Q: I'm curious about the hoof condition known as canker. I was told that my older horse had it after I noticed a smelly discharge that started during a spell of wet weather. I haven't been able to find a lot of information about canker, and my farrier and veterinarian have never treated a horse for this. What are your thoughts?
A: Canker is an infection that invades the surface tissues of the hoof and causes overgrowths, beginning in the frog and extending to the sole and hoof wall, and in severe cases even to the skin of the pastern. Signs include an intact but ragged-looking frog, a putrid odor and a thick, white discharge with a cheese-like texture.
Canker causes the horn growth of the frog to be very friable, and the horse is usually very sensitive when you poke around and inspect these areas. The infection, which can occur in one or multiple feet, can be quite damaging and painful enough to cause lameness. (Especially in its earliest stages, canker is sometimes confused with thrush, another type of hoof infection. One distinguishing difference is that thrush erodes, or destroys, tissue, while canker causes mounding overgrowths.)
We don't know for sure what causes canker, but we have some evidence it could be a papillomavirus. Other studies have consistently isolated spirochete-type bacteria from these lesions. It is possible that both a virus and bacteria contribute to the problem. In that case, it could be that the virus triggers the rapid, abnormal growths, which eventually lead to necrosis and secondary bacterial infections.
Canker is rare and is mostly diagnosed in wet, muddy climates of the southern United States. It seems to be most common in drafts, but I have seen it in other types of horses as well. Although some lesions may be small, about the size of a dime, others can be quite widespread and damaging. I've seen severe cases where large regions of the hoof capsule have sloughed off from the separation caused by the disease. Another serious case involved the digital cushion (the tissue mass that lies above the frog within the hoof capsule) and almost progressed to the deep digital flexor tendon. It is important to recognize and treat canker early before the infection extends that far and gets too serious.
It's best to involve both your farrier and veterinarian in the treatment, which consists of debriding (cutting away) all of the abnormal tissue. And just as you would with an invasive tumor, you need to trim off some of the surrounding healthy tissue as well to make sure all of the diseased tissue is gone. The viruses and/or bacteria may be present in these healthy-looking areas and act as a source for reinfection.
As a precautionary measure, after I surgically debride the area I follow up with laser treatment to the entire surgical site. If it's just a small area I perform this procedure with the horse standing and the foot desensitized with a nerve block, but in more severe cases, or those involving multiple feet, I typically do the procedure under general anesthesia. Some horses may require a shoe with a removable treatment plate to protect the area as it heals.
Many topical treatments are described in the literature, but what seems to have worked most consistently for me is painting the area with the antibacterial solution Tricide and then applying a mixture of the antibiotics tetracycline and metronidazole. I usually put the horse on a systemic antibiotic, oxytetracycline, for three to four days after the surgery.
It's wise for your veterinarian to reexamine the foot during the healing process to make sure that any canker-type tissue is not reappearing. If there is, minor debridement or laser treatment may be required. However, using the technique described, I have successfully treated many cases of canker without recurrence. The healing period depends largely on how badly affected the foot had been but ranges from 60 days to many months in severe cases.
Preventive measures include keeping the feet as clean and dry as possible, routine trimming and medicating any areas that look suspicious. It is possible that thrush can predispose a horse to developing canker, since it too is most common in wet environments. But I have also seen a few cases of canker in horses who were kept in a very clean dry barn with good foot care.
Scott E. Morrison, DVM
Rood and Riddle Equine Hospital