Canker in Horse Hooves: Signs, Causes & Treatment

Canker in horses, also called equine proliferative pododermatitis, is a condition that causes proliferation of the hoof tissues. The exact cause of canker is currently unknown, but may be related to a bacterial or viral infection.

Canker characteristically has a cauliflower-like or ragged appearance and is most common around the frog of the hoof. The tissue is highly sensitive when touched and usually results in lameness in the affected horse. Treatment requires surgical removal of the affected tissue to prevent further growth.

Many horse owners mistake early canker lesions for thrush, as both can have a distinctive foul odor. However, canker identified in the early stages has a better prognosis and is easier to treat.

Mistakenly treating for thrush may worsen the prognosis by allowing the canker to progress further. Changes in your horse’s hoof health should be examined by a veterinarian prior to treatment.

Causes of Canker in Horses

Canker is an infectious condition that causes rapid proliferation of keratin, the main component of the hoof wall and sole. ^[1] The proliferation forms long fronds of keratin, usually around the frog of the hoof. ^[1]

The exact cause of canker is unknown. ^[2] There is some evidence to suggest that a particular group of bacteria called spirochetes and bovine papillomavirus may be involved in the development of canker. ^[2]

Spirochetes

Spirochetes are helix-shaped bacteria that may have a role in hoof conditions in other species, such as digital dermatitis in cattle and sheep. ^[3]

Spirochetes were first suggested as a possible cause of canker when biopsies from three horses showed helix-shaped bacteria on microscopic evaluation. ^[4]

More recent studies compared the amount of spirochetal DNA in canker tissue and normal equine tissue, to determine whether spirochetes are truly associated with canker. From these studies, researchers showed that the spirochetal species Treponema are commonly associated with canker. ^[3][5]

Many of the Treponema species identified in these studies are closely related to bovine digital dermatitis-associated bacteria, suggesting the diseases may be similar. ^[3][5]

Although spirochetes appear associated with canker, how the horses acquire the bacteria, which species of bacteria are most common, and how the bacteria cause disease are still unclear.

Bovine Papillomavirus

In horses, bovine papillomavirus most commonly causes sarcoids, a benign skin tumour characterized by proliferation of keratin in the skin. Based on the similarities between sarcoids and canker, researchers suspected that bovine papillomavirus may contribute to the development of canker. ^[6]

One study evaluated canker biopsies for the presence of bovine papillomavirus, and found that 100% of the biopsies tested were positive for the virus. ^[6]

From this finding, the researchers suggested that antiviral medications or immune modulators that are currently used for treating sarcoids may be successful in treating canker. ^[6]

However, confirming bovine papillomavirus as the causative agent requires inoculation studies, where researchers introduce the virus experimentally to horses to see if they develop canker. At this time, these studies have not been performed.

Risk Factors

The most common risk factors for canker in horses are: ^[1]

• Horses living on muddy or moist pastures year round
• Horses living in wet, unhygienic conditions such as dirty bedding in stalls or
• Draft breeds
• Horses with autoimmune disorders

Although canker is most common in horses kept in wet environmental conditions, it can also occur in horses in clean, dry environments that receive regular hoof care. ^[7]

Symptoms

The initial appearance of canker is a pale growth near the base of the frog that has well-defined edges. ^[1] The tissue usually bleeds easily when damaged.

Some owners may mistake early canker for thrush. ^[7] However, studies show that canker typically gets worse when thrush treatments are applied, and the delay in treatment can increase the risk of canker recurrence by up to 13 times. ^[8]

Canker is most common in the hind feet, although front feet can also be affected. ^[2]

As canker progresses, symptoms may include: ^[1][7]

• Foul odor to the hoof
• Ragged appearance of the frog with numerous fronds on the surface
• Soft, white, cauliflower-like material on or near the frog
• Cottage cheese-like exudate over the affected tissues
• Pain on palpation of the affected tissues
• Lameness

