Controlling Thrush in Horses

Thrush in horses can cause chronic foot problems and lameness unless treated.

Thrush can lead to chronic foot problems and lameness. iStock/Nevena

The tell-tale foul odor of thrush is something no barn owner wants on the farm. Besides the odor, thrush can also lead to chronic foot problems and lameness.

The Disease

In earlier times, before people knew about bacteria, horsemen thought thrush was caused by secretions from the frog itself that collected in the clefts, making the frog moist and foul. Now we know that this “hoof rot” is caused by bacteria commonly found in barnyards and pastures.

The bacteria thrive in wet, decaying material, such as bedding and manure in dirty stalls, or in wet pens and paddocks. Several pathogens cause thrush, but most common is Fusobacterium necrophorum, which can also cause foot rot in cattle, diptheria in calves, and navel ill in calves and foals.

If a horse’s feet are frequently packed with dirt, mud or manure, the lack of air next to the frog and the constant moisture in the hoof make ideal conditions for these bacteria to flourish, rotting the frog tissue. In wet conditions you’ll always see more incidence of thrush, adds Mitch Taylor, director of the Kentucky Horseshoeing School.

The infection affects the clefts of the frog and sometimes the sole of the foot, especially along the white line. In the early stages of thrush, there is merely a little dark coloration and grime around the frog, or dark soft spots along the white line of the sole—plus the bad odor. At this point the problem can be easily and quickly cleared up by keeping the foot cleaner and drier and by applying iodine or a thrush product daily to the affected areas to kill the bacteria.

But, if the horse lives in a muddy pen, boggy pasture or dirty stall and his feet are not cleaned, they never have a chance to dry out. In that case, the condition can slowly progress to the point of lameness as infection eats deeper into sensitive parts of the foot.

One sign of the problem is when the feet are cleaned, you notice the clefts are deeper than normal. The entire frog might be undermined, with large areas becoming loose or rotten.

If thrush is long-standing and deep, the horse will flinch when his feet are cleaned or trimmed. In severe cases, there might be a discharge from the frog and the horse will be lame, with swelling above the hoof due to infection in the foot.

A neglected case can eventually affect the inner tissues, including the navicular joint.

Preventing Thrush

Prevention is the best treatment—keep the horse in a clean environment and clean the feet often.

Riding or exercising the horse regularly is helpful; if he can get out of the wet paddock or pasture and travel on dry ground, his feet have a chance to dry out—inhibiting the onset of thrush.

It is important to keep feet properly trimmed or shod so they are healthy and pliable and don’t grow too long. Horses with contracted feet often have deep clefts (grooves on each side of the frog) where dirt and mud can accumulate.

Steve Norman, a farrier in Kentucky, said that often the foot that develops thrush is one that has too much heel. “The frog is recessed down inside the foot and doesn’t get much frog pressure, and the foot also tends to collect more mud and dirt,” said Norman.

Tia Nelson, DVM, veterin­arian/farrier in Helena, Montana, said healthy feet rarely develop thrush.

Good circulation to the foot helps keep it healthy. If the foot is out of balance or has contracted heels, or if lameness keeps the horse from using the foot normally, that raises the risk for thrush. “I’ve seen thrush in dry, arid places—where you would never expect it—in feet that were not healthy,” said Nelson.

Frequent foot cleaning is important to help prevent thrush. Torn or ragged pieces of frog should be trimmed away, since they provide nooks and crevices where the bacteria that cause thrush can gain a foothold. A properly trimmed frog makes the hoof’s clefts more self cleaning. As the hoof expands and contracts with each step during exercise, this action dislodges mud and debris.


If a horse starts to develop thrush, removing the horse from the dirty/muddy environment, along with keeping the feet clean and treating them with iodine or a thrush product, should clear up the problem. But, keeping a horse on dry ground can be a challenge. It might take daily foot cleaning and care to prevent thrush.

Some horsemen and farriers use hoof hardeners such as Keratex or turpentine to dry up feet or to help harden the soles. Iodine will dry the sole, so some horsemen spread a little iodine daily over the sole and frog to help toughen and harden the sole and keep the frog disinfected. But iodine can damage the proteins in the structure of the hoof wall. It also causes excessive drying, which damages the structure and can lead to cracks.

Bruce Hague, a farrier in Vancouver, B.C., Canada, where conditions are frequently wet, described several ways to harden the hoof.

