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.