Veterinarians at Rood and Riddle Equine Hospital answer your questions about sales and healthcare of Thoroughbred auction yearlings, weanlings, 2-year-olds and breeding stock.
Question: What is white line disease (WLD) and how can it be managed?
Dr. Raul Bras: Horses' hooves are subjected to a variety of influences that can impact soundness, including breed, limb conformation, and environment, which can change the shape of a foot. The function of the hoof can be affected by the environment, discipline, exercise, and farriery. The hoof has the ability to respond relative to its structural characteristics, its natural tolerance of the mechanical challenges, or by adaptation with changes in growth rate and shape. External alterations of the hoof capsule can be evident to the astute eye. However, many crucial changes go unnoticed until lameness or other problems are evident.
Many foot-related lameness involves hoof capsule distortions. Hoof capsule distortions occur when the tensile, compressive, or shearing forces on the hoof exceed the capacity of the hoof capsule components to withstand them. There are three basic situations in which the loading capacity of a structure can be exceeded: normal load on an abnormal structure, abnormal load on a normal structure, and abnormal load on an abnormal structure. The latter is a more reliable recipe for distortion and perhaps outright destruction of the compromised component.
WLD is a syndrome associated with structural defects of specific parts of the hoof and is characterized by progressive hoof wall separation that occurs within the non-pigmented Stratum Medium layer of the hoof wall. WLD describes a set of symptoms, rather than one specific cause of those symptoms.
The name of white line disease should not be confused with the anatomical white line that is visible around the edges of a horse's sole. Rather, it deals with the non-pigmented (or “white”) layer of the hoof wall. The hoof capsule's horn is made of three layers: the stratum externum is the stronger, denser layer of tubules that are pigmented in black feet. The stratum medium is nonpigmented. has a less dense configuration of horn tubules. stratum internum (inner layer) is the laminae and its blood supply that anchors the coffin bone to the wall.
WLD is often confused with laminitis, as the clinical and radiographic evidence are similar with a few key differences. In WLD, the separation seen in the foot will originate from the solar surface, whereas in laminitis, the separation starts in the dermal lamellae and continues distally. The air density line of WLD invades the white softer part of the wall via small breaks in the ground surface, and goes unnoticed for long period of time as the horse shows no evidence of discomfort until the capsular rotation reduces sole depth to a painful level. Capsule rotation occurs within the horn wall with WLD, and within the laminae with laminitis.
WLD has an air density lesion that extends from the ground surface and can have irregular borders, often with considerable debris filling the defect. Laminitis has capsule rotation that occurs within the laminae, and the gas line does not extend to the ground surface even when the coffin bone has penetrated the sole. Only after weeks to months from onset does the defect communicate with the ground surface.
The venogram can provide key information to veterinarians and farriers when diagnosing either laminitis or WLD, as it allows them to see areas that are deprived of blood flow.
WLD and laminitis can occur simultaneously when either is very chronic in nature.
The syndrome was first described by Drs. Ric Redden and Al Gabel in the 1970s after they observed that this defect involved the non-pigmented part of the horn. Biomechanical tests have shown that the non-pigmented portion of the stratum medium is the least rigid area of the hoof wall and has the highest water content, which could play a role in the expression of white line disease.
Redden observed that when WLD happens in one foot only, it tends to be in the hoof with a steeper profile or a club foot shape. Additionally, other researchers found a connection between the symptoms of WLD and a fungal infection called onychomycosis. Since then, we have learned the fungus is probably common in equine hooves but isn't present in all cases of WLD. Additional research has searched for an association between symptoms and different types of bacteria, the theory being the symptoms may be a sign of a bacteria that destroys hoof horn. Of course, like fungi, bacteria are also very common in a horse's environment and the bacteria that have been studied seem to be present in some cases of WLD, but not all of them. It seems more likely that bacteria are secondary opportunists which add more damage to a weakened hoof wall but aren't the main cause of damage
As with many other types of foot-related lameness, prevention is much more effective than treatment. Prevention begins with careful observation. It is important that detrimental changes in the foot be recognized early and dealt with appropriately while there is still a chance to preserve the integrity of the foot. Failure to understand the normal structure and function of the equine foot and to manage the foot and the horse accordingly can have deleterious effects.
Treatment for WLD varies from sheer mechanics to wall removal and debridement of the affected areas. Mechanical benefits that can produce medical benefits appear to be a major key to therapeutic and pathological shoeing. A well-made strategic plan based on the basic mechanical requirements of the foot can greatly improve the outcome and success of many podiatry cases. Understanding the basics principles of podiatry allows the veterinarian and farrier to aid in the prevention of foot related lameness, maintain a sounder horse, and implement therapeutic shoeing when necessary.
Dr. Raul Bras is a certified journeyman farrier and veterinarian in the podiatry department at Rood and Riddle Equine Hospital. A member of the International Equine Veterinarian Hall of Fame, Bras graduated from Ross University and completed the farrier program at Cornell University. He is a partner at Rood and Riddle.
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