Some horsemen and farriers, he said, use a mix of iodine and formaldehyde to seal and toughen the hoof. “Formaldehyde pickles the tissue and hardens it, which helps seal it. Keratex (a hoof product for treating thrush) is similar, but more expensive, and it probably works better. There are also some hoof sealers that help repel all the moisture, acid, manure and urine. These products seal the outside of the foot and make it slicker,” and this creates a better moisture barrier, said Hague.

Kopertox is another alternative that can be used on the bottom of the foot, and, Hague said, this is often better than using Keratex or formaldehyde.

Before using any of these products, he warned, “make sure the foot is very thoroughly cleaned. You don’t want to seal in any thrush or other harmful organisms.”

Even then, be careful when using hoof hardeners, as many contain formaldehyde or acetone, which are drying agents. “Don’t overdo it,” said Taylor. “If you over-dry the foot or dry it out too fast, that can cause problems, too.”

Many horsemen, farriers and veterinarians use Mustad’s Thrushbuster to prevent/treat thrush. Glen Hause at Mustad said the ingredients include isopropanol (isopropyl alcohol), iodine complex and gentian violet dye. “The latter tells you where it’s working. Frequency of application depends on the severity of the thrush. It is applied by squirt bottle and is easy to use,” said Hause.

Tia Nelson said her favorite treatment for thrush is household bleach. “It’s cheap, effective, and a strong oxidizer. I caution people to not get any on the hairline; you don’t want it running along the crevice of the frog onto the heel bulbs, or down the front of the hoof if you have the foot tipped up sharply when applying the bleach,” she said.

“Clorox will burn the skin,” she said. “If it does get on the skin, it should be immediately washed with lots of cold water; be prepared to flush it off if you spill. I recommend applying it with a paint brush (less risk for getting too much on the foot) and some people use a small syringe so they can control the amount being applied.” With a syringe you can squirt it just where you need it and can aim it into the clefts and crevices of the frog.

Nelson said, “Bleach can be used straight, without dilution. Usually a couple applications is enough to kill all the bacteria. If you clean and open up the thrushy areas, sometimes this is enough to resolve it, but I always use bleach as well, for two reasons: It kills any pathogens that are there, and it also helps toughen the frog so it won’t be so vulnerable. I’ve seen feet so thrushy that when you use a hoof pick they bleed; the frog is soft and eaten up by the infection. Bleach helps toughen those areas.”

Protecting the Foot

Horses that wear hoof pads are susceptible to thrush, since the pad holds moisture against the bottom of the foot. Norman does not use pads on a horse unless absolutely necessary, but if a horse needs pads, it’s important to keep the covered foot from getting thrush. “You can squirt Thrush Buster under the pad periodically and hope it goes where you need it,” he said.

If a horse needs the bottom of the foot protected, he prefers to use Equi-Pak—a soft, instant pad material that’s squirted on with a gun and sets up within a minute—rather than a hoof pad. “It seals the bottom of the foot and no moisture can get in there. If you have to use a nail-on pad, you could squirt this between the pad and the foot to keep the moisture out and prevent thrush,” said Norman.

Getting Aggressive

“Where you need to get more aggressive, I often use a product called CleanTrax,” said Norman. “It’s a liquid you mix with water. You can soak the foot in a plastic bag, and this product acts as a poultice.” It tends to penetrate more deeply to clear up a bad case before it gets into the sensitive tissues such as the laminae and digital cushion.

Thrush that invades inner tissues of the foot must be treated like a puncture wound. Large areas of the frog might need to be trimmed away and infected areas opened for drainage. The horse needs a tetanus shot and antibiotics (under veterinary supervision) and daily foot soaking (20 to 30 minutes per soaking) to clear up the infection. A warm water solution of Epsom salts helps. Bandaging between soakings and a protective boot can keep dirt and mud out of the open areas until the inner tissues have healed. The cleft of the frog can be packed with cotton soaked with disinfectant solution each time you re-bandage the foot.

As with most health issues, prevention is key, and thrush is no different. By staying on top of the horse’s stabling and turnout conditions, thrush can be avoided entirely.

Heather Smith Thomas has been shoeing her own horses for 49 years, beginning at age 14 on the family ranch in Idaho. She often writes about hoof care and shoeing. Her book, “Understanding Equine Hoof Care,” was published in 2006.






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