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White line disease often begins in horses with long feet and the weight of each step tends to pry the hoof wall away from the sole. The separation collects dirt and manure that is forced up into the opening, making a moist, dark place for microbes that like to grow in hoof horn. Barefoot horses in mud, with long feet, may suffer wall separation due to mud or fine gravel packing into the bottom of the foot and white line area. As the horse travels and puts weight on the feet, the mud is forced higher, creating a separation where the horn-digesting fungi get started. Anaerobic pathogens (yeast, fungi and certain bacteria) thrive in the white line area because they are keratinophoc (keratin loving), and keratin is the basic component of hoof wall materials. Plus, these pathogens are found in the soil in most geographic regions, thus any hole in the hoof will let them in. The white line area is the vulnerable region and most likely the entrance for these organisms since it has some flexibility and elasticity, being a hinge between the solid hoof wall and the softer sole. There is some movement at the white line to allow for expansion and contraction of the quarters when weight is placed on the foot. Repeated pressure on this "hinge" could possibly weaken the white line. Separation is often found at the back of the foot between the heel and quarter, and the cavity in the white line may be very shallow, or extend as much as halfway up the hoof wall. In determining the cause of white line disease, many elements are taken into account. Environmental factors (wet climate, dirty conditions), mechanical stresses (trauma, hoof cracks, long feet, damage to the laminae) and opportunistic microbes may combine to cause disintegration of hoof horn. A wet environment can soften and weaken the structure, allowing microbes easier access up into the horn tissue. Urine and manure create a chemical action that can dissolve and extract keratin from the horn cells; this breakdown process is accelerated if feet have a high moisture content. Humidity was also thought to contribute to the disease, but in recent years cases have been reported in drier locations, as well. Today more farriers, veterinarians and horsemen are aware of it, and more likely to recognize white line disease. Ironically, it seems to be most common in horses that are well cared for, though it may be that their problems are more likely to be noticed. Research in Scotland, looking at horses with a variety of hoof problems, discovered that all horses diagnosed with white line disease had been on a high concentrate (grain) diet. There is still some debate over the actual cause of white line disease. Exposure to air seems to stop the problem, so anaerobic pathogens (yeast, fungi and certain bacteria) have been blamed. A common pathogenic yeast, Candida Albicans, was the first focus of attention, but treatments for yeast infection did not halt white line disease. The four species of fungi most commonly involved in white line disease are keratinophoc, which means they love keratin. They all have the ability to grow well on it. They are all common in the horse's environment and can be cultured from soil and wood. Some cause disease in roots and tree bark. The inner hoof wall has the perfect chemical composition and environment for these fungi. COURSE The wall separation begins at the bottom of the foot, anywhere along its circumference. It may result in a bulge in the hoof wall at that spot, a gray-black area of debris at the edge of the hoof wall or a shelly, crumbly consistency (hoof material that is very easily pulled off or picked away). There also may be a powdery or greasy spot at the toe, or a triangular or inverted "U" shaped lesion in the front of the hoof, a rotten odor (but not as overwhelming as that of thrush) or black pus oozing from the affected area. As the condition progresses, the affected area expands upward, along the inside of the hoof wall toward the coronary band (with hollow pockets filled with debris), gradually separating the wall from the inner structures of the foot. Tapping the hoof produces a hollow sound over the affected area. The space is quite hollow; you can insert a knife blade up into the region between hoof wall and foot. If you scrape it out, not only will you find dirt and manure packed up inside, but also hoof horn residue that looks like crumbled cheese. The hoof wall has now disintegrated. In a long-standing case, an x-ray of the foot will show a gap between the hoof wall and the coffin bone. The outer wall becomes completely detached and the coffin bone may sink, rotating downward toward the sole of the foot at the toe. This factor (and subsequent lameness) often leads to misdiagnosis as laminitis. Laminitis can also result in separation of the hoof wall, but the separation occurs within the laminae; the sensitive laminae lets loose from the insensitive laminae of the outer wall--the separation is within the attachment mechanism itself. Laminitis is always very painful to the horse. In white line disease, the separation occurs next to the mechanism instead, and is thus not as painful. So the main difference between laminitis and white line disease is that white line disease does not cause lameness unless it progresses to a dropping of the coffin bone. The disease creates no pain as long as the disintegration process is confined to the horn. TREATMENT White line disease can be treated by removing all the diseased hoof horn with clippers, then grinding away all remaining residue of the affected horn and treating the underlying tissue with a good fungicide. This may leave a large hole, so reconstructive shoeing is usually needed to support that part of the foot and prevent further separation of hoof from coffin bone. A patch of fiberglass or some other hoof repair material can be used to help stabilize the shoe while the hoof grows out again. A small separation at heel and quarter can often be treated with less drastic measures. These heel holes can be dug out, removing as much of the decomposed hoof as possible, then filling them with medicated packing. A shoe can be put on the foot to keep the packing in, and prevent further contamination of the hole. Keep the horse in a dry place, have his shoes reset regularly and feed a balanced diet. Remind your farrier to disinfect his tools between horses as a rasp can harbor millions of fungi spores. Light and air are good defenses against this disease, so most veterinarians and farriers advise against sealing off the area. They like to leave it open a few weeks before repairing the hole. Many also recommend a broad-spectrum antibiotic or fungicide such as povidone iodine or bleach (half bleach, half water). Other topical products like copper sulfate, formaldehyde, DMSO, pine tar, turpentine, gasoline and merthiolate have reportedly been used with success. Antifungal-impregnated hoof repair materials are also useful. Early treatment with a topical fungicide such as bleach or iodine can often stop the disease before it spreads, especially if applied every day for a month, then every few days until new healthy hoof horn has replaced the old damaged horn. Supplements containing biotin, amino acids and trace minerals help promote hoof growth and healthy horn if a horse is deficient, but over-supplementation can be damaging and can make a horse more vulnerable to problems. Excess methionine (an essential amino acid often found in hoof care supplements) can cause disintegration similar to that of white line disease. Horses on poor diets can benefit from hoof building supplements, but if overdone, supplements themselves can be the cause of hoof problems. RECOVERY How long it takes for the horse to recover depends on how much hoof wall was lost. An advanced case where much of the wall had to be removed might take six months to a year for new horn to grow out. During this time the foot must be protected against re-infection. With treatment and protection, however, even feet with massive destruction can recover; some horses have lost more than 80 percent of the hoof wall and still grew back a normal, healthy hoof.
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