An Overview Of Equine Canker

Stephen E. O'Grady DVM, MRCVS

Equine canker is a disease in search of a definition since the cause has not been determined. It could be described as a pathological response to an insult to the foot’s horn-producing tissues. Equine canker has been defined as an infectious process that results in the development of a chronic,hypertrophic, moist pododermatitis of the horn producing tissues, generally in the palmar/ plantar sections of the foot. Usually originating in the frog, canker may remain focal or invade the adjacent sole, bars and hoof wall. Canker is generally confined to and proliferates from the epidermal tissue and rarely invades the underlying dermal tissue.Although the etiology of canker remains elusive, it can consistently and effectively be treated and resolved.^1,2,3,4

Etiology Of Canker
The etiology of canker remains evasive, but generally is seen in a frog that is unhealthy and enters through a break in the horn tissue. There is another disease entity termed “coronary band dystrophy,” which appears to have an immune-mediated component that is associated with or can lead to canker (O’Grady SE. “Coronary Band Dystrophy,” 2017, in review). Wet environmental factors may play a role as there is a seasonal incidence of canker during the rainy season in Florida from July until December.¹

Previously, it was felt that unhygienic conditions were causative, but canker is seen in horses that are well cared for and receive regular hoof care. At onetime, canker was commonly seen in draft breeds, but now is diagnosed in all breeds,especially Thoroughbred and warm blood horses. It can occur in one or multiple feet, affect one or multiple horses on a given farm and has no predilection to the age or sex of the horse.

Figure 1: A foot with chronic thrush. Note the unhealthy nature of the frog and the loss of structural mass.

Figure 2: Early canker in the center of the frog. Note the light brown epithelial tissue that is trying to surround the ulcerated lesion.

Clinical Signs Of Canker
Canker generally originates in the frog and can be mistaken for thrush in the early stages. Thrush is limited to the frog and results in a very recognizable deterioration or loss of frog tissue where on the other hand there is a proliferation of tissue with canker (Figure 1).

Canker may present anywhere from a focal area of granulation tissue in the frog that bleeds easily when abraded to long filamentous fronds of hypertrophic horn.Canker is characterized by numerous small finger-like papillae consisting of a soft,off-white material that resembles a “cauliflower-like’ appearance and is covered with a caseous white exudate that resembles cottage cheese (Figures 2 and 3a-3c).

Figures 3A, 3B & 3C: Figure 3A (left) shows filamentous fronds of hypertrophic horn. Figure 3B (center) shows the finger like papillae that presents a “cauliflower” appearance. Figure 3C (right) shows a white caseous exudate that resembles “cottage cheese” beneath the outer hard layer of the frog. The condition is frequently, but not always, accompanied by a foul odor.

The condition is frequently, but not always, accompanied by a foul odor.However, if any type of antiseptic or caustic medication has been used for treatment, the smell will disappear. The affected tissue will bleed easily when abraded and maybe extremely painful to the horse when touched or pressure is applied. Varying degrees of lameness will be present depending on the extent and depth of the infection. Most horses are not lame if the disease is recognized and treated early.

Figure 4: Photomicrograph on the left shows normal epidermis/dermis of frog. Photomicrograph on the right shows the tissue affected with canker.Red arrow is papillary hyperplasia, green arrow is keratolysis, yellow arrow is degeneration of the outer layer of the epidermis and blue arrow is a mixed population of bacterial organisms.

Diagnosis Of Canker
A presumptive diagnosis of canker is based on ruling out the presence of thrush,the gross appearance of the affected horny tissue covering the frog, a fetid odor and the ease with which the abraded tissue bleeds. However, a definitive diagnosis may be confirmed with a biopsy. Biopsy is most useful in recurrent cases or when the lesions do not have the characteristic appearance or location in the foot.

Histologically, the lesion is read as a“chronic hypertrophic moist pododermatitis.” It is characterized by a proliferative papillary hyperplasia of the epidermis with dyskeratosis, keratolysis and ballooning degeneration of the outer layers of the epidermis. A mixed population of bacterial organisms are observed in the stratum germinativum layer of the epidermis of the frog (Figure 4). Cultures per se are unrewarding as they typically produce an assortment of environmental organisms such as “Bacteroides sp.” and“Fusobacterium necrophorum,” the usual opportunistic bacteria found in the solar surface of the horse’s foot.

Figure 5: No canker present in the frog, but note how the foot should be prepared prior to debridement. Note the slope of the bars for exposure of frog sulci.

Treatment Of Canker
Treatment of canker is a realistic example of a joint venture between a veterinarian and a farrier. From a legal and ethical standpoint, working with dermal tissue is the practice of veterinary medicine. The effective veterinary treatment of canker requires sedation, local anesthesia,surgery (debridement without hemorrhage)and medication. Many veterinarians are not familiar with farrier tools or their use,unable to trim and prepare the foot prior to debridement, are unable to explore the hoof capsule appropriately and are unable to provide the necessary follow-up farriery care. This describes the farrier’s role and input. Treatment should be considered a team approach with the veterinarian, farrier and owner/caregiver

Trimming and preparation of foot. An important aspect that is often overlooked is to trim the foot prior to surgery. Try to address any hoof capsule distortion at the onset in order to give the foot a mechanical advantage. All loose exfoliating sole is removed from the solar surface of the foot. The bars and sole of the hoof capsule adjacent to the frog are explored to be sure the horn in this area is not involved. All underrun or loose horn is removed down to solid structures (Figure 5).

Figure 6: Esmark bandage in place.

The heels are trimmed such that the hoof wall at the heels and the frog are on the same plane or approach the same plane.The frog should not be recessed between the heels of the hoof wall at the beginning of treatment or the affected tissue will not heal. I also trim the bars on more of a slope than usual because I want good exposure to the sulci around the frog. After the foot is prepared with the appropriate farriery,soaking the foot with any of the various cleaning solutions or products is unnecessary. The foot is now placed in a bucket of clean water and the foot is scrubbed thoroughly with an antiseptic soap using a firm brush. I prefer to do the debridement standing under local anesthesia but it can be done under general anesthesia if there are behavior issues with the horse or at the discretion of the clinician.

Debridement. A tourniquet such as an Esmark bandage or a length of tire inner tube cut at a 3-inch width is placed starting at mid-pastern to above the fetlock (Figure 6). It is essential to create a bloodless field so the demarcation between normal and diseased tissue can be seen during debridement. All abnormal tissue is removed until the tissue resembles a pink velvet color with numerous pin-point hemorrhages, which are the dermal papilla (Figure 7).

Remember that canker is generally limited to the epidermis, so debridement needs to be careful, gentle, thorough and wide rather than aggressive, deep and radical, which often leads to unnecessary removal of the dermal tissue under the lesion. This tissue is necessary for re-growth of healthy horn and cornification. For debridement, I use a small pair of rongeurs to remove the bulk of diseased tissue, a sharp loop knife and a #12 scalpel blade (Figure 8).

Figure 7: Frog surgically debrided.Papillae can be seen but any hemorrhage has been wiped off. Note thin strips of healthy tissue left intact(red arrow).

Figure 8: Tools / instruments used to facilitate debridement.