In severe cases, the canker tissue may spread to other locations on the hoof, including the sole, bars, and hoof wall. ^[7] Some horses may have multiple affected hooves. ^[7]

Diagnosis

A veterinarian can typically diagnose canker based on an examination of the hoof and the appearance of the proliferating tissues near the frog. ^[1]

In some cases, a biopsy of the tissue may be submitted to a diagnostic laboratory to confirm the diagnosis. ^[1] Under the microscope, the diagnostician can see a thin layer of defective epidermis, the tissue producing the keratin layer of the hoof. ^[2]

Treatment

Treatment of canker can be challenging, particularly in severely affected cases. The mainstays of treatment are debridement of the affected tissue, topical treatment, steroid treatment, and wound support.

Debridement

Debridement refers to removal of affected tissue to prevent further growth and development.

Veterinarians start by trimming the rest of the hoof, to remove any excess sole and to identify the extent of the canker. ^[7] In some cases, additional canker tissue can extend into the bars and sole next to the frog, which may be missed if the hoof is not trimmed beforehand. ^[7]

Canker debridement requires local anesthesia using an abaxial sesamoid nerve block, which blocks the horse’s feeling to their entire foot. ^[7] Some horses may require general anesthesia depending on their temperament.

The veterinarian may also apply a tourniquet to the lower leg, to reduce blood flow to the hoof. Since canker tissue often bleeds profusely, this step can prevent excessive blood from blocking the veterinarian’s view of the hoof surface. ^[7]

The veterinarian begins debridement by carefully removing any abnormal tissue using a hoof knife or scalpel, cutting the tissue back until the normal tissue underneath is exposed. ^[7] In most cases, an area of normal-appearing tissue around the canker is also extracted, to ensure complete removal. ^[9]

As the surgical site heals, it should be carefully monitored for any signs of abnormal tissue growth. Some horses may require an additional debridement treatment to completely remove canker. ^[7][9]

Maggot Debridement Therapy

There is a case report of using maggots to debride canker in a Thoroughbred. ^[10] Human medicine successfully uses maggots as part of debridement protocols. ^[10]

Maggots used for debridement therapy ingest dead or damaged tissue, while leaving behind healthy tissue. ^[10] This can promote growth of healthy granulation tissue for faster healing.

In the Thoroughbred case report, maggots were applied under a treatment plate shoe and replaced every 3 days. ^[10] All of the abnormal canker tissue was removed by 1 month after beginning treatment, with complete healing of the hoof by 4 months after treatment. ^[10]

The researchers concluded that maggot debridement therapy may be a helpful addition to traditional surgical debridement for canker cases. ^[10] However, the use of maggots in veterinary medicine is still limited, and it is unlikely that most veterinary practitioners will have maggots available for this treatment.

Topical Treatments

There are several topical treatments that can help resolve canker in horses. Most treatments include an astringent (drying agent) and an antibacterial product. ^[7] The topical treatment is applied to gauze squares and placed over the surgical site before bandaging the hoof. ^[7]

Common antibiotics used for treating canker include: ^[1][7]

• Chloramphenicol
• Metronidazole
• Ketoconazole and rifampin
• Tricide, gentocin and lincomycin
• Oxytetracycline and metronidazole

Some veterinarians may also include systemic antibiotics, given by mouth or injection, as part of the treatment protocol. ^[7] Usually, the antibiotics used for this purpose are chloramphenicol or oxytetracycline. ^[7]

The most common astringent used for cleaning and drying the surgical site is 10% benzoyl peroxide in acetone. ^[9]

Cryotherapy

Cryotherapy (cold therapy) after complete removal of the canker tissue can also help the treatment process. ^[7] A double freeze-thaw cycle using cryotherapy can remove any residual bacteria from the hoof surface, and prevent regrowth of the canker. ^[7]

Most veterinarians use liquid nitrogen or a coolant spray for performing cryotherapy. ^[9] Usually this treatment is applied once, immediately after the first debridement of tissue. ^[7]

