White line disease is degeneration and separation of the attachments between the outer hoof wall and the underlying structure of the hoof. The cause of this disease is still debated but is likely due to agents that breaks down the normally tight attachments, the laminar layers. Once this happens, bacteria and fungal agents invade and digest the attachments, continuing the process up under the wall toward the coronet.

Genetics, poor nutrition and environmental factors all likely play a role in this condition. In addition, this disease often occurs secondary to another illness or injury. If the disease process has not progressed to the point of lameness, it is often discovered by a farrier during routine trimming. In advanced cases of this disease, it is often accompanied by laminitis.

In mild cases, treatment involves regular soaking and cleaning. In advanced cases, treatment may include ongoing removal of under-run hoof wall, application of topical agents, a hoof cast and corrective farriery. In either case, it is important to leave the hoof uncovered to expose the bacteria and fungal elements to air.

This disease is commonly known by a several other names. Under the name Hoof Wall Separation Syndrome (HWSS), a variation of this disease is thought to be caused by an autosomal recessive gene in Connemara Ponies.