White line disease in horses is a keratolytic condition affecting the deeper layers of the stratum medium of the hoof wall. This progressive pathological process is characterized by a crumbling decomposition of the horn at the white line, leading to separation of the hoof wall.
Etiology and Pathophysiology of White Line Disease in Horses
The exact etiological agent of white line disease in horses is unknown. Both anaerobic bacteria and fungi are thought to be involved in the disease process. The primary cause may be fungi, or it may be an opportunistic pathogen that invades after other pathogens have caused the initial damage.
The keratolytic process creates characteristic cavities within the hoof wall that contain air and degenerating horn. White line disease may affect only one hoof or multiple hooves. The disease can occur in any region of the white line of the hoof capsule. Only rarely are the lesions associated with inflammation of the underlying tissues.
In severe cases of white line disease, with a large proportion of the hoof wall affected, mechanical support of the laminae may be lost and the coffin bone may become displaced.
Clinical Findings of White Line Disease in Horses
White line disease in horses is insidious: it is often not recognized in the early stages, because the hoof capsule appears very normal and there is usually no associated lameness. Subclinical cases are usually incidentally found during routine farrier visits.
Lameness due to white line disease is usually associated with sole bruising as a result of displacement of the coffin bone or debris filling the cavity to the extent that it exerts excessive pressure on the underlying sensitive laminae.
Diagnosis of White Line Disease in Horses
In horses that have white line disease, affected horn on the surface and margins of the defect is soft and chalky, and it can readily be scraped away with a blunt object. The size and appearance of the defect on the ground surface of the foot often underrepresent the amount of cavitation within the hoof wall. Hollow areas may be audibly located by gentle percussion of the hoof wall.
Radiographs taken in a horizontal plane tangential to the defect are most useful to reveal the proximal extent of the cavitation, which appears as radiolucent areas within the hoof wall. Multiple radiographs at various angles may be needed to identify the circumference of the area involved.
Differential diagnoses of white line disease in horses include laminitis and hoof abscesses.
Treatment and Management of White Line Disease in Horses
Methods of treatment for white line disease in horses vary depending on the extent of the disease. The most important aspect of treatment is the removal all the undermined hoof wall to expose the underlying tissue to air.
Courtesy of Dr. Brian Beasley.
The undermined portion of the hoof wall does not provide effective support to the coffin bone and can usually be removed without appreciable consequences. Care should be taken not to damage the underlying tissue when removing the undermined wall. When a large portion of hoof wall must be removed, the hoof capsule may need to be stabilized with a special shoe or plate.
Without complete removal of affected hoof wall, white line disease is likely to progress. After complete resection of the affected area (see resection image), the foot should be properly (ie, in a bag sealed at the top) and frequently (2–3 times per week initially, then as needed) soaked in a commercially available chlorine dioxide product. Horses with lameness associated with displacement of the coffin bone should be shod the way a horse with chronic laminitis is shod.
White line disease usually can be completely eliminated. Horses in which the coffin bone has not displaced should be able to return to their previous level of performance. The amount of time before returning to work depends on the amount of hoof wall that was removed and the growth rate of the foot. Horses in which the coffin bone has displaced still have a much better prognosis for return to work than do horses with rotational laminitis.
Horses with a history of white line disease should be monitored closely because they are at higher risk for recurrence of the disease.
Key Points
White line disease is a keratolytic condition affecting the deeper layers of the stratum medium of the hoof wall.
Both anaerobic bacteria and fungi are thought to be involved in the disease process.
The most important aspect of treatment is removal of the undermined hoof wall to expose the underlying tissue to air.
For More Information
Pritchard L, King A. White line disease in horses. Vet Times. 2015;45.31:12-14.
Also see pet health content regarding white line disease in horses.
