Problems With Hooves
It takes nearly a year for the hoof to grow down from the coronary band to the ground. Therefore, anything that happens to the horse -- whether specific trauma to the foot or a systemic problem -- can affect the health of the hoof for a long time. Add into that equation that many horses genetically have feet that are not equipped to handle the day-to-day stresses of their work, and a bad shoeing job every now and then, and you have the ingredients for disaster.
The wall of the hoof grows down from the coronary groove from sensitive papillae. These cells are cornified (hardened) and form the horn wall of the hoof. Each papilla grows one tubule, taking approximately one year for that tubule to grow from the coronary band to the bottom of the foot. The matrix (a cementing substance between the tubules) that surrounds the tubules also is cornified tissue that grows from the secondary laminae, forming a network with the tubules.
The three basic layers of the hoof wall from outside to inside are the stratum externum, stratum medium, and stratum internum. The stratum externum is only a few millimeters thick and is somewhat rubbery near the coronary band and gives the exterior of the foot a "waxy" look. The purpose of this wax-like coating is to prevent dehydration of the hoof structure. It is important not to remove too much of this protective layer. Stripping of this protective layer can be caused by caustic chemicals, such as turpentine used to remove hoof polish, or excessive removal of the material by rasping during the shoeing/trimming process.
The bulk of the hoof wall is made up of the pigmented horn tubules of the stratum medium.
The non-pigmented stratum internum consists of the approximately 600 laminae that interdigitate (connect in a hand-in-glove-like or Velcro-like means) with the sensitive laminae of the laminar dermis.
The dermis of the sole is firmly attached to the undersurface of the coffin bone and produces a mixture of horn tubules and intertubular horn. The junction between the sole and the wall is the so called "white line" (zona alba). The white line includes some of the non-pigmented stratum medium of the wall, the distal ends of the horny laminae, and, between these, pigmented horn produced over the terminal papillae of the laminal dermis.
There are several nutritional factors that are important to normal hoof growth. The protein structure of the hoof is loaded with sulfur-containing amino acids, so elemental sulfur is important to normal hoof growth. The sulfur-containing amino acids methionine and cystine are two of the more important ones. In addition to sulfur protein, calcium and the vitamin biotin are very important to normal hoof growth.
There are many supplements on the market specifically formulated to promote normal or "better" hoof growth-many containing methionine, biotin, sources of sulfur, and other trace minerals important to the enzyme machinery that makes hoof material. I must note here that it has never been scientifically demonstrated that supplementation with any of the necessary building blocks for healthy hoof will increase hoof growth or its quality in horses which are receiving the required amounts of all of the essential components from adequate nutrition.
Whether the nutritional requirements of your horse are adequate with respect to these essential components is another question.
There is strong evidence that failure to meet the nutritional requirements with respect to protein and other essential components will have a negative effect on hoof growth. It has been shown that in weaning-age ponies being fed a nutritionally deficient diet, there can be as much as an 80% reduction in hoof growth.
The mature horse's foot grows an average of 0.25-0.35 inches per month. This growth rate varies seasonally. Growth is slowest when the weather is the hot or the cold.
"Shelly feet" is a vague term describing a condition where there are a brittleness and hollowness to parts of the hoof wall. There are other conditions that sometimes happen concurrently with shelly feet, including "seedy toe" and "white line disease." If the composition of the hoof wall is abnormal (such as from nutritional deficiencies), it cannot withstand the forces that the normal hoof is intended to withstand.
The hoof is constructed to be a pliable structure and has elastic and displacing qualities that allow the foot to deform under force in an effort to allow it to withstand those forces. In addition, the physical movement that the hoof experiences while under load is one of the major ways that blood is pumped throughout the structures of the foot.
Inappropriate foot care with respect to shoes and trimming can have a great influence on how well the foot deals with normal work. Many domesticated horses do not wear down their feet at a normal rate due to inactivity, which makes trimming necessary. If the feet are not kept appropriately trimmed, the wall length will become excessive and increase the forces displaced up the front of the wall when the horse is in motion. This increased force can have an effect on the structure of the hoof wall as it grows down and can lead to problems such as abnormalities in the horn structure.
Shoes have become necessary for many horses, but the shoe will have an effect on how the hoof displaces under load. Failure to keep appropriate shoeing intervals can lead to abnormalities in hoof structure.
Therefore, the overwhelming factors that contribute or detract from a healthy foot are nutrition and management. In many horses with shelly feet, at least in the early stages of the problem, there is no lameness, and the part of the hoof wall affected is the stratum medium layer. The early subtle changes generally are a loss of normal integrity of the horn structure starting at ground level and working up toward the coronary band.
