White Line Disease, also called onychomycosis, is an infection of the hoof wall of horses. Onychomycosis does not attack the outer wall but the area of the insensitive white line that joins the outer wall to the sensitive tissues of the foot. The result is a disruption of the supporting tissues of the hoof wall. Over time the infection works its way from the bottom of the hoof wall up creating a cavity that may be hard to detect initially. This disease is frequently confused with focal infections of thrush that have become established in defects in the white line. This article discusses the appearance, diagnosis, treatment and prognosis of this disease which seems to be growing more common.

White line disease is an insidious disease that starts at the solar surface of the white line and spreads upward and out under the wall often undetected until large amounts of wall are undermined by the infection. Lameness is not common until late in the process when the wall breaks away placing the sole on the ground. However white line disease can be detected early by a observant farrier as a chalky off-white to very light yellow color. As it is dug out from the white line with a shoe nail, think of the color and consistency of a firm to hard white cheese. Though the disease does not spread rapidly the fact the white line is not remarkably change allows this disease to go on until other signs appear, usually lameness but not always.

If the undermined wall breaks away or allows the sole to drop to the ground lameness from bruising may be the first detected sign. Some cases of mild lameness seem to be due to the increased weight bearing on smaller sections of functional wall and may represent laminar tearing. A third cause of lameness is seen when thrush gets established up underneath the wall and digests horn down to sensitive tissue in the wall setting up a poorly draining wall abscess.

Sometimes the first detected sign is the farrier hears an unusual hollow or bell like sound when the shoe nail is hammered. By this time the disease has hollowed out a large area of insensitive white line and the cavity has become quite extensive.