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. Opportunistic Invasion
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. Clinical Signs
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. Diagnosis
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.”