Keratosis is a rare equine skin condition in which patches or bands of skin become thickened, rough, dark, and oily. The patches are not painful, itchy, or contagious, and do not impact the overall health of the horse.

There are two forms of keratosis: cannon or linear. Cannon keratosis appears as patches of thickened skin on the front side of the lower part of the horse’s hind legs, along the cannon bone. Cannon keratosis is usually present on both sides of the horse’s body.

Linear keratosis appears as bands of thickened skin on the ribs, neck or back of the horse. Occasionally patches of linear keratosis are noted on the chest, legs, or hips. They typically appear only on one side of the horse’s body.

Treatment for keratosis is not usually necessary, except in cases of secondary infection and non-healing wounds. Some owners may use topical treatments for cosmetic reasons, with variable success. The prognosis is excellent although the disorder usually lasts for the duration of the horse’s life.

Keratosis in Horses

Skin is made up of several layers, the uppermost of which is called the epidermis. [1] The epidermis is composed of cells called keratinocytes that change characteristics as they move from the bottom to the top of the layer. [1] The top-most layer of the epidermis is called the stratum corneum. [1]

The stratum corneum is a two-way barrier. [1] It keeps the water in to prevent dehydration and it keeps infectious microbes (bacteria, fungi, protozoa) out to keep the horse healthy. [2]

It also encourages the growth of beneficial microbes, defends against UV rays, helps maintain body temperature, and is the first line of defense against deep injury to epidermis and other systems in the body, among other functions. [1]

The cells in the epidermis die off and regenerate continuously. [1] In the horse, the renewal time of an epidermal cell is approximately 17 days. [2] As new cells migrate to the top of the epidermis, the dead cells on the outermost layer of the skin fall away naturally.

Keratosis occurs when cells in the stratum corneum multiply too rapidly or in a disordered way, or do not fall away quickly enough to keep the skin smooth and healthy looking. [1]

Keratosis is not contagious and is considered cosmetic as it does not impact the overall health of the horse. [2]

Types of Keratosis

There are two characterizations of keratosis in horses: cannon keratosis and linear keratosis: [1][2][3][4]

  • Cannon keratosis, also known as keratinosis, stud crud, cannon crud, cannon hyperkeratosis, and cannon keratitis, is a skin condition that appears as well defined patches of crusty, thickened skin on the lower legs of the horse along the bone between the knee and the fetlock known as the cannon bone.
  • Linear keratosis typically affects the neck, shoulder, or rib cage of the horse but has been reported less commonly on the legs, hips, or chest. It appears as vertical bands of crusty, thickened skin. It is distinct from cannon keratosis because of its location and shape of the patches.

Risk Factors

Cannon keratosis, although relatively rare, is reported to be the more common form of seborrheic dermatosis in horses. [3] It typically occurs in middle aged or older horses. [4] It does not seem to be related to the breed or sex of the horse. [4] An earlier theory that it was more common in stallions has been discredited. [4]

Linear keratosis is a rare disorder that is usually noted in young horses, particularly those under five years of age. [2] Some horses are born with the disorder, and some develop it shortly after birth. [2]

It affects Quarter horses, Thoroughbreds, and Standardbreds more often than other breeds, although all breeds are susceptible. [4] There does not appear to be a greater risk due to sex. [2]

In general, skin disorders are more common in horses that are in poor condition, especially those that are undernourished. [1]

Horses with inadequate shelter are also more susceptible to skin disorders due to continuous sun or rain exposure. [1]

Horses that are exposed to irritants such as pesticides or detergents are also at greater risk of skin disorders. [1]

Causes

By the time keratinocytes reach the top of epidermis they are mostly made up of the protein keratin. Keratinocytes in this form make up the stratum corneum, which is the robust top layer of the skin. [2]

To remain smooth and invisible, the cells of the stratum corneum must regenerate and slough off in a coordinated manner. [2] If this cadence is disrupted, the stratum corneum becomes thickened and visible. [2] This is known as a disorder of keratinization or cornification. [2][3]

Keratosis is a cornification disorder where the skin cells do not function properly, resulting in the thickening of the skin. [1][2] The dead skin cells that remain on the surface of the horse’s skin appear scaly. [3]

Dysfunction in the stratum corneum is often accompanied by disruption to other natural processes occurring in and on the skin including secretion of fats, oils, and waxes, which results in an oily or greasy appearance to the skin. [2]

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Pathophysiology

The process of skin development is complex and not entirely understood, and there are a variety of abnormalities lead to thickening of the stratum corneum. Given this, it is difficult to determine the specific processes underpinning the development of keratosis. [3]

Cornification disorders appear to have a genetic component. Other possible contributing factors warranting further investigation include: [3]

  • Drug interactions
  • Skin damage
  • Hydration issues
  • Hormone imbalances
  • Preexisting internal illness or infection
  • Metabolic issues
  • Autoimmune disorders
  • Tumor growth

Despite these proposed contributing factors, the precise mechanism of both forms of keratosis is not well understood. [2]

Since cannon keratosis is more prevalent on the hind legs, it was historically assumed to be the result of urine scalding, particularly in stallions. [3] This is why it was given the nickname stud crud. [2] Urine scalding has since been definitively ruled out as a cause of cannon keratosis. [3]

Linear keratosis is also not yet well understood. [2] It is thought to have a genetic component because it is noted in certain breeds including: [2]

A genetic link is also suggested since some horses are born with the condition. [2]

Symptoms

Cannon keratosis appears as patches of rough, thickened skin on the cannon bones of the horse. [3] The patches tend to be greasy with sparse or patchy hair. [3] The lesions are typically darker and bumpier than the skin around them. [3]

