There are two disease processes commonly associated with a horse's frog. The most common of the two is thrush, which is a bacterial infection that occurs on the hoof of a horse, specifically in the region of the frog. The bacteria involved occur naturally in the animal's environment — especially in wet, muddy, or unsanitary conditions, such as an unclean stall.
Canker is much less common, and it's much more challenging to treat. This disease starts out looking very similar to thrush, but it can be very difficult and frustrating for everyone involved to deal with if not treated appropriately.
What Is Canker in Horses?
Canker in horses is an infectious process that causes a chronic hypertrophy (i.e., enlargement or increase) of the horn-producing tissues of the equine hoof. The disease generally originates in the frog, but if left untreated, it can spread to the adjacent sole, bars, and hoof wall. It is seen commonly in draft breeds, but it can occur in any breed of horse. One or multiple feet can be affected at the same time, but it tends to affect the hindlimbs more frequently.
Canker was initially thought to be stimulated by unhygienic and/or wet conditions, much like thrush, but it has been seen in horses that are well cared for and have regular hoof care provided. Etiology is not well known, and anaerobic bacteria were originally thought to be a stimulus. A recent study found the presence of bovine papillomavirus in canker lesions on horses. The pathogenesis seems to involve a number of factors.
What Are the Signs of Canker?
Canker can often be mistaken for thrush in its early stages. Whereas thrush is a disease that causes loss of tissue and is limited to the lateral and medial sulci and/or the base and sulcus of the frog, canker causes a proliferation of tissue and can spread to the heel, bars, and hoof wall. In the early stages, canker will often appear as a focal area of granulation tissue on the frog that bleeds easily when abraded, surrounded by light brown or gray tissue.
If left untreated, the disease will spread to the frog, bars, sole, and hoof wall. The infection appears as small, fingerlike, off-white projections that resemble cauliflower. This is caused by abnormal keratin production (dyskeratosis) stimulated by the infection. It may or may not be accompanied by a foul-smelling odor and can be covered with a white discharge that resembles cottage cheese. The infection will generally undermine the frog and spread under the horny portion. The affected foot is painful and the horse will appear lame.
How Is Canker Diagnosed?
Generally, canker can be diagnosed based on the appearance of the tissue and foul odor, but the definitive diagnosis is achieved via biopsy. Canker is histologically characterized by proliferative papillary hyperplasia of the epidermis with dyskeratosis, keratolysis, and ballooning degeneration of the outer layers of epidermis. Several anaerobic bacteria are usually observed in the layers of epidermis of the frog.
How Is Canker Treated?
Treatment can be challenging, but successful therapy is based on the following principles:
1. Debridement of all abnormal tissue down to the normal corium is critical. This can be performed under general anesthesia or standing while using regional anesthesia of the limb. Electrocautery or excision followed by cryotherapy allows for complete destruction of the diseased tissue. Excessive corium should not be removed as it will slow the cornification process of the hoof and may cause a decrease in quality and depth of the new sole growing in.
2. The treated area must remain clean and dry, which can be achieved using dry bandaging.
3. Topical treatments that have been proved most successful in treating canker are daily cleanings followed by application of 10% benzoyl peroxide in acetone c, then packing the defect with crushed metronidazole tablets. Cleaning the area daily removes surface bacteria, the benzoyl peroxide and acetone keeps the tissue dry, and the metronidazole effectively kills the anerobic bacteria in the lesion.
Prognosis is always guarded with canker, but owner compliance to perform the necessary daily foot care is essential to the successful treatment of this disease process. Horses have variable responses to treatment, with some cases healing within a week to 10 days and others lasting for months. Once the tissue is healed, the disease rarely recurs. But if treatment is halted before healing is complete, canker often returns — much to the frustration of the veterinarian and owner.
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