White Line Disease

When we think of the white line disease (onychomycosis,) or Hollow Hoof syndrome, we need to realize that the fungus is opportunistic here and will continue to be problematic until we debride and treat the areas affected.

There is much more to the underlying condition than just hoof wall separation, or change in environment that has caused the fungus to have an ideal environment in which it has become pathogenic. Even though described at times to mimic fungal infection experienced by humans, its main difference lies in its etiology.  Though similarities in acquisition of the organism, the fungi in WLD are not just looking for warm moist environment in which to live. They actually attack the intermediate layer of the stratum corium known as the stratum medialis and digest the membrane responsible for securing the wall (as we know it) to the sensitive areas deeper into the center of the hoof. The resulting hollow sound is produced because this digestion results in 2 living layers of tissue separated by air and debris rather than the normal cartilaginous layer joining the two distinctive tissue types. The understanding as to why is still open for some debate, but in general is found to be related, if not directly a result of the way we house and care for our horses at present. Although reported in early manuscripts, WLD was not considered common place as it is today. The late Burney Chapman long ago stressed the importance of treating these aggressively considering the overall picture rather than just the pathology that resulted in the intervention. The multi-factorial cause and effect relationship is probably best narrowed to three components: DIET, ENVIRONMENT and HOOF CARE. All three aspects of treatment are critical to success, and any one of these ignored can cause an otherwise good plan to fail.

We know many of these horses are insulin-resistant and there are several options to consider when purchasing feed types and sources. Diets that tend to be lower in carbs and fructans are important to consider when dealing with foot problems such as white line disease. This is critical, as feeding habits of confined horses have increasingly become more tailored as monogastric diets rather than herbivore diets. This has led to conditions referred to as insulin resistance (IR) and metabolic syndrome, experienced by many horses in confinement these days. This has brought about much discussion by experts as to pathological conditions that IR might lead to. Diets low in certain types of carbohydrates (CHOs) are considered essential to maintaining proper digestive balance and metabolic health. Metabolic Syndrome that is associated with IR horses can be directly linked to excessive supplementation of sugars in feeds and or supplements, and therefore should be monitored when deciding what supplements or grains one intends to feed.  The repercussions of excessive supplementation could be far reaching, as is the case with hoof diseases such as WLD, laminitis and poor hoof growth. Supplements should, among other things, offer low sugar content and adequate levels of Chromium and Magnesium to help decrease the level of insulin resistance. Insulin resistant horses often have poor circulation, so turnout and pasture time are critical to overall success in diet management, as well as type and quantity of forage available. Involving a veterinarian with an understanding of hoof biomechanics and physiology can be very helpful in managing the pathology and etiology of laminitis and/or white line disease to ensure success and decrease the likelihood of recurrence.

Housing environment is often the least addressed yet makes the most difference in long term success. Let’s begin with pasture or paddock and stall. Stall environment rather than paddock environment may be the single biggest compounding factor as it pertains to the pathogens associated with white line disease. Horses moving even in smaller paddocks are much better equipped to overcome the detriments of opportunistic feet pathogens than those living in a stall environment. Conditions such as Thrush, WLD and Laminitis can be improved with better hoof circulation. Hoof form and function is developing into an entire field of study and as we gain knowledge pertaining to current practices. We have begun to rethink our current relevant animal husbandry guidelines or practices. Bedding types and footing (pea gravel vs. sand etc.) are being examined as to how they affect foot growth and function. WLD and Laminitis have similarities in mechanisms within the form and function of the hoof and its integrity. A recent study concluded that changes in confinement and footing materials were superior to current techniques employed to deal with WLD and laminitis. Though not repeated to date, it does indicate we still have much to learn. The approach is largely directed at changing the environment as well as nutritional support to more closely mimic that of the wild horse or mustang. Volumes are being written on this subject and the answers we seek will have considerable relevance here.
A veterinary and farrier team is best suited to address individual situations so we will cover only general principles here. Get  the hoof open and dried out with a topical such as chlorine dioxide products or gentian violet-containing products. The degree of damage will dictate the amount of debridement required and care must be exercised to not damage healthy tissue or create more damage to the integrity of the wall than is prudent for weight bearing. Treatment is aimed at killing fungi and spores rather than trying to burn or harden sensitive tissue as the latter actually slows the overall healing process. Support P3 by either casting or shoeing as required and get rid of any extra foot flares, seedy toe, laminar wedges etc. Finally and  often overlooked, get the horse landing heel-first and weight-bearing over the entire solar surface of the foot. This is based on current knowledge of hemodynamics within the foot.

Dr. Preston Hickman

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