Cannon keratosis lesions typically appear on both sides of the body in approximately the same location on both sides, which is typically at the front of the lower hind leg. [2]

Linear keratosis appears as bands of rough, thickened skin on the neck, shoulder, or rib cage of the horse. [2] Occasionally, the bands appear on the legs, hips or chest. [2] Bands of linear keratosis tend to be greasy with sparse or patchy hair. [2] The lesions are typically darker and bumpier than the skin around them. [3]

In some cases, the skin puckers slightly at the edges of the bands, resulting in an appearance that can be mistaken for a scar, scratch, or whip mark. [2] The marks are usually vertical and typically only occur on one side of the body. [2] The bands are very slender (0.25-3.5 cm / 0.1 – 1.4 in wide) and of varying length, with reports of bands as short as 5 cm (2 in) and some as long as 70 cm (28 in). [2]

Both forms of keratosis are neither painful nor itchy to the horse. [3]

Linear keratosis lesions tend to appear gradually, beginning as multiple small raised bumps on the skin that that continue to emerge until they have joined into a band. [2]

In both types of keratosis, the disorder has no symptoms other than the lesions themselves and typically does not affect the overall health of the horse. [2][4] Cannon and linear keratosis are considered cosmetic defects that only affect the horse’s appearance. [2][4]

Severity

In rare cases, cannon keratosis lesions become inflamed, resulting in broken skin. [2] In these cases, the risk of infection increases. [2]

Secondary skin infections may have the following symptoms: [3]

  • Ulcerated skin
  • Non-healing wounds
  • Oozing sores
  • Redness around the wound
  • Fever

In severe cases, cannon keratosis lesions appear on the front limbs as well as the hind ones. [3]

Linear keratosis is often accompanied by linear alopecia (hair loss) which is sometimes related to immune disorders where inflammation damages the hair follicle. [4] It is uncertain whether these cases are variants of the same disease or separate, concurrent disorders. [4]

Diagnosis

Diagnosis of both types of keratosis is based on their distinct appearance upon physical examination. [4]

Other diagnostic strategies are aimed at ruling out the presence of microorganisms and other possible causes for skin damage. [1]

Diagnostic tests include: [1][3][4]

  • Skin scrapings
  • Adhesive tape skin samples
  • Skin swabs
  • Biopsy
  • Blood work
  • Urinalysis

Differential Diagnosis

Other skin conditions to rule out include: [2][5]

Treatment

Treatment is typically deemed unnecessary as there is little to no risk to the overall health of the horse and treatment will not permanently resolve the disorder. [2][4]

If treatment is pursued and is initially successful, the patches or bands on the skin tend to return once treatment stops. [3]

Since there is no cure for the condition, treatment options tend to be aimed at reducing symptoms by eliminating scales and flakes, reducing oil build up, and returning the skin to a smooth, healthy appearance. [2][3] [4] Rarely, further intervention is required to mitigate infection or inflammation. [4]

In these cases, treatments include: [3][4]

  • Manual debridement (the removal of dead or damaged tissue from the lesion)
  • Shampoos, creams, lotions, or ointments to remove built up skin flakes and oil
  • Anti-inflammatory creams or ointments

In cases where secondary infections are present, antibiotics and wound care may be required. These are generally effective as long as there is no other pre-existing condition preventing normal immune system function. [3]

Wound management may include: [3]

  • Daily bandage changes
  • Topical medication
  • Oral antibiotics

Severe cases of infected skin wounds may require support from a veterinarian or a veterinary technician to manage.

Prognosis

The prognosis for keratosis is excellent as this disorder is cosmetic and does not affect the health of the horse. [3] However, even with treatment, cannon and linear keratosis are lifelong conditions. [3][4]

Since the underlying disease process of keratosis is not fully understood, there are no effective preventive measures available. Due to possible genetic causes, removing affected horses from the breeding stock is recommended. [1]

Horses with chronic skin and coat issues may benefit from omega-3 fatty acid and vitamin E supplementation. Consider adding Mad Barn’s W-3 oil to your herd’s routine, especially if you are dealing with chronic keratosis or other skin issues.

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Summary

Keratosis is the general term for two skin disorders, Cannon Keratosis and Linear Keratosis, in which patches or bands of thickened, bumpy, dark, greasy skin appear either on the horse’s hind cannon bones, or its back, neck or ribs. The lesions are not itchy nor painful and they do not affect the horse’s overall health.

  • The causes of keratosis are not fully understood at this time, but they are the result of a cornification disorder.
  • Symptoms are limited to cosmetic changes to the horse’s skin.
  • Diagnosis is based on their characteristic appearance.
  • Treatment, when deemed necessary, aims to improve the skin’s appearance as no treatment is curative and the disorder is not dangerous to the horse.
  • Because of the possible genetic component of the cause of keratosis, it is recommended that the horse be withdrawn from the breeding stock.

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References

  1. Lorch, G. The “Scaling Flaking” Horse: Addressing Unwanted Desquamation. 2011.
  2. Scott, D. W. and Miller, W. H. Chapter 11 – Keratinization Defects. Equine Dermatology. W.B. Saunders, Saint Louis. 2003.
  3. Scott, D. W. and Miller, W. H. Chapter 4 – Bacterial Skin Diseases. Equine Dermatology. W.B. Saunders, Saint Louis. 2003.
  4. Hilton, H. et al. Evaluation of Four Topical Preparations for the Treatment of Cannon Hyperkeratosis in a Horse. Veterinary Dermatology. 2008. View Summary
  5. Knottenbelt, D. C. The Approach to the Equine Dermatology Case in Practice. Veterinary Clinics of North America: Equine Practice. 2012. View Summary