Updated: Apr 22, 2020
Indepth Equine Podiatry Symposium Notes Written and presented January 2009 by R.F. (Ric) Redden, DVM
Canker foot carries a wide variety of meanings. The basic characteristics of the disease are white, proliferative finger-like projections along the coronary band, a musty (fungal) odor and/or deep necrotic, non-sensitive involvement of the sensitive frog, digital cushion, sole corium and laminae. There is very little if any useful data in our current text books to help define the various stages of canker or the many different forms it can manifest. Older texts written before veterinary medicine was a reality are also vague, designating many different pathological processes as canker. Canker falls under the broad category of pododermatitis, which is descriptive to some degree but very misleading for the case with very extensive necrotic damage to the deep structures of the foot. The fact is, this disease occurs throughout the world yet remains quite a mystery, as we do not understand the challenges of the various forms that all appear to be related. Over the years I have dealt with many cases categorized as canker, from its initial to advanced stages.
Contrary to popular belief, in my experience this disease is not limited to horses kept in low hygiene conditions. Some of my most difficult cases have come from top level racing and training stables that maintain the highest level of hygiene. It does not appear to be contagious either, as I have only seen one instance in which more than one horse from the same farm or stable developed canker. In that case, several show quality Clydesdales were presented to me with very extensive, advanced lesions, some in one foot and others in all four. They were from a farm in Ohio that had previously been a major hog farm, but any possible connection would be speculative.
Draft breeds are apparently more prone to canker, possibly because of the long feathers and dense, coarse hair that is characteristic of the breed. The lack of air to the skin in the lower leg and coronary band along with the cumulative effects of bacteria and fungus that persist in this area could be a direct cause of the increased incidence in these breeds. However, it certainly does not explain why other breeds that have extremely short hair and often get daily baths can have such a serious incident, often appearing to originate within the digital cushion and frog sulcus. I have seen this condition in warm bloods, Thoroughbreds, Standardbreds and only a few mixed breeds. Possibly other breeds have this condition with the same incidence, but I cannot say that from experience.
Clinical Differences Between Thrush and Canker
During the initial stages of canker, most horsemen almost invariably feel they are treating thrush that simply won't go away. The majority will use caustic agents to dry it up, which works quite well in most thrush cases. However, when canker is the culprit, products that burn or cauterize the sensitive tissues seal the canker inside, frequently causing it to spread throughout the digital cushion.
Thrush is caused by an organism initially referred to as xerophilic Nectria, but now called spirochaeta. Both terms have destructive connotations, but they are simply low virulent organisms that have frog lysing properties when conditions allow them to multiply. Thrush starts along the sulci of the frog. The deep crevasse that appears periodically in a variety of foot stereotypes can harbor moisture and causative organisms that find it ideal for growth. The result is thrush. The area is tender to a hoof pick but does not cause a lameness concern. The unpleasant odor is typical of this organism. Canker also has a rank odor, but it is more of a musty odor that we commonly find when fungi are involved.
Canker appears in many forms and has been thought to involve the skin of the lower leg as well as the foot. I will focus on the disease as it affects the foot. Canker may involve one foot or all four. The very mildest form appears as an inflamed coronary band with a villainous growth of tissue that appears as soft, white projections with a fungal odor. More extensive cases can have a non-sensitive, necrotic appearance that can destroy the sensitive frog, digital cushion and sole corium in various degrees. It can also appear as a yellow, cheesy layer of tissue that involves the sensitive frog and digital cushion.
Both problems get very little respect at the onset as the horse is not lame. Thrush can be successfully treated simply by cleaning up the foot with soapy, hot water and packing dry gauze in the deep confines to prevent air from getting to the deeper areas. I prefer to use a 50/50 solution of iodine and glycerin to treat thrush, as it doesn't burn the tissue and quickly kills surface organisms. Canker on the other hand, does not respond to this treatment, and as stated goes rampant when sealed behind caustic burns.
When canker appears in the heel area or reaches the surface through an existing quarter crack or angle of the heel, some may mistake it for an abscess and treat it as such. However the horse seldom shows any sign of lameness and a large majority of cases continue in training relative to the size and extent of the lesion. This is non-typical of any and all abscesses that most always cause a painful response.
As a rule, it is helpful to know what organism we are dealing with, but with canker that remains a mystery. We can culture corneybacteria, which is most likely a contaminant. Spirochaetas are also routinely found within the necrotic tissue. Other researchers say it closely resembles neoplasm. The bottom line is that we have no clue how this problem starts or why it is manifested in so many different forms.
