What is white line disease? Why does my horse have it? How can I get rid of it? These are commonly asked questions when a horse is first diagnosed with white line disease. White line disease is not completely understood. It is characterized as a progressive separation of the inner layers of the hoof wall. This separation occurs on the solar surface (bottom) of the foot and can start at the toe, quarter, and/or heel. The separation is usually invaded with opportunistic bacteria or fungi. This disease process often arises secondary to a hoof-wall separation. The disease has been termed seedy toe, hoof-wall disease, yeast infection, Candida, and onychomycosis.
The hoof wall consists of 3 layers:
- stratnum externum (external layer)-smooth and shiny appearance of hoof
- stratnum medium (middle layer)-densest part of the hoof wall
- stratnum internum (inner layer)-non-pigmented
There are bonds present between the various layers in the hoof that form the white line or zona alba at the sole-wall junction. The white line is actually yellow in color and is the junction between the stratnum internum and the horn of the sole. In most cases the separation occurs between the stratnum medium and internum layers.Only in advanced cases will it involve the “white line”.
White line disease can affect a horse of any age, sex, or breed. One foot or multiple feet may be involved, they may be shod or barefoot, multiple horses on a farm can be affected or just one.
Numerous causes have been proposed, although none have been proven. Some possible causes are excessive moisture (softens the foot allowing dirt and debris an easy entry into an existing separation), excessively dry hooves (cracks or separations form in the hoof wall, allowing pathogens to migrate inward), mechanical stress placed on the inner structures of the hoof (leads to a separation in the hoof wall), excessive toe length, under-run heels, sheared heels, clubbed foot, and other hoof conformation issues can also predispose a horse to contracting white line disease.
Clinical signs are not extremely obvious in this disease. Most often, white line disease is noted by the farrier during routine hoof care. In the early stages, the only noticeable change is on the solar surface, the sole-wall junction (white line) is wider, softer, and has a chalky texture. The horse may be tender to sole pressure, have flat soles, and will occasionally have heat in the affected feet. As the separation progresses, a concavity or “dish” may form along one side of the hoof and a bulge may be present on the opposite side above the affected area. A hollow sound is sometimes noted when the outer hoof wall is tapped.
Often times the disease may go undetected until the horse shows discomfort. White line disease may not cause lameness until damage between the bonds in the hoof wall are significant enough to cause displacement of the coffin bone.
Diagnosis is made based on clinical signs and by further exploring the chalky area at the sole-wall junction. Exploration of this area usually reveals a separation filled with white/grey powdery material. Further probing reveals the depth and extent of the cavitation present. Radiographs should also be performed. The extensive hoof wall damage that occurs can mimic laminitis, both clinically and radiographically. Cultures of the separation have been performed; most organisms obtained have been fungal and are believed to be contaminants and not truly the cause of the disease.
White line disease must be treated aggressively. The organisms that invade the hoof wall are anaerobic or microaerophilic; meaning they don’t need oxygen to grow, or can grow with very little oxygen. The way to kill these organisms is to fully expose them to air.
Completely opening the outer hoof wall of the affected area is necessary, and this procedure (called resection) should only be done by an experienced veterinarian or farrier. The diseased areas need to be exposed to the air all the way up to the point where there is a solid attachment between the outer hoof wall and the external laminae.
After resection, topical medications (such as merthiolate, gentian violet, or 2% iodine) are good disinfectants, and the dyes provide an added benefit by showing where new or developing tracts of the disease are forming. Medications should not be applied more than 2 times per week–more frequent treatment can cause the exposed laminae to become too hard.
The affected hooves should be closely re-examined at 2 week intervals. In severe cases, a special shoe may be needed to provide extra support during the treatment/healing process. The extent of damage to the hoof wall will be the deciding factor as to whether the horse is able to continue work during this time.
The horse’s feet should be kept as dry as possible during the period of time it takes for the hooves to grow out. Clean and dry bedding is essential, as is maintaining a strict 4 week shoeing schedule.
Keeping the farrier and veterinarian involved and actively monitoring the healing process is extremely important.
Since the exact cause of white line disease is unknown, prevention of the disease is difficult. In fact, horses can have a reoccurance of white line disease after being disease-free for 1 to 2 years, even though they have strong hoof walls and show no outward signs of hoof wall separation.
We do know that proper trimming and shoeing are essential for producing a strong hoof wall, which helps prevent separation of the layers of the hoof, which offers protection from disease.
Remember: “No hoof, no horse”!