THE WHITE LINE AND BEYOND
Despite the name, “white line disease” does not actually affect the white line itself---the border zone around the edge of the sole where the inner surface of the hoof wall connects to the tissue that lies underneath. Instead, the area affected is the layer of hoof wall just outside the white line, a structure called the stratum medium.
Sitting between the stratum externum (the smooth, thin outermost layer of hoof wall) and the stratum internum (where the keratinized and soft laminae interlock) the stratum medium consists of parallel horny tubules that give the hoof wall its strength. The stratum internum’s finely pleated laminae--hornlike structures lining the interior of the hoof wall and sensitive laminae covering the coffin bone---work together to hold the hoof wall firmly in place. The dermal laminae are tipped with projections called terminal papillae, which fit into corresponding sockets in the stratum internum and generate new keratinized cells that fill the spaces between the structures. Below the leading edge of the coffin bone, this keratinized material expands to plug the gap between the interior surface of the hoof wall and the edge of the sole---this is the white line.
In a healthy hoof, the white line is impenetrable, but if its integrity is compromised, a pathway opens for invading microorganisms. “These microbes don’t attack a healthy, strong, solid foot. They can more easily enter a weak area,” says Travis Burns, CJF, assistant professor of practice and chief of farrier services at Virginia–Maryland College of Veterinary Medicine.
A number of factors can predispose a horse to white line disease, but in virtually all cases the infection occurs in the wake of injury to or abnormal stresses on the hoof. “Leverage on the hoof wall starts pulling the wall away from the bone. Stretching of the laminae creates little separations,” Steward explains. “There’s no longer a healthy junction, so in come the microbes.” Various types of microorganisms may be involved, including both bacteria and fungi.
Once the organisms are inside the hoof, destructive processes begin. “The