There are many instances where veterinarian-farrier collaboration provides a great service to a horse and owner. A farrier might contact a veterinarian about a horse on which he or she has worked that is showing some separation of the white line or a localized void of sole, causing a concern that the horse has developed white line disease.
Although the term “white line disease” is used loosely, it isn’t an accurate representation of the pathology that is occurring within the foot—the affected area lies just in front of the epidermal lamellae, not at the white line itself. White line disease (WLD) occurs secondary to an area of separation of the hoof wall at the inner part of the stratum medium, which happens to be the non-pigmented, softest part of the hoof wall. Such mechanical separation results from environmental conditions that allow entry of microorganisms such as fungi, yeasts and bacteria into a weak spot with the potential to proliferate and extend more proximally into the hoof wall.
The infection usually tends to be a mix of both bacteria and fungi. Any of the feet or all four feet can be affected. There can be extensive areas of diseased tissue or the problem area can be localized within a small area of a foot.
Conditions Conducive to White Line Disease
Moisture is thought to be an inciting cause because it softens the foot, allowing entry of dirt and microorganisms. Although white line disease is more prevalent in wet conditions, horses in arid and hot climates can also develop the problem. Cracks that form in dry hooves similarly set up favorable conditions for invading microorganisms.
Mechanical stressors on a hoof are primary risk factors for white line disease. Hoof wall separation results from abnormal loading patterns of a foot and/or hoof distortions such as long toe and underrun heels, club foot, sheared heels or overgrown hooves. It can also occur after a hoof abscess. In addition, laminitis is notorious for causing hoof wall separation.
Signs of White Line Disease
In the early stages of infection with white line disease, a horse rarely shows pain or lameness until the diseased tissue becomes more extensive and/or invades into sensitive tissues. More advances cases might compromise laminar attachment to the hoof wall and result in coffin bone rotation or sinking.
In most cases, what is observed is a lightly visible, chalky or crumbly area just dorsal to the junction of the hoof wall and sole. A horse might also show a positive hoof tester response around the affected area, but many don’t experience tenderness.
The hoof capsule can develop asymmetric dishing or bulging, representative of distortion forces on the hoof. The more advanced the case, the more likely it will form a hollowed-out area within the hoof wall that might be audible when tapping on the hoof wall with a hammer or hoof testers.
Unless a horse’s white line disease progresses to the point of discomfort, it is most likely identified by a farrier during hoof care. Removal of dead sole with nippers or a hoof knife allows visualization of the hoof wall-sole junction on the bottom of the hoof.
In a case of white line disease, an abnormally deep cavity within the inner layer of the hoof wall is visible. Early recognition is important to prevent spread of infection upward in the foot, which is more difficult to manage.
Any concern about an abnormality associated with hoof wall separation should prompt veterinary consultation and radiographic imaging of the hoof from different angles. This is especially true if the horse is lame, painful and/or is sensitive to hoof testers. This helps to identify white line disease and also to differentiate from laminitis.
Treatment and Prevention
A critical element of treatment is to address and resolve causes of hoof wall distortion and to unload the diseased area of hoof damage. Diseased tissue is debrided and removed to get back to healthy tissue with a solid attachment of lamellae to the hoof wall. Infection begins at the ground and moves upward, so debridement follows this path.
Usually, debridement does not extend into bleeding tissue since white line disease tends to affect non-sensitive tissues of the foot.
Exposure to air is good for restoring and maintaining hoof health; this also applies to opening up hoof cracks that tend to collect debris.
Growth of healthy hoof horn relies on continuity of pressure—one strategy is to affix a horseshoe that bridges missing portions of the diseased hoof that have been removed. A horseshoe also minimizes weight bearing on the sole. It is recommended to have a plan in place for applying a shoe prior to removal of extensive portions of hoof wall.
Another important strategy is to back up the toe by setting the shoe back a little from the end of the toe. This moves the breakover to relieve pressure on the toe and minimize pinching at the junction of normal hoof wall and the resected area.
Depending on the location of diseased tissue, an egg-bar shoe might provide a protective advantage, or a glue-on shoe might be the appliance of choice.
Although it is tempting to replace a hoof defect with polymethylmethacrylate composite, this tends to be counterproductive in these cases. Heat from curing of a composite material potentially drives organisms deeper into the hoof, and infection can “simmer” beneath the composite without notice until the horse becomes overtly lame from a building infection.
In addition, it is important to keep the horse’s feet dry by eliminating wet areas in the paddock or turnout, such as puddles by the water tank from gutter run-off or puddled wet spots that collect on uneven ground.
The horse should not be turned out into wet or dewy grass, mud or pasture along creek areas. Provide clean bedding and change it frequently to avoid build-up of urine and manure, which contribute to degradation of hoof horn through contact.
Foot cleaning is accomplished using a wire brush and debridement when necessary. Although plenty of commercial topical medications are available, studies have demonstrated that a white line disease-affected hoof benefits more from wire brushing than from topical treatments, provided bad tissue is debrided and the hoof is kept clean and dry.
At times, the use of iodine, methylene blue or Thrushbuster as a dye marker might help to outline diseased areas that need further debridement.
Prevention relies on maintaining a clean, dry hoof along with regular farrier care, relief of hoof distortions, and exploration of any suspect area of the sole that appears compromised.
Horseshoes can hide insiduous problems, so a farrier has an important role in identifying syspect areas within the sole and near the white line when shoes are pulled and the hooves are trimmed. Horses that have developed white line disease are also prone to recurrence, so those horses must be monitored regularly.
Returning to Work
The amount of time off for a horse with white line disease depends on the extent of hoof wall loss from the resection, how much support the coffin bone still has, and how many feet are affected. Normal hooves usually take nine to 12 months to grow out, with the heels and quarters taking less time. In most cases, a horse is pulled from active exercise to facilitate healing without setbacks or other hoof damage. This might take as long as four to six months for full recovery in a horse with resection halfway up the hoof wall.
Yet, there are instances where a horse can be put back to work prior to full regrowth of new hoof. If the resection only extends about a third of the way up the hoof, then the horse can return to work provided that there is no pain or lameness following resection. The horse should be fitted with an appropriate, protective horseshoe that distributes
the load and promotes easy breakover to relieve pressure on the resected hoof tissue.
The Bottom Line
The cause of white line disease is not always evident. Treatment objectives rely on debridement and removal of all diseased tissue. Ample time is needed to allow hoof growth before returning a horse to work. Veterinarians and horse owners need to remember to practice patience.
Preventive measures for white line disease rely on maintaining the strength of the hoof and hoof capsule, as well as minimizing hoof distortions through competent farrier care.
Horses living in moist conditions should be monitored regularly, and wet ground conditions need to be resolved. Healthy environmental conditions, exercise, good nutrition and regular farrier visits all contribute to management that optimizes a horse’s genetic propensity to grow solid, healthy hooves.