Look up “white line disease” in your equine veterinary book, and you might not find it. This name for the condition was first coined in 1990, and the disorder is also known as seedy toe, hoof or stall rot, hollow foot, yeast infection, Candida, wall thrush, and (incorrectly) onychomycosis. But when white line disease came to be recognized as a unique condition what it’s called is of no consequence if a serious case of the disease puts your horse out of commission. In its worst stages, white line disease can leave a horse with little hoof wall and can cause permanent loss of athletic ability.
Simply stated, white line disease refers to hoof wall separation within the hoof’s non-pigmented layer. Explains Tracy A. Turner, DVM, Dipl. ACVS, professor of Large Animal Surgery at the University of Minnesota, “White line disease is characterized by progressive hoof wall separation that occurs in the non-pigmented horn at the junction between the stratum medium (middle layer of the hoof capsule) and laminar horn. The separation is usually progressive and typically involves most of the toe and quarters. The name is a misnomer, as the white line is not actually involved, but rather the deepest layer of the non-pigmented stratum medium.”
The cause of white line disease is unknown, but it’s thought to be a multifactorial condition linked to an invasion into the area by infectious organisms such as bacteria and/or fungi. Stephen O’Grady, DVM, MRCVS, of Northern Virginia Equine in The Plains, Va., says these organisms might be secondary opportunists that invade the area after some sort of hoof wall trauma.
“One researcher hypothesizes that bleeding from laminar tearing—which can be caused by bruising, a long toe, etc.—may set up a very, very good medium for bacteria to grow in,” he says. “In other words, white line disease may occur from the inside and work its way out. This theory makes sense to me, as opportunistic bacteria and fungi live within the foot.”
Other causes that have been linked to the disease include environmental conditions, mechanics, and nutrition.
“Most commonly, white line disease is seen in times of moisture changes,” Turner states. “Moisture—or lack of it—causes weakening of the weakest part of the hoof wall. As the hoof is weakened, mechanical factors such as long toes, clubby feet, or any condition that increases the stresses on the toe or quarters can cause the wall to break apart or separate. The subsequent crevice that forms is perfect for bacteria or fungi that normally inhabit the ground to invade this area. The microorganisms further digest the keratin in the wall, which exacerbates the weakness. Poor nutrition or diets that do not have enough biotin or sulfur-containing amino acids may result in a weaker wall that is further predisposed to this condition.”
O’Grady says the disease is very common, almost to epidemic proportions.
Indications of white line disease are numerous and varied. In mild cases, the horse might remain sound and show no discomfort; in severe forms, the horse will be lame with the hoof wall displaying massive undermining and separation. Says O’Grady, “White line disease offers no threat to the soundness of an animal until damage is sufficient to allow mechanical loss of the attachment between the laminae and the inner hoof wall, resulting in displacement of the coffin bone in a distal direction (rotation). Only then does the horse begin to show discomfort. In many mild cases, the horse isn’t off at all and his owner is unaware the animal has the disease.”
Early clinical signs include:
Small white or grey crumbly or powdery area along the hoof wall-sole junction;
Tender sole via hoof testing;
Heat in the foot;
Increasingly flat foot;
Formation of concavity along one side of the hoof and a bulge on the opposite side directly above the affected area at the coronary band.
As the disease worsens, hoof wall growth might slow and the hoof wall becomes weak, chipped, or shelly. A hollow sound might be heard when the outer hoof wall is tapped with a hammer, and lameness can develop. Wall separation between the hoof wall and sole at the white line becomes apparent.
The disease can occur in one or more feet and affect the toe, quarter, heel, or any combination thereof.
Often, white line disease is first discovered during routine hoof trimming. “The farrier or veterinarian can take a knife, explore around the white line or sole wall junction, and it will look absolutely normal,” O’Grady says. “He takes a few more passes with the knife, and all of a sudden uncovers an unexpectedly separated area.”
Closer examination of the solar surface of the hoof should confirm diagnosis. “Exploring the inner hoof wall that lies dorsal to the white line will generally reveal a separation filled with a white/grey powdery horn material,” he says.
“Further exploration with a probe will give the depth and extent of the cavitation (formation of cavities). There might be a black serous drainage from the separation.”
Lameness might not be present. “If lameness exists, a thorough lameness examination should be performed, including diagnostic nerve blocks and a complete radiographic examination of the hoof,” Turner states. “Radiographs may allow visualization of the wall separation. This will be seen as an air density between the hoof wall and third phalanx. Pedal osteitis or third phalanx rotation may also be noted on some occasions.”