Other diseases causing founder or laminitis will have an effect on the integrity of the hoof wall, causing damage to the deeper layers of the foot due to a loss or decrease of the blood supply. This loss of integrity can lead to various degrees of separation of the different layers of the hoof wall structure, infections, "cracks" in the wall, and, eventually, lameness.
White Line Disease, et al.
This process occurs in varying degrees and generally starts out as a gradual loss of hoof structure integrity from one or many different causes. White line disease has been a highly debated issue for a number of years and has been a source of contention among veterinarians, farriers, and horse owners. 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 lower centimeter or so of the wall, but often extends upward and can be as high as the coronary area. The term "seedy toe" or "shelly" 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" is probably 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 innermost zone of the stratum medium can be un-pigmented and can, therefore, appear white in color when viewed from the solar surface.
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, excessive toe length, and nutritional deficiencies as previously mentioned.
A great number of bacteria and fungi have been incriminated in the development of the disease and loss of hoof wall integrity, but sample collection techniques are either unreported or can be criticized regarding contamination of the samples. Farrier Bernie Chapman showed there were 25 different species of fungi incriminated in affected feet. The species Pseudallescheria and Scopulariopsis are two of the more common isolates. There is one report in veterinary literature that demonstrates, 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, to what degree are the predisposing factors causing a weakness in the integrity of the hoof wall involved?
A prospective clinical study currently is underway at Cornell University attempting to answer some of these questions. Our hypothesis is that the term onychomycosis, as offered by Chapman in much of the existing literature, is appropriate with the disease's being a fungal infection of the hoof wall secondary to mechanical stresses related to poor management or other disease processes.
All horses presented to our farrier service are evaluated for the clinical signs of "white line disease," "seedy toe," or "shelly feet." If feet are identified with an irregular white line area (consisting of thickening, an abnormal percussion up the wall, and/or a dry or chalky texture) and consent is granted, the horse is selected for the study.
Five feet have been evaluated thus far in this program, with the horses' average age being five years old (ranging from one to eight years). The feet were of four geldings and one mare. Clinical signs at presentation consisted of the aforementioned horn quality in the white-line area and variable horn cracking, lameness, and hoof wall separation. We also selected four horses which were determined to have completely normal feet.
An attempt was made to locate the leading edge of the lesion by using hoof wall concussion. In all suspected cases, there was a distinct hollow sound to percussion that disappeared half the distance or greater from the sole to the coronary band. It was this junction that was assumed to be the leading edge of the lesion and was chosen for sample collection. An attempt was made to sterilize the hoof wall with formaldehyde prior to sample collection. Then, a 1 cm diameter portion of the stratum externum was removed with a sterilized dremel bit and formaldehyde application was repeated, with a final swabbing of concentrated iodine placed in the created wall defect.
The hoof wall and pastern were covered with an iodine-impregnated surgical sticky drape and a small window was cut out over the created wall defect. Sterile surgical gloves were worn. At that point, sterile bone curets were used to remove the first few millimeters of the stratum medium, which was discarded. Then approximately 50-70% of the stratum medium was removed from the hole. The stratum medium collected for culture was dry/chalky and not considered to be of normal texture. The material was transferred into a sterile container for culture.
The four control feet did not grow any bacteria or fungi. The five affected feet did not grow any bacteria, 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 the fungi are known to have the primary ability to cause disease, although the fungus 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, and Gliocladium sp (in two feet).
We feel that because all of our controls were negative for bacterial and fungal growth it validates our sample acquisition as a methodology for collecting sterile samples of the stratum medium for culture. We also feel that this was 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 in culture 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.
It was difficult to determine if these horses had inappropriate management of the feet in the year preceding our evaluation. All were show horses and were not reported to live in environments that would predispose to loss of hoof integrity (excessive contact of the feet in water, muddy pasture, areas of excessive urine/fecal contamination). Although the horses appeared to be in good health, their specific nutritional status was unknown and no testing was performed to determine if there were any nutritional deficiencies-an area necessary to explore, but limited by financial constraints in this study.
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 certain bacterial infections, but 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. In addition, it has been demonstrated that the application of some of the counterirritant products (i.e., Reducine) applied to the coronary band will increase hoof growth. One study that applied the product once every three days reported a 21-41% increase in hoof growth rate. The stimulation of hoof growth might reduce the recovery time. Nutritional management also should be evaluated and corrected if thought to be deficient.
As with most problems, prevention is the key. Therefore, evaluation of nutritional and hoof management might help you prevent these problems, or at least catch them before they become serious.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.