The white line is the area on the underside of the hoof where the outside of the hoof wall meets the sole. Despite its name, the white line is actually more of a cream colour. White line disease (WLD) is an infection that causes progressive separation of the hoof wall. Although it may start at the white line, it actually wreaks its harm in the internal structures of the hoof.
Damage to the white line at the toe, quarters or heel creates an entry point into the hoof for fungus and/or bacteria that are commonly found in the natural environment, but go wild in the damp, dark inner reaches of the hoof. As the layers of the hoof wall separate, the infection spreads. In its early stages, white line disease is not painful, but if left to develop, causes severe lameness and, at its worst, rotation of the cannon bone.
“There is a great deal of disagreement regarding the causes,” said farrier Cole Henderson, who sees a lot of WLD in this Vancouver Island practice. “Historically, it was always seen as a sign of poor stable management.”
One of the foremost authorities on WLD, the late American farrier Burney Chapman, also initially thought WLD only struck horses that lived in poor paddock or stable environments. Eventually, however, Chapman questioned this belief, having encountered many cases in horses that lived in clean, well-maintained stable environments.
After noticing a significant increase in white line disease cases in both the US and the United Kingdom in the mid-1980s to early 1990s, Chapman asked farriers to send samples of affected hooves to be analyzed at Mt. Sinai Medical Centre. Based on the results, he defined WLD “as a fungal infection of the hoof wall secondary to mechanical stresses related to poor management or other disease processes.” He referred to the disease as onychomycosis (which, in human medicine, is a fungal infection of the fingernails and toenails).
“The common denominator of the disease is high humidity or a high moisture environment. The guilty organisms are opportunistic and invade quite rapidly when conditions become suitable,” said Chapman in his report, The Outbreak of Equine Onychomycosis (White Line Disease in the U.S. and the U.K., 1985 through 1992.)
Chapman’s theory now prevails among veterinarians and researchers, but it has still not been proven which fungi are to blame. Many experts suspect it’s a variety of combinations of fungi and other opportunistic microorganisms such as bacteria and yeast, that begins the complex biological processes that break down the hoof wall.
This could also explain why the disease presents differently from horse to horse and why certain treatments that are effective in some cases don’t work in others.
“Whatever the cause,” said Henderson, “the fungus thrives in a damp, anaerobic environment and when that is present, that will start it off. Having said that, there is research around now that claims that there are aerobic strains that have not been ruled out.”
The primary mechanical stresses that allow the infection to take hold in the first place can include: poor hoof conformation, concussion on hard ground, long toes/underrun heels, ill-fitting shoes, trauma (even from horseshoe nails), overweight horses and prior conditions such as abscesses and laminitis.
Dr. Susan Kempson, of the Edinburgh Veterinary School in Scotland, has researched the effects of nutrition on WLD. She suggests either too much or too little of certain nutrients in a horse’s diet can break down the hoof wall making it easier for WLD to establish itself.
WLD from the Inside
WLD mainly attacks the stratum medium, the dense, non-pigmented middle layer that makes up the bulk of the hoof wall. Advanced white line disease, however, can reach into the stratum internum, the inner hoof wall that joins the hoof capsule to the coffin bone, despite the fact this part of the hoof is usually quite infection-resistant due to its oxygen-rich lamellar tissue that inhibits and battles off invaders. The outer hoof wall, or stratum externum, is not usually affected by WLD.
The hoof wall is comprised of parallel strands of dead, hardened cells that protect underlying tissues called keratinized epithelial cells. They grow downward from the coronary band.
When the hoof undergoes mechanical stresses, separation between the strands of epithelial cells can occur. The area of separation then fills with rocks, dirt, manure, etc. that contain the offending bacteria/fungi. Every time the foot bears weight, the material exerts more pressure on the epithelial cell strands. That, in turn, causes further separation and more material enters. The damage process continues until, eventually, the coffin bone is affected and, in the very worst circumstances, begins to rotate.
Diagnosis and Treatment
Caught early, WLD is reasonably treatable. It is usually first detected by a farrier during routine trimming. “A knowledgeable horse owner would recognize the symptoms, but many don’t,” said Henderson. “There is nothing to beat early diagnosis and early treatment.”
The first indication is a visual separation and softening of the sole and the wall at the white line when the hoof is viewed from the bottom. When probed, the area will look like dry, crumbly cheese often with a grey or black tinge, revealing the separation of the hoof layers. The affected area might sound hollow when the hoof wall is tapped on. Sometimes there is a sharp odour. If more extensive damage is suspected, radiographs will show the extent of the hoof wall separation.
