"White Line Disease, " "Onychomycosis," "Stall Rot," "Hollow Foot," "Wall Thrush," "Yeast Infection," "Seedy Toe" - What is it?© Richard Dunivant First posted here June 2000, then published in the October 2000 Issue of Anvil Magazine "White Line Disease,"Onychomycosis," "Stall Rot," Hollow Foot," "Wall Thrush, "Yeast Infection," "Seedy Toe" - what is it? All of these names have been used over the years to refer to a fungal infection in the hoof wall. DEFINITION White Line Disease, as it is commonly referred to, has been labeled by the late Texas farrier Burney Chapman as "Onychomycosis" - the Latin root, "onyx," meaning nail; "myke" a fungus, and "osis" being condition of. To define this condition as a "disease" sends most horseowners into a state of panic. A disease and an infection are two different things. An infection, as defined by Tabor's Medical Dictionary, is "A state in which the body, or part of it, is invaded by a pathogenic agent (microorganism or virus) which, under favorable conditions, multiplies and produces effects which are injurious." The name White Line Disease is also misleading because the white line structure itself (zona lamellatum) is only involved in advanced cases. Basically, this hoof wall condition is caused by one or more fungi acting alone or in combination with bacteria that infiltrate, feed upon and destroy the keratin tissue of the hoof wall. It can occur in one foot or all four and is found among all horse breeds. SYMPTOMS Symptoms of hoof wall infection are numerous, often vague, and may differ in each horse. They include:
HOOF WALL STRUCTURE 
In order to gain a better understanding of this condition, it is necessary to review the anatomy of the hoof. There are three basic layers that compose the insensitive hoof wall: the stratum externum (external); stratum medium (middle) and the stratum internum (internal). The stratum externum is the outermost layer visible to the naked eye. The stratum medium makes up the bulk of the wall containing nonpigmented tissue comprised of microscopic horn tubules imbedded in tissue named intertubular horn. Horn tubules are responsible for the hoof's strength and elasticity. Inside the tubules are cells that absorb water, which allow them to bend, flex and compress. The stratum internum consists of 600 or more "horny laminae" (finger-like tissue extensions) that interlock with the sensitive lamina which lies adjacent to it. These horny laminae of the inner layer are called stratum lamellatum. The sensitive lamina lies between the stratum internum and the coffin bone. It is engorged with blood vessels and covers the coffin bone, lateral cartilages and the sensitive bars on the bottom of the hoof. Similar to the finger-like extensions found in the stratum medium (horny lamina/stratum lamellatum), the sensitive lamina has its own fingers. The sensitive lamina's "fingers" and the stratum internum's "fingers" interlock together like teeth on meshed gears. In other words, the horny laminae (stratum lamellatum) and the sensitive lamina fit together like tongue and groove. Because the coffin bone is attached to the sensitive lamina, which in turn is attached to the stratum internum, the coffin bone is, literally, suspended. Located at the end of the fingers of the sensitive lamina are small clusters called papillae that house cells which produce the horny laminae, middle wall and hard sole. These cells are called the stratum germinativum. THE WHITE LINE STRUCTURE So what exactly is the white line structure? The white line (zona lamellatum) is technically the junction of the sole and the hoof wall. The white line runs all the way up the inside of the hoof but is only visible to the eye on the sole of the hoof. It consists of a combination of horn produced by the stratum germinativum (of the papillae of the sensitive lamina) and horny lamina (stratum lamellatum) of the middle wall. The white line actually appears yellow in color. In order to create a link between the hoof wall and the sensitive sole as the wall grows down, the white line structure has to have an elastic property to allow for expansion and contraction of the hoof quarters. The weak link in the white line is the medullary cells of the horn tubules since they are the farthest away from any blood supply. Here is where bacteria and fungus enter the stratum medium. PREDISPOSING FACTORS Microorganisms do not ordinarily attack healthy hoof tissue. They usually gain entry via a hoof crack or fissure at the white line. Other predisposing factors include:
It has been demonstrated that urea can extract keratin from horn cells and the chemical action of manure acts to remove the substance that binds horn cells together. It has not been proven that a farrier can transmit the disease from one horse to another, but an understanding of a septic technique makes one believe it could be possible. After trimming a horse with suspected or known infection, I always wipe my tool down with an antifungal-bactericidal foam spray. WHITE LINE DISEASE The term White Line Disease is confusing. The majority of cases diagnosed are restricted to the stratum medium, but the causative organisms can infiltrate the stratum internum and the white line structure itself can become affected. Involvement of the sensitive lamina can result in laminitis and subsequent rotation of the coffin bone. The stratum medium, with its horny laminae and keratin tissue, provides an excellent host for microorganisms to thrive: a good food supply and a moist, dark, oxygen-free environment (anaerobic). A study performed by Burney Chapman identified 25 different species of fungi in tissue cultures obtained from affected horse hooves. All fungal species were common environmental contaminants found normally in soil and wood. His findings, identifying the most common fungi were Pseudallescheria and Scopulariopsis. Another study performed by Dr. Richard Shakalis and Dr. John Pautienis, researchers at SBS Equine Industries, has shown this condition is caused by two different types of organisms existing in a symbiotic relationship, rather than just one acting alone. The tissues that they cultured consisted of at least one destructive bacterium and one fungus. Symptoms and the severity of the disease vary among horses depending upon the specific strain of fungus and bacterium present. Infinite numbers of combinations are possible. The bacterium and fungus live independently, but mutually benefit from each other's presence. Both these microbes produce enzymes and exotoxins that break down the protein and collagen structures found in the hoof. Their metabolites feed off each other. Two type of fungus exist: heterotrophs that consume dead organic matter and saprophytes that feed off of living tissue. As the fungus becomes embedded in the hoof wall, it sends out filaments called hyphae that absorb nutrients like a plant root. Fungi can produce spores which can lie dormant and erupt later when growth conditions are ideal. Bacteria reproduce quicker than fungi because of their ability to divide quickly. The same bacteria and fungi cultured in affected hooves are known to cause virulence in man. Researchers at SBS Equine Products have identified some organisms that cause diseases in both man and horses as listed in the graph on page 30. The makeup of the colony's organisms can help determine the rate and degree of destructiveness. An aggressive fungus coupled with a virulent bacterium could result in a fast-growing, difficult-to-treat case. A less invasive fungus and a slow, proliferating bacterium may allow for easier treatment. All colonies will grow faster in a dark, moist environment. TREATMENT The best way to treat white line disease is early detection followed by aggressive treatment. Removal of the affected hoof area, exposure to light and air, and sterilization of the underlying tissue using broad-spectrum bacterio-fungicidal agents is necessary. Hoof wall resection should be wide enough that the margins of the affected area are free of poor horn tissue and the debridement continued until there is a solid junction between the keratinized wall and the sensitive tissue. If removal is not thorough, viable fungus and bacteria still present can continue to invade new areas or reoccur in newly formed tissue. The defect should be kept as dry as possible and treated with a topical agent three to four times daily, or as directed, for two to three weeks. Since many of the fungus species produce spores, it is imperative that the topical agent used remains active for a long period of time. It may be possible to keep unkilled organisms from advancing until successive trimmings and growth remove the area with time. There are numerous topical products available including Fungidye, Thrush-Buster, Save A Hoof, merthiolate, benzoyl peroxide and a homemade mixture of iodine, bleach and formaldehyde. A topical solution called Clorpactin WCS-90, a brand of oxychlorosene sodium that is used to treat human fungal and bacterial infections, is being experimented with in my area of Florida. Two grams of powder mixed in one quart of warm water or saline solution is applied topically daily for two weeks. Small, affected areas can be treated with resection and topical agents as described, then trimmed and shod normally. If more than a third of the hoof wall is resected, it is helpful to place a heartbar shoe for bony column support, especially if x-rays demonstrate the infection is close to the coffin bone. An eggbar shoe will support the weight of the wall circumferentially. There is controversy in the equine medical and farrier community as to whether a large, debrided hoof wall defect should be repaired with an acrylic composite, due to the danger of trapping