Chemotherapy

There are reports of using cisplatin, a chemotherapeutic agent, in the treatment of canker. ^[11]

Bovine papillomaviruses are a possible cause of equine canker, and these viruses produce papillomas, a type of benign tumor. Applying a chemotherapeutic agent like cisplatin may target the virus-infected cells, allowing for direct treatment of the canker. ^[11]

In one clinical trial, application of cisplatin after debridement showed a low rate of recurrence within 12 months of treatment. ^[11] None of the horses experienced side effects in this study. ^[11]

Chemotherapy application requires strict safety measures for personnel handling the horse, as skin or eye contact with the chemotherapeutic agent can be detrimental to human health. ^[11] Horses treated with cisplatin must be hospitalized for the duration of their treatment, increasing costs for owners. ^[11]

Steroid Treatment

Some veterinarians recommend treating horses with oral steroids during their recovery from canker. ^[7][8]

Oral steroids act as an anti-inflammatory and immunosuppressant. There is some evidence that suggests canker may cause a local autoimmune disorder, which can be reduced by steroids, and result in faster healing. ^[8]

Usually oral steroids are given for 21 days, with a decrease in dose every 7 days. ^[7] One study showed a significant decrease in hospitalization time in horses treated with oral steroids compared to untreated horses. ^[8]

Wound Support

The exposed tissue beneath the debrided canker is very sensitive and requires careful management to prevent re-infection.

Many veterinarians create a cover for the affected area using impression material, to protect the surgical site from further contamination. ^[7] Other benefits of an impression material cover include: ^[7]

• Keeping topical medications in place for longer than an uncovered surgical site
• Applying mild pressure to reduce the risk of excessive granulation tissue (proud flesh) developing
• Maintaining the normal physiologic function of the frog in returning blood from the lower limb to the body

This impression material becomes part of the hoof bandage used to keep the hoof dry and clean during the healing process. ^[7] The bandage should be changed daily or every second day to allow reapplication of topical treatments and careful monitoring of the surgical site.

Alternatively, some veterinarians may apply a specialized horseshoe with a treatment plate, a metal plate that covers the entire sole of the hoof. ^[7] However, treatment plates generally do not prevent contamination of the sole surface, and may allow moisture into the surgical site. ^[7]

Treatment plates are most effective when used after initial healing has occurred, to protect the surgical site as the keratin strengthens. ^[7]

Prognosis

It can take several weeks of routine bandaging and topical treatments for the surgical site to heal over. ^[7] Most horses successfully recover if they are treated early in the development of canker. ^[1]

Canker cases that involve invasion of the sole, bars, and hoof wall, or horses that have multiple hooves affected, have a poorer prognosis. ^[1] One study suggested that only around 75% of severely affected horses have an acceptable long-term outcome. ^[8]

Prevention

Although the exact cause of canker is unknown, it seems that horses living in wet or unhygienic conditions are predisposed. Strategies to prevent canker include:

• Regularly cleaning stalls, including providing fresh, dry bedding as frequently as possible
• Providing dry “hard standing” areas in pastures and paddocks to encourage horses to stay out of muddy conditions
• Turning out into an arena or other indoor area as an alternative to muddy or wet pastures and paddocks
• Drying hooves thoroughly when bringing horses indoors after turnout
• Routine farrier care to remove excessive sole growth that may harbor bacteria

Additionally, regularly inspecting horses’ hooves can help identify canker lesions early, leading to more rapid treatment. Cases caught in the early stages of development have a lower recurrence rate and a better prognosis. ^[8]

Summary

• Canker is a proliferation of hoof keratin that can cause lameness in affected horses
• The cause of canker is currently unknown, but may be related to bacterial or viral infections
• Treatment involves surgical removal of all affected tissue and a lengthy recovery process
• Most horses recover fully after appropriate treatment