Hoof Wall and White Line Disease
The topic for this month's "Step by Step" is white line disease. Nothing would please me more than to write a helpful, informative article on white line disease for horse owners, but at this point, that article would have to be very, very short. We know very little about white line disease. Many people have ideas about what it is and how to treat it, and some people are very successful. I would suggest, however, that there is a great deal more to this whole problem than any of us know.
HOOFCARE & LAMENESS
The elastic white line is the link between the inner sensitive laminae and the insensitive outer hoof wall.
First of all, on the humorous side, let's blame the Germans for writing their anatomy texts and research papers in German instead of English. Only now, as many as 20 years after several important papers were first written, are we obtaining translations of key studies of the structure of the hoof wall and analyses of hoof wall defects.
Second, let's blame the entire state of Florida. In the 1980s, Americans started shipping horses to Florida to race, show, and be bred. Once there, many contracted a hoof wall defect Floridians called "yeast infection" (although they know it's not yeast), and they brought it home. Suddenly, even though we know the problem isn't contagious, we had a national crisis on our hands, and hoof walls everywhere were caving in, collapsing, rotting away, or just plain smelled awful. Since so many horses had at one time or another been to Florida, we decided it was some sort of tropical foot rot, rising like a swamp monster to compromise the soundness of our best horses. It was the best answer we could come up with, at the time.
Third, whose problem is it? Your horse has a hollow hoof, or maybe an eaten-up outer wall, and your veterinarian tells you to call the farrier. The farrier tells you to call the veterinarian. No one really knows whether it's a medical problem or a mechanical problem, but neither professional wants to be the one to tell you that he or she doesn't know what it is. Ultimately, it's your problem. Your farrier can tell you at least 10 things that might be causing it...but you haven't changed your horse's routine in three years, and suddenly you're looking at a big gap in his hoof wall.
The more you read about white line disease, and the more you talk to farriers and veterinarians about it, the more you think there might be some wisdom to those who shrug and say, "White line disease? Never see it. Not in any of my clients' horses." For years, I puzzled over this response, but now I can see some value in the fine art of denial.
What we don't know about the horse's hoof wall could fill a library. We don't know how or why a horse founders to the point where its hooves fall right off its feet. We don't know how a hoof maintains (or loses) moisture, so we constantly are trying to moisturize dry feet and dry out soggy feet. We don't know exactly how to evaluate how or if a horse's hooves are utilizing nutrients, or which nutrients might be in short supply or be blocked by other nutrients, slowing hoof growth or causing shelly, cracked feet. We don't know a lot about the hoof, when you come right down to it. But we have our own individual theories, pet treatments, and second-guesses at what is wrong.
Hoof Wall Construction: Undressing The Foot
The outer hoof is designed to protect the fragile bones, tendons, and blood supply inside the hoof capsule. Think of the outer hoof wall as a thick Gore-Tex survival suit that you'd wear if you were headed into the Arctic, or climbing Mt. Everest. Raps against jump rails, constant thumping of rocks on the trail, that insistent "bam!" of the hind foot on the stall wall to remind you that it's feeding time...the hoof wall is on the job, making sure (at least most of the time) that the hard shell takes the lumps. The inner foot might be jarred, but it is rarely seriously injured.
Often, inner injuries like coffin bone fractures are found on horses with thin hoof walls. Nature's protective shell is compromised. If you look at wild horses, you would be surprised to see how thick the hoof wall is. Perhaps natural selection has given a thick, tough hoof wall a high priority rate for survivors.
Have you ever noticed the striped feet on an Appaloosa? The stripes always are vertical. Have you ever seen horizontal color stripes on a hoof? Probably not. This is because pigment follows the horn tubules, which run vertically down the hoof wall. This is also why most cracks are vertical.
To oversimplify the construction of the hoof wall, think of piano keys. The white keys are "horn" tubules; the black keys are what is called "inter-tubular horn."
Cuddled up to the outer wall is the inner hoof wall. This would be the layer of polar fleece that you wear under your survival suit. It is designed to wick moisture away from your body, circulate air, and more or less moderate your body temperature. The horse's foot functions in a similar way. If you think of the fuzzy nubs on polar fleece, imagine them reaching inward to the "body" of the hoof's sensitive structures. These nubs are epidermal (insensitive) laminae, that link the "nonliving," bloodless outer wall with the living inner structures and fill the space from the coronet to the ground, extending inward from the parallel tubules.