remaining fungi and bacteria in an anaerobic environment, facilitating their reproduction. Acrylic composites can be applied with the addition of an antimicrobial powder, but this will slow the cure rate of the material. This decision should be discussed and agreed upon by the farrier, horseowner and veterinarian, keeping in mind the horse's environment, extent of disease and size of the defect. Prevention, early detection and aggressive treatment are essential when treating hoof wall disease. Research is currently underway to understand this entity and provide the best treatment options possible. REFERENCES 1. Ball, Michael DVM, and Wilderstein, Michael. "Examining White Line Disease," American Farriers Journal, July/Aug. 1998, pp.79-82. 2. Cohen, Jonathan and Peacock, Robert. "Update On Hoof Wall Disease," Anvil Magazine, vol. XX, #5, May 1995, pp.94-95. 3. Shakalis, Richard and Pautienis, John. "Taking the Mystery Out of White Line Disease," Anvil Magazine, vol. XX, #5, May 1995, pp.90-93. TABLE
SIDEBAR THE USE OF SODIUM OXYCHLOROSENE AS A TOPICAL TREATMENT FOR WHITE LINE DISEASE There are numerous products marketed for topical application to treat white line disease, a fungal and bacterial infection of the middle hoof wall. Which agent combines a high degree of activity against the most resistant microorganisms, is non-toxic in nature and free of allergic or systemic reactions? In the 1930s chemists began studying the germicidal efficiency of chlorine derivatives on fungi, viruses, spores and bacteria. It led to the development of a stabilized organic derivative named sodium oxychlorosene. As of 1955, a solution called Clorpactin WCS-90 was approved and introduced for surgical irrigation in operating rooms for all types of surgical specialties. Although Clorpactin WCS-90 is approved only for human use, its active ingredient, sodium oxychlorosene, is found in veterinarian-approved topical agents currently available on the market. It is an over-the-counter product that can be purchased (if not readily stocked by your local pharmacist) from a wholesale pharmaceutical supply house or from a veterinarian. Sodium oxychlorosene is a chemical complex formed by combining the sodium salt of dodecylbenzesulfonic acid and hypochlorous acid. Hypochlorous acid has the outstanding power to kill microorganisms because of its ability to diffuse through the cell wall into the cell's protoplasmic contents. Through the process of oxidizing, this chlorine derivative is stronger than any chlorite or peroxide agent available. The active ingredient, sodium oxychlorosene, is bactericidal for both gram positive and negative bacteria - virucidal, fungicidal, sporicidal - and effective against yeast and molds. When added to a topical solution in concentrations from 0.2 - 0.5% (depending on conditions), it is destructive to all known microorganisms. This makes it a highly effective, broad-spectrum topical antimicrobial to treat white line disease, after resection has been performed on the affected hoof wall. Because oxychlorosene has a normal pH of 6.5 to 6.9, it is nontoxic in nature and free from allergic reactions, making it useful for repeated topical applications. It has no adverse effect on normal tissue or systemic toxicity, no matter how long the solution is used or in what quantity. A sodium oxychlorosene solution can be applied to a hoof wall by irrigation, instillation, spray, soaks or wet compresses and left in contact with the site for at least three minutes, one to three times daily for up to two weeks. Two grams of powder are mixed with a liter of cool (not hot) water. It should never be used directly in dry powdered form - direct application of the strong oxidizing powder will cause burns to healthy tissue. Mixed solutions will lose their potency if exposed to excessive heat and light. Therefore, the solution should be placed in a plastic or glass bottle that is wrapped in foil and kept in the refrigerator for up to ten days. Because of the unusual combination of properties, the use of sodium oxychlorosene or a hypochlorous acid component can be used to treat localized bacterial/fungal infections, particularly when resistant organisms are present. When looking for a manufactured topical solution for use on a hoof wall affected with white line disease, check the active ingredients listed for sodium oxychlorosene or a chlorine derivative to ensure broad-spectrum antimicrobial activity. REFERENCES 1. MERCK INDEX; Tenth Ed., Oxychlorosene (Sodium oxychlorosene: Clorpactin WCS- 90) 6824, 1983. 2. Sconce, J.S.,Ed.in Chief: CHLORINE: Its Manufacture, Properties and Uses; P.535, Reinhold Publ. Corp. 1962. 3. Swanker, W.A.; The Use of Clorpactin WCS-90:44-46, July, 1955. Return to the White Line Disease Articles Index page. Return to the Farriery Articles listing page.
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