RISK FACTORS FOR WHITE LINE DISEASE
Various conditions that compromise the integrity of the hoof may open the door to the opportunistic microbes that cause white line disease.
• HOOF IMBALANCES OR ABNORMALITIES The stresses associated with hoof imbalances or conformation issues can stretch the structures of the white line area and open pathways for microorganisms. “Any area that is overloaded, flaring or compromised may be invaded,” says Travis Burns, CJF, assistant professor of practice and chief of farrier services at Virginia–Maryland College of Veterinary Medicine. “Horses with chronic laminitis or with club feet are prone to white line disease in the toe region. Horses with long-toe, low-heel conformation are also at risk. Horses who are base wide are prone to white line disease on the side where the hoof is flared.”
• WET CONDITIONS Chronic moisture softens and expands the hoof tissues, and constant changes from wet to dry environments can lead to minor cracking, much like chapped skin. “Moisture makes a hoof more vulnerable—soft and expanding—allowing microbes to invade. Then when the foot dries it shrinks/ closes back up and the microbes are in there,” says Burns. “In moist, humid regions we see more cases of white line disease. We see it a lot in Virginia, even in periods without rain. We have morning dew so feet get wet every morning and then dry out by afternoon.”
• MECHANICAL STRESSES Work on hard surfaces and other factors that stress the feet can increase the risk of infections. “Horses are out there stomping flies, putting more stress/ trauma on the bottom of the feet, making it easier for microbes to enter,” says Burns.
Obesity can also play a role. “We are
all too fat and don’t get enough exercise,” says Michael Steward, DVM, of Shawnee Animal Hospital in Shawnee, Oklahoma. “Many horses are carrying too much weight on their feet, putting more stress on the support structures.”
• INJURIES OR NATURAL IMPERFECTIONS “Many horses tend to get toe cracks, and this can be where white line disease gets started,” says Todd Allen, CJF, APF, a farrier in Vandergrift, Pennsylvania. “Some feet have imperfections in the dorsal sole/wall junction of the laminae, which corresponds with the dorsal notch of the coffin bone. At the end of the coffin bone there is a little dip or dimple. As a result of that dip, there may be a little imperfection at the junction of the laminae, reflected in the hoof wall.”
Even seemingly minor injuries in an otherwise healthy foot can enable infection. “Imagine a small puncture in the foot, introducing the microbes. Over time they slowly eat their way into a larger and larger space. Even if the bottom of the foot is not compromised, a prick through the wall can start a problem,” says Steward. “Perhaps the horse jumped a fence and banged a foot hard enough to cause a severely bruised bone that compromised the blood supply; this could be the opportunity needed by the microbes.”
• INSULIN RESISTANCE LOW GRADE LAMINITIS Insulin resistance is a metabolic disorder, similar to type 2 diabetes in people, in which cells become less sensitive to the action of this hormone, and so adequate amounts of glucose are not moved into the cells for use as fuel. “If cells in the hoof don’t have the ability to take up glucose because of insulin resistance, the cells are damaged and die,” Steward says.
As the laminae cells die, the connection between the coffin bone and the hoof wall loosens, and the sensitive laminae stretch, allowing the coffin bone to begin to sink. “This is similar to what happens in founder, but in this case it’s not due to laminitis; it’s simply due to a stretching of the laminae,” Steward says. When the stretched-out laminae grow down to the level of the white line, that structure also widens and becomes less structurally sound.
• GENETIC PREDISPOSITION Some horses simply carry genes for less robust hoof structures that are more vulnerable to the stretching and pulling that can lead to white line disease. Larger, heavier breeds are also at a higher risk. “For example, draft horses become flatfooted, losing the natural cup [concave sole] they had at birth, because they are so heavy,” says Steward. “A lot of heavy horses—including Quarter Horses and warmbloods—lose that arch and foot support due to chronic overload.”