Seedy-toe and white line disease (WLD) are simply different names describing similar infections. Seedy toe and WLD look the same and involve the same opportunistic fungal and bacterial invaders. Neither are primary hoof diseases; both are secondary problems caused by mechanical hoof problems or whole-horse health issues.
The terminology doesn’t matter so much as understanding the primary causes, so read on.
What causes the infection?
There are lots of theories, but no absolutes.
WLD can affect horses of any age, breed, or sex. One hoof or all can be diseased as well as one horse amongst a herd. It is believed that soft, wet feet are more prone to infections of the hoof wall; yet even horses in arid conditions suffer. Poor management is often cited as a cause, yet infected hooves can be found in well-managed horses.
The only consensus I have found between hoof authorities is that chronic laminitic horses are prone to developing white line disease.
White line disease is fungi, bacteria, or a combination of the two that break down the keratin in the inner hoof wall. These infectious organisms are considered opportunistic. It seems they have to have the right conditions to cause a problem. How do they gain entry? It seems likely that the wall/sole junction is the gateway.
Three red flags
Three primary conditions appear to set the hoof up for infection. The first is poor hoof mechanics. Long toes or wall flares really stress the connection between the hoof wall and sole. Wall separation, splits, and cracks are potential entryways to infection. Luckily, most breaches do not become problematic or infected. If infection does occur, it is usually minimal.
The second reason may surprise: It is because the coffin bone has a small notch in it. This is what I refer to as “seedy toe.” It almost always occurs at the toe and is generally confined to a specific area. It rarely causes problems, but this area will always be prone to recurring infections.
The third condition may be the most revealing, and important to address: possible laminitis. The most common cases of seedy toe involve laminitic horses. There are multiple reasons these horses are the most susceptible. The inflamed laminae form a weak connection between the hoof wall and coffin bone. Any hoof imbalances are problematic. Excess pull from long hoof walls can tear the wall from the coffin bone. These tears start out as microscopic, but accumulate over time. If damage continues, the sole gets pulled and eventually the white line stretches. Circulation can be diminished causing bone loss. All of these problems make the foot vulnerable to bacteria and fungi. Remember, WLD is a secondary infection. It does not thrive in a healthy hoof.
Hoof wall anatomy
Anatomy of the Equine, LLC
White Line Disease
The white line is the connection between the inner hoof wall and the sole. It grows from the end of the laminae.
Within the hoof is the coffin bone. In a healthy foot, the hoof walls are tightly connected to the coffin bone by the laminae.
The hoof wall has two distinct layers. The outer wall grows down from the coronary band. This is the layer of wall that is pigmented and gives each hoof its color. The outer wall is thicker and stronger than the more hydrated inner wall. The inner layer of wall is produced by the laminae and is un-pigmented (white in color). It has the very important job of attaching the outer hoof wall to the coffin bone. This layer is susceptible to opportunistic bacterial and fungal infections (WLD or seedy toe).
To make things even more confusing, there is a structure in the hoof called the white line, but it is not actually white; it is a brownish-yellow. The ends of the laminae are called the terminal papillae, and they produce the white line. The white line is the connection between the inner hoof wall and the sole.
Ironically, white line disease does not infect the white line; WLD causes problems in the inner hoof wall.
Mechanics and infection
The better the attachment between the hoof wall and coffin bone, the stronger the hoof. A good trim that keeps the walls well aligned with the coffin bone goes a long way towards preventing white line disease.
When the walls are allowed to grow too long, the connection between the hoof wall and bone is stressed.
The softer, more hydrated inner hoof wall can separate from the white line. It’s possible the white line will also stretch. This leaves the inner hoof wall vulnerable to infection. The damage is not usually pervasive in hooves with healthy laminae.
Proper trimming and diligent attention to the infected tracks generally take care of most cases of white line disease unless there is internal damage in the foot. Laminitis plays a much bigger part than most people realize.
Leverage on inflamed, weak laminae has devastating effects. Because the inner and outer hoof walls are no longer securely attached to the bone, the hoof will begin to distort. The walls will flare and the coffin bone may shift and create a gap. The laminae will compensate by producing more epidermal cells (inner hoof wall), but they will not be as organized or healthy. The white line will also grow more rapidly (stretch) to fill the gap that is occurring. As the shape of the hoof changes, even the sole can get involved. This is the beginning of a lamellar wedge. It’s bad for the hoof, but the bugs love it!
Coffin bone remodeling is prevalent in horses with chronic laminitis. In advanced cases there can be significant bone loss. Between the outer hoof wall and missing bone, there can be a mass called a lamellar wedge. The lamellar wedge is a mix of damaged tissues made up of inner hoof wall, white line and sole tubules. It becomes hard to distinguish one tissue from another. White line disease and lamellar wedges are often seen together.
It is perfectly normal for the coffin bone to have a notch in it. It is more common in front feet, but can occur in one or all four feet. The notch has a name, crena marginalis, and it is typically at the toe.
More extensive bone loss from chronic laminitis will also continually cause problems and they can be severe. WLD can create another level of damage that undermines the hoof beyond laminitis. Since bone cannot be replaced, these infections will be chronic. Finding and treating the cause of the laminitis is extremely important! Even with bone loss, controlling the inflammation can help manage the infection.
Now you know
Isolated tracks of white line disease are not abnormal. The hoof is exposed to extreme wear and tear on a daily basis, so it is understandable that the wall/sole junction would be occasionally breached. These pockets should resolve with attention.
If infection occurs in a more general fashion, multiple feet, multiple horses, this is a red flag. Either the feet are not being trimmed well or laminitis is the problem.
If you are concerned by a persistent crack or infected hollow in the hoof, a good set of x-rays can help determine if the problem is related to bone loss or poor hoof mechanics. You may have to ask your vet to take a picture of the margin of the coffin bone.
Small notches of bone loss are not particularly worrisome where as extensive bone loss requires diligent attention.
Paige Poss is a professional hoof care provider (www.ironfreehoof.com) and educator. Her passion is teaching the practical and applicable aspects of anatomy. Paige has joined forces with fellow professional, Jenny Edwards (www.all-natural-horse-care.com) to create dissection-based educational material for the professional as well as the dedicated equine owner. One of their most recent projects, Exploring Laminitis 1, uses comparative anatomy dissections to give insight to the devastating effects of chronic laminitis on the hoof: www.anatomy-of-the-equine.com/exploring-laminitis.html
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