canker, the condition is gone. I have never seen it recur. Before healing is complete, however, canker might try to return. So, just when you think you've whipped it, there it goes again.

About the Author

Tracy A. Turner, DVM, MS, Dipl. ACVS

Tracy A. Turner, DVM, MS, Dipl. ACVS, is a veterinarian with Anoka Equine Veterinary Services in Elk River, Minn. He was inducted into the International Equine Veterinarians Hall of Fame in 2004.

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