Treatment consists of several steps geared toward providing support while the affected hoof area grows out. The entire undermined outer hoof wall is removed to expose the diseased area, and the affected areas of the horn are debrided until good horn attachment can be identified.
“Thorough exploration and debridement of any remaining tracts should take place at 10-day intervals,” O’Grady says. “Some form of dye marker can be placed on the resected area to highlight the remaining tracts or fissures. When all tracts are removed, a thorough examination is indicated at re-shoeing intervals every four to five weeks. A wire brush is used daily to keep the resected area clean.”
The most important treatment is to correct or eliminate the primary cause that led to the disease. “If you have a long toe or a club foot, treat the condition,” O’Grady instructs. “If there’s an old abscess, clean up the area. If there is chronic laminitis, give the horse the best foot conformation you can by shoeing.”
Provide therapeutic shoeing to support the frog and weakened hoof wall. “If the defect is small, the hoof can be shod accordingly,” says O’Grady. “If the toe is involved, fit the shoe so breakover is moved back under the toe toward the apex of the frog. This will remove pressure on the resected area of the toe. If the resection is extensive and/or if rotation of the third phalanx is present, a heart bar or egg bar-heart bar combination shoe can be used to provide support to the heel area.
“An alternative method would be to use a bar shoe or open shoe combined with some type of impression material, with or without a pad,” he continues. “The impression material can be applied to the entire solar surface of the foot as long as it is molded thicker at the heels to provide the necessary support.”
Turner notes that glue-on shoes of other alternatives should be used if nailing a shoe is too painful for the horse. “For major loss of hoof wall,” he says, “large clips can be welded to the shoe and short screws used to attach the clip to the remaining hoof wall. Alternatively, fiberglass casting tape can be used to create an artificial wall so that a shoe may be applied.”
Experts disagree on the effectiveness of topical medications. O’Grady says there was no difference in a group of 10 horses which he debrided and treated with topical medication versus a group of 10 which were just debrided. “The hooves in all of them grew out just fine,” he says.
Turner observes that some experts report good results with topical medications such as povidone iodine solutions (undiluted or mixed with dimethyl sulfoxide), copper sulfate, Venice turpentine, and merthiolate. He notes, though, that phenol, formaldehyde, and strong tincture of iodine can damage sensitive tissues and lead to permanent hoof damage.
Both Turner and O’Grady warn against covering the resected area with acrylic. “Microorganisms can become trapped under the repair and continue to destroy the hoof wall,” O’Grady states. “There is no medication in the acrylic that is effective for fungi or many of the causative bacteria.”
However, the need to get into the show arena might cause an owner to ignore that advice, in which case Turner recommends covering the hoof wall with an antibiotic hoof acrylic. “I caution the horse owner that covering it could make it worse,” he says. “To date, we have not seen this happen while utilizing the medicated Equilox under these conditions. The medication is an antimicrobial that is most effective against anaerobes (bacteria that proliferate in a non-oxygenated environment); I feel that it provides an environment that prohibits these microorganisms from proliferating.”
As the hoof grows out, the affected area will continue to need debridement and treatment. “Maintain a shoeing schedule of four-week intervals,” O’Grady recommends.
During this recovery period, the feet should be kept as dry as possible. Avoid turn-out in rain-soaked and dewy pastures, and keep bedding clean and dry. “A well-balanced diet with the addition of biotin and methionine can also be helpful,” O’Grady adds.
Exercise during recovery is permitted, depending upon the severity of the damage.
Prognosis and Prevention
In most cases, prognosis for recovery to full soundness is excellent, although it may take up to 10 months for the undermined area to grow out, depending on the amount of hoof wall removed. The only reason that a horse might have a poor prognosis is if rotation is involved. Also, O’Grady notes that white line disease can return in some horses, even those with strong hoof walls and no signs of separation. Therefore, horses which have had white line disease need careful monitoring for future episodes.
Try to minimize risks by avoiding large moisture changes to the feet and turn-out in wet pastures. Keep the toe short to avoid stressing the hoof wall-sole junction. Maintain a proper trimming and shoeing schedule in order to preserve hoof integrity and to catch small problems before they become big problems.
ABOUT THE AUTHOR
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She’s schooled in hunt seat, dressage, and Western pleasure.
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