Learn about this common condition from farriers and horse owners who have managed it

Thrush, an infection of the horse’s hoof tissue known for the black, necrotic (dead), stinky material it causes, is one of the most common hoof issues horse owners encounter. According to the 2020 American Farriers Journal Farrier Business Practices survey, more than half of farriers see a case of thrush each week during the year. Another 20% see thrush monthly, while 8% only deal with it a few times a year. For some farriers, thrush is only a seasonal concern when rains create persistent muddy conditions.

“Thrush thrives in dark, damp, warm environments,” explains Mike Isles, CF, APF-I, owner of MFI Farrier Service, near Saratoga, New York. “The bottom of the horse’s hoof is a petri dish for thrush.”

The hoof’s underside doesn’t get much light because it’s in constant contact with the ground, which is a natural host for millions of microorganisms. When mud, muck, and manure pack into the hoof and remain there, anaerobic bacteria can flourish, infecting the frog tissue and eating away at it.

Unsanitary living conditions and excessive hoof dampness are primarily to blame for thrush, so keeping the hooves clean and giving the hoof time to dry is essential.

“The biggest thing with thrush is to clean out the horse’s feet,” says Isles. “Ideally, once a day, but at least whenever you see your horse, especially if you’re using a topical treatment for thrush. Medication needs to get on the tissue, and if the hoof isn’t clean, the dirt is just soaking up the treatment.”

Thrush might sound like a straightforward condition that can be remedied with good horsekeeping skills. But mud, dirt, and manure are not the only culprits.

“Thrush is a bit of an enigma,” says Steve Kraus, CJF, head of farrier services at the Cornell University College of Veterinary Medicine, in Ithaca, New York. “It’s interesting because some horses can live in terrible conditions and never develop it, while horses in good conditions can. So, thrush is not necessarily caused by bad health care or bad conditions but aggravated by it.”

Horses that have compromised immune systems are susceptible to thrush, says Kraus. Horses with genetically weak hooves or horses that have an inability to absorb key trace minerals such as zinc, which is needed for healthy hoof tissues, might also be more likely to develop thrush, he added.

A Deeper Look at Thrush

Despite the frog’s small size, it plays a critical role in keeping horses sound. A healthy frog absorbs concussion and helps with load bearing. In healthy hooves frog tissue expands and contracts with each step, pushing dirt and debris out of the collateral sulci—the grooves on either side of the frog.

Thrush develops in any part of the sulci (the central sulcus—the groove in the center of the frog toward the heel—or the collateral sulci) when there is chronic tearing of the frog tissue, which allows anaerobic bacteria to invade, Kraus explains. Uneven hoof loading created by unbalanced conformation damages the frog tissue. A horse with an upright club foot or contracted heels can develop thrush because the deeper sulci collect more debris than those in a hoof with a flatter angle. A steep hoof angle is also likely to develop thrush due to the frog atrophying from lack of stimulation, he adds.

“I think you’re more likely to see thrush in horses with poor conformation,” says Kraus. “For instance, a horse that toes out lands on the lateral toe and then loads a medial heel, tearing the frog tissue with each step. That leaves an opening for an invasion of microorganisms. A healthy foot that loads the heel properly and stimulates good frog pressure doesn’t do that.”

Kraus also points out that thrush is distinct from another type of hoof infection called canker. While thrush is a destructive, necrotic process (the bacteria kill the tissue), canker is a proliferative process whereby bacteria cause abnormal growth of the horn, the hard, outer layer of the hoof.

Treating Thrush

Thrush can be mild, moderate, or severe. Knowing what to look for and treating at the first signs of infection is critical for preventing it from progressing further.

“My new farrier was great about showing me what to look for and how to treat it,” says New Jersey-based horse owner Heather Wallace. Her gelding battled thrush and white line disease (another type of opportunistic infection, whether bacterial or fungal, of the area just in front of the epidermal laminae, which connect the hoof wall to the coffin bone) for two years despite living in a dry lot with a sand base.

Conversely, a hoof with thrush has a gooey look to it with pitting or pockets in the frog. Thrush can also cause a frog to disintegrate and create the unmistakable odor horse owners notice when caring for affected horses’ feet.

“In the early stages of thrush, it can be challenging to identify because a pocket of thrush can look like a pocket of mud, dirt or debris,” Isles says. “In some cases, a horse will be sensitive to the hoof pick, and if the owner is really digging into the hoof, it might bleed.”

Even if you’re unsure about whether your horse has thrush, Isles recommends using a topical thrush treatment product of your choice and pointing out the condition on your next farrier visit.

“If the frogs appear too soft, or the heels look like they might be thrushy, I hit it hard for a few days (with a topical treatment),” Wallace says.

There are many topical options from which to choose. An effective one has active ingredients mixed with a stain so it sticks to the tissue, says Kraus. He prefers povidone-iodine products and recommends applying them once or twice a week in mild to moderate cases.

It is not uncommon to hear someone suggest using bleach (sodium hypochlorite), but beware because it can do more harm than good.

“Bleach kills the microorganisms, but it burns the tissue; you can do more damage to the tissue,” he explains. “However, chlorine dioxide, a stabilized version of chlorine, is a bacterial killer just like bleach but doesn’t harm tissue, and that is available under multiple brand names.”

In moderate to severe thrush cases, farriers might use copper sulfate as a treatment, but they must do it carefully. The Environmental Protection Agency¹ and the National Pesticide Information Center² caution users about health risks associated with the product.

Wallace says it took trial and error to find products that worked best for her horse.

“I use iodine from time to time but found that applying Absorbine Hooflex Thrush Remedy every few days is a great preventative,” she says. “If the frogs appear too soft, or the heels look like they might be thrushy, I’ll use Dry Cow (mastitis treatment ointment) instead and hit it hard for a few days.”

While Wallace’s horse lives in a dry pasture, it has been an extremely wet year that has made even the driest lots muddy, so thrush is rampant, requiring frequent applications of topical treatments. Her efforts are paying off. During a recent visit, her farrier complimented her on her horse’s progress.

“Routine has made the biggest difference,” she says, but it was a hurdle at first. She hired someone to pick his hooves and apply the treatment when she can’t be there. “The most frustrating thing is that no matter how hard I work, if I treat the thrush and it goes away, there’s always a chance it comes back.”

When Thrush Progresses

In most cases a combination of routine farriery and topical treatment based on the individual horse’s needs can be enough to cure thrush. But in some scenarios it becomes a complicated situation of bacterial infection invading sensitive tissues within the hoof. In these cases working with a veterinarian is necessary.

Take-Home Message

Luckily, horse owners can manage most cases of thrush with topical treatments and routine farrier care. But in some situations more intervention might be necessary. Owners dealing with thrush in their horses’ feet should contact a veterinarian if it doesn’t clear up easily.

References:

1. Reregistration Eligibility Decision (RED) for Coppers; U.S. Environmental Protection Agency, Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, U.S. Government Printing Office: Washington, D.C., 2009: 11.

2. General Fact Sheet: Copper Sulfate. National Pesticide Information Center, 2012; npic.orst.edu/factsheets/cuso4gen.html.