The white line is the junction between the hoof wall and the sole on the bottom of the horse's hoof. White Line Disease is a widening of the white line, or a separation of the hoof wall from the sole. This separation is then invaded by bacteria and fungi, leading to infection that progresses up towards the coronary band which produces a white or gray powdery material. It is usually first noticed by the farrier during a routine trimming because only severe cases actually cause lameness.
Because mechanical factors such as poor hoof conformation and various hoof imbalances (long toes, underrun heels, club foot) as well as primary hoof problems like laminitis are thought to lead to the initial separation, treatment begins with correcting the primary cause. If a large section of hoof wall has been separated from the inner tissue the detached wall may need to be removed. Any infection should be treated and frequent trimming used to make sure new hoof wall grows correctly. A hoof supplement containing biotin and methionine may be helpful to ensure healthy new growth.
Once thought to be due to a wet or dirty environment, hygiene is now being questioned as a cause of White Line Disease. Not only can any age, breed and gender of horse develop this hoof condition, a horse can have it in only one hoof or more while a farm can have it in only one horse or more. This shows that it has more to do with the individual horse and hoof than with the living conditions.