Thrush is very common, and it typically is a mild disease that can be easily treated. More importantly, it can be prevented with adequate foot management and good stable husbandry. However, if you choose to ignore these preventive measures, or you come into possession of a horse with thrush, it needs to be addressed immediately and aggressively as complications and chronic lameness issues can result. Thrush can be very insidious in the early stages, and it can become severe enough to cause permanent lameness.
First, let's discuss some descriptive anatomy of the foot to better understand the problem. When looking at the foot from the bottom, the hoof wall circles from the lateral (outside) heel around the point of the toe (the dorsal surface) and on to the medial (inside) heel. The frog is the brownish/ black rubbery-textured triangle with the flat base at the heel and the point two-thirds of the distance to the toe (see page 92).
The remainder of the whitish structure on the bottom of the foot is the sole. The frog has a central invagination or groove called the central sulcus, and the deep grooves at its junctions with the sole are the lateral and medial sulci (sometimes called the paracuneal grooves). The tissue of these structures is considered epidermis (skin). As it grows out toward the surface of the foot, it goes through the process of keratinization (toughening to a horn-like consistency), making it firm and durable. The frog is softer than the sole because it is not completely keratinized. The keratinization process basically is the conversion of the live (and therefore sensitive or pain-feeling) deeper layers of the foot's epidermis to the dead (and therefore non-sensitive) superficial layer.
Keratin is a protein that's rich in sulfur and is the principal constituent of skin, hair, and nails. The keratin tissue of the foot is also often referred to as the horn tissue. The deeper layer of the bottom surface of the foot that produces the keratin tissue is called the corium. The corium of the sole is attached to the bottom of the coffin bone, and the corium of the frog is attached to a thick, spongy layer of tissue called the digital cushion.
The digital cushion is a golf ball-sized mass of connective tissue that is responsible for shock absorption and is extremely important in blood circulation throughout the foot. As the horse's weight compresses the digital cushion (by compressing the frog in contact with the ground), it actually functions as a pump for blood circulation within the foot. Beneath the digital cushion is the deep digital flexor tendon passing on its way to attach to the coffin bone, and deep to that are the navicular bursa (the cushioning sac between the tendon and navicular bone) and the navicular bone.
Where Thrush Strikes
With routine thrush, the areas of concern are the central, lateral, and medial sulci of the frog. Thrush is a bacterial infection of this tissue. The clinical signs of thrush are disintegration of the tissue, an extremely offensive odor, and a grayish-black discharge. If the deeper layers of the foot are affected by the infection, bleeding, inflammation of the deep flexor tendon, inflammation of the navicular bursa, and lameness can be additional signs.
No one bacteria specifically causes this condition, and if an infected foot is cultured, many different bacteria are grown. The bacterium Fusobacterium necrophorum is a common player in these infections, known to be keratinophilic (keratin-loving), and has the ability to infect, invade, and digest keratin. The foot is hardly a sterile environment, so this is an opportunistic type of infection where the bacteria are there in the environment all the time and are just waiting for conditions to be right to start an invasion.
The best tool for preventing and detecting thrush is the common hoof pick--poor foot cleaning is a common cause of thrush. Stabled horses should have their feet cleaned at least once daily. For pastured horses, weekly foot cleaning should be a minimum, and more frequently is preferable. Timely farrier care is also an essential factor in preventing thrush, as it often is the farrier who first discovers the traces of early thrush. You should respond immediately if your farrier says your horse has or is developing a thrush problem.
There are many factors that can predispose a horse to the development of thrush. Individual conformation can play a role as horses with deep, narrow sulci and/or narrow/contracted heels are more likely to develop thrush because the deeper cavities don't self-clean with horse movement as well as shallower, wider ones. Water and manure are also factors in the environment that can predispose a horse to thrush. Basically, the longer the contact time between the manure/muck (major bacteria population) and the deep frog/sole tissue, the greater the chances are for tissue invasion and the development of thrush.
Management practices that keep horses' daily living areas as dry and manure-free as possible will minimize these risk factors. Frequent stall cleaning, pasture cleaning, and ensuring good pasture drainage will also help prevent horses from developing thrush. Keep in mind that regular exercise also helps prevent thrush; with movement of the foot comes some natural cleaning. "Weekend warriors" are often the victims of thrush, especially if they live during the week in a stall or small paddock.
There is no magic bullet for preventing or curing thrush. There are numerous commercial products on the market and many "home-brew" concoctions, but there is no strong evidence that any one is significantly superior to another. Products sometimes are used as daily preventives. I typically only recommend the use of something as a preventive if there are predisposing factors, because the chronic use of many of these products will dehydrate and excessively harden the sole and frog.
What You Might See
In mild to moderate cases of thrush, there is no lameness because the tissue affected is not living. However, as the infection progresses into the deeper layers of the foot, the bacteria begin to involve the sensitive corium and cause pain. If the thrush has gone unnoticed until this point, the first signs might be lameness, increased digital pulses, and even leg swelling up to and above the fetlock. These signs could be caused by a foot abscess or other inflammatory conditions, but the possibility of thrush should always be explored.
