The white line is the area of fibrous connective tissue that joins the rigid hoof wall to the more resilient sole. Since the white line is soft, it is more vulnerable to penetration by foreign material. Dirt-filled cracks and fissures in the white line are not uncommon in normal feet but when abscesses develop under the sole, lameness follows. Factors predisposing to white line disease are continuously wet feet and animal with previous bouts of laminitis which result in poor horn quality and decreased white-line strength.

White line disease occurs most frequently at the heel-sole junction of the abaxial lateral hind claw. This is the area of greatest concussion on impact and the white line is wider in this area. Lesions are often bilateral. When white line disease occurs on the front feet, the lesions are located at that abaxial border near the toe as this area receives greatest concussion on impact.

A serious sequela to white line disease is infection of the navicular bursa. Although the bursa is protected from direct penetration or infection by the flexor tendon it is still vulnerable at the edges of the tendon. Occasionally, infection extends up along the wall, abscesses in the navicular bursa and drains above the abaxial coronet (called gravel).

White line abscesses cause lameness. The animal may walk or stand with the heel slightly raised or the limb abducted (again, in an attempt to shift weight to the medial claw). Hoof testers help localize the lesions that are not obvious. Black lines, cracks, and fissures should be followed and pared out. Opening of an abscess will often result in release of watery black exudate and gas. If the navicular bursa is involved the heel will be swollen, hot, and painful. The sole should be pared to allow adequate opening and drainage of the abscess and the lateral wall trimmed to prevent packing of dirt and manure back into the abscess hole.