Thrush in Horses

A great many farm and ranch boys and girls of my generation grew up with the time-worn admonition that "Cleanliness is next to Godliness." Usually, the words came from a determined mother who was shepherding her brood toward the bathtub. There was a good reason for mothers to be determined that their young charges cleanse hands and bodies after a busy day in the dirt, dust, and animal waste of a farm or ranch. Bacteria could cause any number of problems, and some of the powerful antibiotics we know and take for granted today were still on the horizon. Cleanliness was, and remains, a solid defense against bacterial infection.

Nowhere in the equine world is this more true than with the nasty and potentially dangerous affliction called thrush. If one were to compare thrush with a human condition, it would be more like athlete's foot than anything else.

Athlete's foot certainly isn't life-threatening, although it can be extremely irritable. Thrush is not life-threatening either, but if left unchecked, it can cause some serious foot problems in the horse.

The location on the foot where thrush bacteria (many species, but most commonly Spherophorus necrophorus) normally attack is the frog. Specifically, the point of attack often is the sulcus or commissure -- that groove along the frog between it and the bars. For some reason, thrush seems to show up in the rear feet more frequently than in the front. One reason could be that the horse carries more of its weight up front than in the rear. Thus, there would be more concussion in the front feet that might jar loose manure and dirt lodged next to the frog.

When manure or moist material is lodged against the frog for an extended period of time, the anaerobic bacteria flourish. They attack the frog and begin a destructive process that can have dire consequences if left unchecked. The thrush bacteria will "eat" into the frog, rendering it incapable of playing its role as one of the shock absorbers of the foot. The product of the infection is usually moist, black and very strong-smelling. Anyone who has ever had the odor waft past his or her nostrils will never forget it.

The good news about thrush is that, unlike many other equine maladies, it is usually preventable. In general, all it takes to prevent thrush is cleanliness on a regular basis. The reason this is true is that thrush is caused by bacteria that can thrive only in an anaerobic atmosphere. This means that the bacteria can live only in an environment where oxygen is not present. Once oxygen arrives on the scene, the bacteria die.

However, thrush is not always so easy to eradicate. Long-standing, severe thrush infections are hard to get rid of because the bacteria have eaten away a substantial amount of healthy tissue, leaving abnormally deep sulci next to the frog. These deep sulci are even more hospitable to the thrush bacteria than the healthy, shallower ones were, and are also harder for us to clean and medicate thoroughly.

In these long-term or severe thrush cases, the bacteria haven't stopped at just carving deeper sulci, however; they may have also eroded the frog from the sides of those sulci and the central sulcus. This erosion can allow the bacteria to invade the sensitive structures of the hoof underlying the frog, causing lameness. One re-sult of this lameness can be an altered gait (fa-voring the sore hoof or hooves) that reduces concussion and thus knocks less dirt and manure out of the hoof, perpetuating the problem. There is no research on this phenomenon, but practicing veterinarians see this frequently -- the lame foot staying packed with more dirt.

If the infection is not extensive, has not been present for a long period of time, and the horse has no predisposing factors to thrush (more on that later), treatment is not difficult. It begins with cleaning out the packed substance that allowed thrush to get a start in the first place. If the infection is in the very early stages, this might be the only treatment required. Keeping the foot clean allows oxygen into the afflicted area, which kills the harmful anaerobic bacteria.

If the infection has a good head start, one might want to trim away the infected portion of the frog in order to get rid of the bacteria immediately. However, unless you have had some farriery experience, you should not take a hoof knife and begin paring away the frog. It's better to leave this to a veterinarian or farrier. The danger is that you might cut away too much of the frog and wind up doing more harm than good.

A treatment protocol advised by many veterinarians is the use of diluted Formalin applied topically to the affected area, says Glen Gamble, DVM, of Riverton, Wyo. This is a commercial substance with formaldehyde as the base ingredient. It not only kills the bacteria, he says, but also serves to harden the foot. This drying effect provides a less hospitable environment for the bacteria.

Other treatments that have an astringent (drying) effect can also help kill the thrush bacteria and make the hoof a less hospitable home for them. Some of these include treatments based on copper sulfate and tincture of iodine.

The problem with untreated thrush is that the bacteria can wind up penetrating the sensitive structures of the hoof.

We should pause to take a quick look at how the hoof is constructed so that we can better understand how something as simple as thrush can cause such serious problems.