In the early stages the hair along the bulbs of the heel and coronary band will stand out instead of lie down over the coronary crest. Looking closely you will see small, white hair-like projections that can easily be removed with an abrasive cloth or tool. There is a distinct odor typical of fungi once this tissue is debrided. The hair-like projections can be removed every day and still reappear the next.
The coronary band can also appear quite hyperemic and can easily bleed with abrasive massage. When this stage appears it behooves us to closely examine the frog and adjunct horn tissue for any involvement. Often the organism will be embedded deep within the sensitive frog and digital cushion and can only be detected in a very small area on the surface.
Clean hooves daily with hot, soapy water.
Physically remove the surface hair-like projections with abrasive action.
Cover the diseased area with a thin coating of tetracycline sulfamethazine paste. Tetracycline is apparently effective in eliminating the spirochaeta, and SMZ is a broad spectrum antibiotic for commonly found bacteria. This product was developed by my colleague, Dr. Carrie Long, and I now use it on all my cases.
A small, thumb-sized area of tissue with a yellow, cheese-like appearance that does not appear to be part of the frog or digital cushion is indicative of a more moderate invasion of canker. This area needs to be surgically removed to include a couple of millimeters of surrounding healthy tissue. I have had little luck being conservative when the digital cushion is involved. On occasion, even the smallest surface area will only be the tip of the iceberg. The entire frog and a large majority of the digital cushion and sole corium and even laminae in the heel and quarter area may also be involved.
Apply a hospital plate shoe prior to surgical debridement. The plate serves as a bandage and offers a means to apply pressure to the surgical site, which is a prerequisite for healing. Pressure also suppresses granulation tissue. I've found this to be one of the most important aspects of treatment.
Using a tourniquet and local anesthetic, surgically remove all diseased tissue plus a thin layer of healthy tissue. Pack the area firmly with gauze soaked in 2% Betadine. Change it daily.
When the non-sensitive frog and surrounding horn is undermined, the organism has most likely penetrated deep within the foot. It is not uncommon to expose the DDF simply by debriding with dry gauze. I often wonder how the horse can remain non-painful as such a devastating organism destroys the foot.
Trim the foot, avoiding all necrotic tissue until the shoe is applied.
Apply a hospital plate that reaches up around the bulbs of the heel. This is important as most advanced cases also involve the bulbs, necessitating firm pressure post surgery.
Sedate the horse and block the foot with an abaxial block.Apply a tourniquet.
Using a pair of Allison forceps snapped onto the tissue that needs to be removed, start in one area and attempt to dissect the diseased tissue from the underlying healthy corium and cushion, taking approximately 2mm of healthy tissue. Removing it in one piece greatly facilitates our ability to get all of the diseased tissue. Know before you start that this is not a little debridement. You may be removing a large majority of the structures of the entire posterior half of the foot. Be very careful not to involve the navicular bursa, tendon sheath or coffin joint.
Pack the area with 2% Betadine soaked gauze and fill the remainder of the area with Advance Cushion Support before applying the hospital plate. Note there will be considerable hemorrhage once the tourniquet is removed. I prefer to over-pack the foot for the first few hours post op, then remove a very small area of gauze or trim the plate side of the ACS. We need the excess packing only until hemorrhage has ceased. Too much pressure for 24 hours traumatizes the sensitive tissue, but too little allows granulation tissue to grow rampant. Experience will help you get it just right.
Remove the plate, rubber and gauze on day two. I prefer peroxide as a means to soften the clot adhering the gauze to the sensitive area.
Make a paste using LA 200 tetracycline and SMZ tablets. Put a thin layer on a gauze pad and cover the area. Fill the remainder of the foot with ACS.
Cut the bottom 1/3 of the ACS off with every change and add a new layer of ACS that will fit the conformation of the surface as it heals. Then make a new filler pad when indicated. This continually suppresses granulation. Keeping direct but firm pressure assures optimum results.
The most severe case requires a minimum of 60-90 days to heal. Most cases go on to be sound athletes. Occasionally the growth centers of the horn are drastically distorted, resulting in a variety of horn growth abnormalities.
This sounds like a radical approach to a problem that doesn't make the horse lame. Keep in mind the majority of my cases have come to me as a last resort following months of more conservative treatment.