Meanwhile, the sensitive structures of the foot are surrounding the bones and tendons. There is a complex blood supply of tiny capillaries that supply each sensitive lamina, which in turn "locks into" matching insensitive laminae, like the right hand of a glove linked to its left when holding hands. Think of the sensitive laminae as reaching out with a warm woolen, life-filled glove; think of the insensitive hoof wall as clasping it with a cold, lifeless leather glove. Whatever the mechanism or the magnetism between the two, it works to protect the inner foot and provide the horse with a somewhat flexible but strong traction device to move it quickly across the ground.
The Ground Surface: A White Line
A freshly trimmed foot clearly shows a white line circumnavigating the sole; it forms a dividing line between the sole and hoof wall. The line is usually of uniform thickness, and if examined closely, will appear to be slightly yellow closer to the sole and whiter toward the wall. Farriers use the white line as a convenient entry point for their nails; they know that anything inside (toward the frog) the white line is "alive" and sensitive, so all nails must go into the white line, then be turned out to exit through the wall. This is why horseshoe nails are bevelled; they naturally arc as they are driven into the shoe.
Why is the white line white (or yellow)? The laminae from the hoof wall (insensitive) are not pigmented, and they are interleaved with tubular horn from the sensitive inner foot.
Remember the wool glove and the leather glove? They are still at work inside the wall, from coronet to just above the ground. But if the wool glove's finger touched the ground, the horse would feel pain, and it would be very vulnerable to injury. The remarkable engineering of the horse's hoof manufactures empty extensions of terminal horn tubules in the white line, instead of the living fingers. These are the darker, or more yellow, portions of the white line.
Even within this seemingly narrow area of the hoof there is a series of layers--the stratum internum and the stratum lamellatum sections of the outer wall, then the white zone itself, and the sole.
The recent publication of extensive studies on the white line by German anatomist Dr. K. D. Budras has shed new light on how the white line works--and how it can be compromised.
White Line Workshop
The sensitive foot has to be defended, and the white line is designed for that purpose. At a certain point of juncture between sole and wall, the insensitive leather-gloved wall still sends out its fingers. The wooly glove must fill in the spaces, or else be vulnerable to pain and infection from the ground. So, it creates horn tubules to fill the gaps.
Professor Budras has shown us that the white line has an elastic property, to create a link between the solid hoof wall and the softer sole. He likens the white line to a hinge that is involved with the expansion and contraction of the quarters of the foot. Repeated pressure on the hinge might mechanically weaken the white line. He has identified the medullary cells of the terminal horn tubules as the weak link in the white line.
An analogy be-tween a brick wall and the medullary cells of the horn tubules is obvious-as the foot grows, the horn tubules are farther and farther away from any blood supply. Their "mortar" thins and weakens without nutrition; mortar links between the tubules break down, and bacteria can enter the white line, even on a healthy foot.
The Germans tested this hypothesis by soaking hoof wall material in urea and liquid manure. Urea was shown to extract keratin from the horn cells. Amazingly, the manure's chemical action on the horn cells was to remove the intertubular "mortar." Together, manure and urine were destroying the horn cells! This destructive process was found to be accelerated in feet with a pre-existing high moisture content.
While progress has been made in better understanding the equine foot, and in particular white line disease, there still is much to be learned.
A Detailed Examination Of "White Line Disease"By Michael A. Ball, DVM; Michael Wildenstein, Certified Journeyman Farrier; and Sang Shin, PhD
"White Line Disease"--Or Is It?
White line disease has been a highly debated issue for a good number of years and has been a source of contention between veterinarians and farriers. The disease/syndrome is observed to create a soft/chalky horn tissue in the stratum medium; in severe cases, the stratum internum can be affected. The abnormal horn can be limited to the distal centimeter or so of the wall, but often extends proximally and can be as high as the coronary area. The term "seedy toe" has been used to describe this disease/syndrome due to the "hollowness" of the affected tissue when percussed. Given the fact that, technically, the "white line" is the junction between the stratum internum and the horn of the sole, the term "white line disease" probably is inappropriate in most cases. In advanced cases, the white line can be involved, but the majority of cases (especially in the early stages) are primarily restricted to the stratum medium; the inner-most zone of the stratum medium can be unpigmented and can therefore appear white in color when viewed from the solar surface.
The horse health literature and farrier trade journals contain a plethora of anecdotal reports and uncontrolled studies postulating a great number of causes and predisposing factors. Some of the suspected predisposing factors include excessive moisture/drying, lamellar damage (laminitis), acute trauma/abscess development, hoof cracks, and excessive toe length.