The superficial bottom of the foot can appear normal if the crevices of the sulcus are deep within the tissue of the frog, masking abnormal tissue. If the horse has contracted heels, there can be a big crevice between the heel bulbs where infection can hide.
Care must be taken when cleaning the hoof--you aren't "digging" into the tissues to clean. But you need to be sure that the deepest areas are free of manure and debris. For horses with contracted heels, I use a piece of gauze like a strand of dental floss between the bulbs to clean this area.
One of the most important aspects of treating thrush is having as much infected tissue trimmed away as possible by your veterinarian or farrier. No anti-bacterials (even pure antibiotics) will work in an environment contaminated with organic material and necrotic (dead) tissue. Having the infected tissue trimmed will also widen the cracks of the sulcus, allowing easier cleaning and treatment. In addition, opening up the areas will allow greater penetration of air/oxygen. Many of the bacteria responsible for these infections require a low-oxygen environment, so providing air circulation is an important part of treatment.
After debridement of the foot, one of the products for treating thrush should be used, strictly following directions. Your veterinarian should guide you on this choice, as some chemicals might be too harsh for your particular case and actually cause damage.
One of my favorites is a 10% diluted solution of chlorine bleach applied to the cleaned foot two to four times a day depending on the severity of the thrush. Remember, just because you are using something from your household doesn't mean it is harmless. I also use one of the penicillin-type cattle mastitis medications topically on the affected foot tissue.
It's tough to evaluate is when treatment is working. Thrush doesn't clear up instantly and unequivocally. The best indicator of improvement is the disappearance of the characteristic stench and black discharge. Sensitivity to cleaning of the hoof is not always a good measure of treatment success.
Again, remember that some products contain strong chemicals and can cause inflammation with resulting soreness (more on this later). Continuing treatment in these cases causes irritation from the medication in addition to the initial infection. You can put these chemicals on a healthy foot every day as a thrush preventive without much trouble (but they'll dry out the foot). However, on a foot that's damaged with sensitive tissue exposed, these chemicals can cause burning/irritation.
I recommend treating thrush with chemicals for no longer than seven days, then having your veterinarian re-evaluate the foot after a one- to two-day treatment break.
The more invasive the infection, the more intensive treatment will need to be. For most routine cases of thrush, systemic antibiotics are not indicated and would be of little clinical value since the infected tissue does not have a direct blood supply to get the antibiotics to infected tissue. Not using an antibiotic in this situation is important due to growing bacterial resistance to antibiotics and the relatively limited number of them we have to use on the horse. We don't want to waste them in a situation where we know they aren't of any value.
In more advanced cases of thrush, your veterinarian might recommend soaking the foot in various medicated solutions; I will often use a 10% warm epsom salt (magnesium sulfate) soak or 25% chlorine bleach for 15-20 minutes daily for five to seven days. Take care to avoid excessive contact of the coronary band with these medications/soaks, as they can irritate the skin. In addition, packing the sulci with medicated cotton and covering them with a bandage is necessary for more advanced cases. This will keep some medication in contact with the affected tissue and keep the foot clean between treatments. I have found that a foot-covering product such as an EasyBoot works well for this purpose. Care must be taken not to apply any treatment that could irritate the skin in the heel bulb area.
One complication that I frequently see is over-treatment. There are many products that contain relatively high concentrations of antiseptic dyes, copper sulfate, potassium permanganate, various alcohols, chlorine compounds, phenol, and even formaldehyde. These compounds can be useful in treating thrush, but care must be taken not to cause chemical burns.
If pain can be induced when picking out the foot, or lameness persists for longer than three to five days, you should have the foot re-evaluated by your veterinarian.
Typically, if the affected foot tissue can be made to bleed when cleaning or there is lameness and/or lower leg swelling, systemic (oral, intramuscular, or intravenous) antibiotics will be part of the treatment. When thrush induces lameness, it is invading the deeper layers of the foot, which can cause significant complications if not treated aggressively.
In very severe cases of thrush, the infection can invade the digital cushion, the deep digital flexor tendon, and even the navicular bursa--all of which greatly increase the difficulty of treatment and the risk of long-term lameness.
Treatment of these cases will involve systemic antibiotics, various degrees of surgical debridement of the frog/sole, and likely the application of a "treatment plate" shoe. This shoe has bolt holes on the bottom so that the plate covering the foot can be removed. This allows treatment material to be held in place and foreign materials to be kept out.
This severity of thrush is relatively uncommon, but it does happen and should remind us that even mild cases of thrush should be taken seriously.
Thrush is typically a mild disease that can be easily treated, but more importantly, it can be prevented with adequate foot management and good stable husbandry. With the occasional case developing complications and chronic lameness issues, it is well worth having a hoof pick hanging on the front of every stall, using it daily, and using it well.
WHERE DID THAT NAME COME FROM?
"Frog" and "thrush" both appear to be derived from "frush," which was a term used by Blundeville, the first major English-speaking author of farriery treatises (1566). Blundeville appears to have corrupted the term from the French "fourche" (fork)--a descriptive indication of the frog's division as it moves toward the heel.
This information/explanation is documented in Henry Heymering's annotated bibliography on farriery On the Horse's Foot... (published in 1990 by St. Eloy Publishing, Cascade, Md.).
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.