Attending to advanced cases will prove a lengthy process that may involve a veterinarian as well as the horse owner and farrier. This might involve debriding, or paring away all of the infected tissue, to create an environment where the microorganisms can’t re-establish and flourish. In some instances, the hoof wall over the affected part will be removed.
In this case, for the year or more while the hoof grows out, extra support will be required. Applying egg-bar or heart-bar shoes, with or without a bevel or rocker, are often used to take pressure off the toe and promote proper breakover. Special casts, boots or shoes might also be necessary, particularly if a large amount of hoof wall has been removed and the shoe can’t be attached by nails. Henderson has found success with glue-on shoes. “Expensive treatment,” he noted, “but it works very well when owners let me use it.”
After the infected tissue has been removed, topical anti-fungal and or anti-bacterial solutions are prescribed. Some of these include iodine, bleach, hydrogen peroxide, benzoyl peroxide, copper sulfate, formaldehyde, even antibiotic ointments, athlete’s foot cream and honey. A host of commercial products are also available. Horse owners must discuss what products/materials to use with their veterinarians as damage can result from indiscriminate use.
Some practitioners suggest packing the area with medicated dressings or with an acrylic patch in larger spots, but others maintain the resulting damp, dark area will cause infection to persist.
“We live in a time when there is a great range of products to use for this, but as farriers we are still limited by how much the owner is willing to spend on treatment,” said Henderson. “There are products that will keep a horse completely sound through the process of re-growing the hoof, or we can go cheap and have the horse out of commission for several months.”
Of course, correcting the issues that caused the mechanical stresses in the first place is critical.
Other after care can include an exercise program, consisting of light walking or riding, to promote blood flow to the area and hasten healing and hoof growth. Diligent attention to keeping the feet dry and clean both in the barn and on turn out is important too.
Horse Owner’s Responsibility
White line disease treatment is as much of a mystery as the condition itself. It is frequently noted in the literature that results are widely variable. But Henderson has another idea why WLD treatments work in some cases and not others.
Much of the recovery process is up to the owner, he said. And it’s hard for farriers to rely on the data they receive from owners how a treatment is working. “In a word, some horse owners lie to us. I suspect not for any other reason than they are embarrassed to admit sometimes that they have not carried out the course of treatments we have asked them to do. It makes it very difficult to say what works effectively and what doesn’t. Someone can have great success with a product and another will say it didn’t work,” he said.
He said it is rare that the treatments he prescribes don’t work if they are applied regularly and properly and the owner attends to suggestions he has made regarding cleanliness and stable management. “I see attentive, good horse owners having success in treating this problem 99 per cent of the time, and I see the less-attentive owners spin me stories at every trim session as to how they have no idea why there is no improvement. It can be quite frustrating, but we can’t judge, as people all lead busy lives and they do try their best.”
In addition to owners following through on treatment, it’s also important that the horse be examined regularly by a farrier to ensure the infection remains at bay because white line disease does tend to reoccur. “I always advise preventative treatments ongoing as it always seems to come back,” said Henderson.
Prevention of white line disease
Prevention can go a long way to ensuring white line disease doesn’t strike a horse in the first place.
Keep stalls and paddocks as dry as possible. Pick feet daily and be aware of weather conditions – i.e. double hoof hygiene and stable cleanliness efforts when it’s rainy and wet.
Ensure the horse’s diet is nutritionally balanced. Dr. Kempson, the nutritional expert from Edinburgh’s veterinary school, recommends a high-fibre diet with a ratio of 1:6 parts calcium to phosphorus; alfalfa for horses who have a hard time absorbing calcium; avoiding too much of vitamins A and D and selenium and carbohydrates (especially from molasses-based commercial feeds). Add a commercial hoof supplement, if necessary. Dr. Kempson said those containing biotin, zinc, iodine and methionine help stimulate strong hoof horn. (See the March/April 2013 issue for information on hoof supplements.)
Trim or reset shoes on a regular schedule to avoid strain caused by imbalances. This also offers the farrier an opportunity to find and begin treating an infection early should white line disease have set in. “The worst cases we see are owners who do not keep to regular trimming schedules or owners whose trimming schedules are greater than every two months. If a horse is trimmed regularly, every six to eight weeks, and the owner is on top of it, this is never a problem,” stressed Henderson. “And if it is treated early it is easier to fix. Just a little time spent in the early days can make a big difference.”