A great number of bacteria and fungi have been incriminated in the development of the disease, but sample collection techniques either are unreported or can be criticized regarding contamination of the samples. In one of the most complete studies in the literature (by Bernie Chapman), there were 25 different species of fungi incriminated in affected feet, with Pseudallescheria and Scopulariopsis being two of the more common isolates. There was one report in the veterinary literature (Pathomorphological findings in a case of onychomycosis of a racehorse, by Kuwano, et al., from the Japan Racing Association, Journal of Veterinary Medical Science, 1996) that demonstrated, with tissue evaluation under both a regular and electron microscope, disease of the white line area caused by a fungus.
The questions remain: Is this disease/syndrome a primary or secondary infection with bacteria, fungi, or both, and, if it is secondary, what are the predisposing factors?
An Attempt To End Controversy
A prospective clinical study currently is underway at Cornell University with the support of Steve Bloom (Across The Anvil East, Inc.), Bob Peacock (Farrier Science Clinic), and the New York State Diagnostic Laboratory. Our hypothesis is that the term onychomycosis, as offered by Chapman in much of the existing literature, is appropriate with the disease being a fungal infection of the hoof wall secondary to mechanical stresses related to poor management or other disease processes.
In our study, four control feet did not grow any bacteria or fungi. Five affected feet did not grow any bacteria on either aerobic or anaerobic culture, but all were positive for the presence of a fungus in the deeper tissues of the stratum medium. The individual species of fungi isolated were variable, and are all considered to be common environmental contaminants. None of these fungi are known to have the primary ability to cause disease (although the recovered species Gliocladium is well known for its ability to cause disease in plants, specifically roots and tree bark).
The individual fungal species isolated are as follows: Trichoderma sp, Mucor sp, Aspergillus glaucus, Gliocladium sp (in two feet).
The fact that all of our controls were negative for both bacterial and fungal growth validates our sample acquisition as a methodology for collecting sterile samples of the stratum medium for culture. The authors feel that this is a weak link in many of the other studies reporting the presence of fungus in hoof biopsy material. In much of the literature, the sample techniques are either inadequate to ensure against contamination, or were not stated to allow for evaluation of the technique. In addition, many of the previously reported studies did not include control hoofs or failed to include bacterial cultures.
The absence of bacteria and positive growth of fungi supports the role of a fungal infection and, therefore, supports the term "onychomycosis" in describing this disease/syndrome. An interesting fact is that all four species of fungi cultured are known to, potentially, be "keratinophilic" (literally meaning keratin "loving"). This does not mean that they have the ability to cause primary keratin disease, but have the ability to grow well on keratin.
We continue to hypothesize that mechanical stress created by excessive toe length, laminitis, poor management, and/or generally poor hoof health predisposes to the collection of environmental contaminants within the stratum medium. All of these identified fungi are environmental organisms and can be cultured from soil and wood. The inner environment of the wall horn (chemical composition, oxygen tension, and the limited availability of microbial substrates) selects for species of fungi that can survive and grow well on keratin. It is this collection of factors that likely leads to this condition.
Removal of the affected tissue, sterilization of the underlying tissue, and protection/support with the appropriate hoof repair material and shoeing appear to be the most effective treatment. There are numerous topical products reported to have varying degrees of efficacy including: iodine, bleach, formaldehyde, copper sulfate, DMSO, metronidazole, pine tar, Save-a-Hoof, Thrush-Buster, turpentine, gasoline, merthiolate, benzoyl peroxide, Fungidye, miconazole, clotrimazole, cleantrax, and many others.
The findings of our studies to date support the use of antifungal treatments, with the development of antifungal-impregnated hoof repair materials being of great interest. It must be mentioned that the antibiotic "metronidazole" is most useful in anaerobic bacterial infections and has little activity against fungi. The bleach products, DMSO, Fungidye, and other more classic antifungal drugs such as miconazole, clotrimazole, and itraconazole should be of benefit.
Thanks to Michael A. Ball, DVM, of the Departments of Pharmacology and Large Animal Medicine; Michael Wildenstein, Certified Journeyman Farrier, the Department of Farrier Science; and Sang Shin, PhD, the New York State Diagnostic Laboratory .
About the Author
Fran Jurga is the publisher of Hoofcare & Lameness, The Journal of Equine Foot Science, based in Gloucester, Mass., and Hoofcare Online, an electronic newsletter accessible at www.hoofcare.com. Her work also includes promoting lameness-related research and information for practical use by farriers, veterinarians, and horse owners. Jurga authored Understanding The Equine Foot, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.