Anatomy Of The Foot

The hoof is divided into sensitive and insensitive structures. The hoof wall is insensitive, meaning that it does not contain any blood vessels or nerves. The portion of the foot that it encompasses and protects contains many blood vessels and nerve endings. These sensitive structures within the hoof wall also provide nourishment necessary for hoof growth and function. Any insult or damage to these sensitive structures can bring intense pain to the horse, and a diminished ability to perform.

A few anatomy terms can help us understand hoof construction:

Hoof wall -- This insensitive structure is constructed in three layers, the stratum internum, stratum medium, and stratum externum. The hoof wall is about 25% water, and serves as the primary bearer of the horse's weight. Hundreds of interlocking thin membranes called laminae attach the sensitive inner portion of the hoof to the hoof wall.

Ground surface -- The ground surface of the hoof wall is divided into the toe, quarters, and heels. At the heels, the wall turns forward to form the bars that converge toward one another. The sole, comprising most of the ground surface of the hoof, conforms to the inner curvature of the wall and to the angles formed by the wall and the bars.

Sole -- The structure of the sole is similar to that of the wall, with tubules running vertically. These tubules curl near the ground surface, which accounts for the self-limiting growth of the sole and causes shedding. The sole is not designed to bear weight from the ground surface, but is designed to bear internal weight.

Frog -- The frog is a wedge-shaped mass occupying the angles bounded by the bars and sole. The frog is divided into the apex, which is the forward-most point; the base, which is the wide rear portion of the frog; and the central sulcus, which is the central groove. The frog also covers the sensitive digital cushion. The frog is 50% water and quite soft.

The walls, bars, and frog are the weight-bearing structures of the foot, with the majority of the weight borne by the walls.

These aren�t all the structures of the hoof, but they are the ones most important to our discussion of thrush. Suffice it to say that these outer structures protect the inner hoof, which is extremely sensitive and complicated. It is not designed to handle insult of any kind, especially an invasion by bacteria. Thus, we have to keep the outer structures of the hoof in good condition to deny bacteria entrance to the sensitive areas.

Who�s At Risk?

Horses most apt to develop thrush are those confined in a moist area, such as a dirty box stall or a paddock that is often muddy and filled with trampled manure. Even under those adverse circumstances, however, thrush can usually be prevented if the hooves are cleaned daily. Simply using a hoof pick to clean out debris often will prevent the harmful bacteria from setting up shop.

However, if the conditions in which the horse is kept are so dirty that a horse's hooves get packed with damp manure and mud the moment he returns to the stall or paddock, a daily hoof cleaning might not suffice. Cleaning and drying up the horse's environment might be required to prevent thrush from developing. There is no substitute for cleanliness and good housekeeping in stalls and paddocks.

Geography and habitat can also figure into the equation. It would be rare, for example, to ever encounter a case of thrush among the wild horses which roam desert-like rangelands in the west. The lack of moisture, plus traveling over hard ground that tends to dislodge dirt and manure from the foot, serve as basic deterrents.

It would be the same for domestic horses in that type of environment. They would be far less likely to develop thrush than would a horse in Florida, for example, where the ground is often soft and moist.

A moist and/or dirty environment isn�t the only predisposing factor to thrush, however. Certain hoof conformations can also invite this infection, such as feet with naturally (or shoeing-induced) deep sulci. These deep, narrow grooves next to the frog do not clean out as easily with normal gait concussion as the wider, shallower ones in normal hooves, allowing dirt and manure to remain packed in them longer. This, of course, makes it easier for bacteria to gain a foothold (pardon the pun).

Chronically lame horses might also be more at risk for thrush on the lame leg(s), as they tend to avoid the hoof impact with the ground that normally helps knock dirt and manure out of the hoof.

Bacterial invasion of the sensitive structures can have serious consequences. Once thrush has become established, ongoing cleaning and treatment is required to make certain that the harmful bacteria are killed and that there is no new invasion of the healing tissue. This might mean shoeing the horse with a pad or even a removable plate for protection.

However, the key point is that such treatment should never be necessary because of thrush. This problem should never be allowed to establish itself in a healthy foot, and, even if it does, it should be cleared up quickly before it has an opportunity to penetrate to the sensitive inner hoof.

Cleanliness might not be next to Godliness, but it certainly can prevent thrush from occurring.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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