Proportionally speaking, the frog is close to one of the smallest structures in the equine body. Yet, despite its size, it plays a central role in maintaining healthy, sound horses. A healthy frog shares in the load-bearing function of the hoof and absorbs concussion each time the hoof contacts the ground.

In healthy hooves, the triangular-
shaped frog tissue expands and contracts within each stride, pushing dirt and debris out of the crevice known as the frog sulci. The sulci crevices serve as a self-cleaning mechanism to “pop” foreign materials out of the hoof.

When the central sulci or the grooves on either side of the frog are compromised, the hoof structure is predisposed to developing thrush. When the sulci are deep and if the cornified frog has ragged edges or pockets, the frog has an increased risk for anaerobic bacteria to invade and deteriorate healthy tissue.

Unsanitary living conditions and excessive hoof dampness are largely to blame for thrush, so encouraging horse owners to keep horse hooves clean and give the hoof time to dry out is a fundamental horse-keeping skill.

“We see a lot of thrush here in Missouri because of the snow and mud with our often wet and climate conditions,” says Amy Rucker, an equine veterinarian from Columbia, Mo. “We have quite a bit of affordable land and so our horses are out a lot. The biggest place I see thrush develop is when horses stand around hay bale feeders because the footing is a compacted combination of rotted hay, manure and mud.”

More than half of farriers see a case of thrush each week during the year, according to the 2020 American Farriers Journal Farrier Business Practices survey. Another 20% see thrush monthly, while 8% only deal with it a few times a year. Another 20% indicated thrush is only a seasonal concern.

“Far too many farriers ignore it and say, ‘It’s not my issue,’” says Mike Wildenstein, FWCF (Hons), CJF, from Sharon, Vt. “They see it as a maintenance issue and blow it off, but I think it needs to be brought to the attention of the team caring for the horse. Poor maintenance destroys the integrity of the bottom foot and takes away the insensitive tissue that protects sensitive tissue.”

Unsanitary environmental conditions are not the only culprit. Horses living in the filthiest conditions might never develop thrush while those receiving “best practice care” develop infections over and over. The best key to preventing recurring cases is to encourage a healthy foot through basic trimming — and shoes as necessary — to increase the odds of a hoof fighting off an infection, says Scott Fleming, DVM, CF, of Rood & Riddle Equine Hospital in Lexington, Ky.

“If a hoof is strong and able to clean itself, or allow itself to be cleaned,” he says, “the foot has a lot better chance of fending off infection.”

With thrush being among the most common hoof ailments, farriers of all levels of expertise must be able to recognize the earliest signs of infection. Responding with trimming and treatment protocols can keep infections from progressing to more serious hoof conditions.

Most horses have frogs with a different appearance on all four feet. They are affected by the width of the heels, the strength of the digital cushion and the angle of the coffin bone in relation to the ground. The left foot has a 14-degree positive palmar angle (palmar surface of the coffin bone in relation to the ground). The central sulci mimic that angle. The narrow heels, as well as the high palmar angle, create deep collateral and central frog sulci. The ragged frog tissue is susceptible to thrush; however, this horse was housed in a meticulously cleaned, deeply bedded stall using bagged kiln-dried shavings and had no thrush. The pockets and flaps were debrided using a sharp hoof knife to help prevent thrush from developing. Feet with wide heels and zero-degree palmar angle, such as the one on the right, generally have a broad and flat frog with a shallow central sulci. They are less prone to packing material in the sulci, so they might resist thrush.

Identifying Thrush

Traditionally, a dark discoloration and sometimes a strong odor are the most recognizable signs of thrush. But Wildenstein explains that thrush is a broad term to describe an infection that can present itself in various ways, including pain.

“Identifying thrush starts with recognizing any irregularity in the sole or the frog,” says the International Horseshoeing Hall of Fame member. “If the tissue does not look normal, chances are it isn’t.”

Fleming adds, “Thrush is obvious when I pick out the foot, and it is black and cruddy. I also look to see if any normal tissue like the frog or sulci is starting to degrade.”

In healthy hooves, a frog is broad and well-formed. Conversely, an unhealthy frog is shrunken and shriveled. As the frog withers away, the sulci deepens. Organic material that accumulates in these deep grooves, creates conditions where anaerobic bacteria can thrive.

While researchers have not linked a specific organism to the cause of thrush, Bacteroides sp. and Fusobacterium necrophorum are suspect. These two anaerobic bacteria naturally occur in the environment and are commonly found in the bottom of the horse’s foot.

Thrush can be mild, moderate or severe in the level of active infection. For example, a frog that looks “ratty” or has deep cracks or fissures is mild thrush. However, when the protective outer horn is weakened, the frog tissue becomes more susceptible to penetration by the bacteria and, consequently, the development of the disease.

“Even if it is dry and looks good at the moment, but you pick up the hoof and recognize the architecture of the frog is not normal or healthy, that’s time to start addressing it,” Fleming says. “Waiting even a day or two can set back the time to heal the hoof.”

In many cases, farriers can successfully treat thrush. Fleming adds that if the infection is deeply seated in the central sulcus and sensitive tissue and involves the bursar tissue, it might be time to bring in a veterinarian.

“In these cases, antibiotics might be required,” he says. “I’ll often use an intramammary mastitis treatment in these cases.”

Rucker cautions that a horse that isn’t responding to treatment for thrush might actually have canker. When the tissue looks warty, soft and bleeds, the underlying issue is likely canker.

“I’ve only seen a couple of dozen cases myself and most horse owners never have a case of canker, but it is possible,” says the International Equine Veterinarian Hall of Fame member. “If someone thinks they are treating thrush and it is not getting better, they need to consider canker.”

Trimming Basics

While farriers can’t control a horse’s living conditions, they can influence the hoof capsule. Since an abnormal hoof capsule is another primary culprit, farriers can reduce or prevent thrush during scheduled visits. Upright feet or a club foot are more prone to thrush because the structure allows manure and dirt to pack in and trap bacteria, Wildenstein adds.

Extremely contracted heels and deep sulci increase the chances of a thrush infection. The key to healthier and stronger hooves starts with the caudal aspect of the frog and digital cushion while also mitigating or reducing negative plantar or palmar angles.

“I try to minimize the distortions and model the foot into its strongest self to maintain healthy architecture and function,” Fleming says. “It’s important to recruit load-bearing structures evenly to maintain the healthiest for that individual.”

In mild cases and depending on the horse’s conformation, rasping the heels might be enough to bring the structures back onto the same plane. In more advanced cases, additional trimming is necessary.

“In trimming, I’m trying to mirror the sensitive structures with external structures,” he adds. “At the same time, I don’t want the heels to get excessively long to the point where packing of environmental debris can happen. My goal is to improve that architecture in the back of the foot.”

The healthier the foot, the better its odds of fighting off infection and external factors such as environment. If the hoof is strong and able to clean itself or allow itself to be cleaned, there is a much better chance of fending off infection.

In the 2018 literature review, “Three common equine hoof ailments: Laminitis, thrush and navicular disease,” Jennifer A. Stoltz identifies a lack of exercise as another contributing factor to thrush.¹ Stalled horses move less, thus allowing dirt and debris to stay in the sole. In the worst-case scenario, Stoltz notes that thrush could develop into white line disease.

Treating the Infection

Encouraging a healthy hoof capsule is part of a long-term maintenance hoof-care plan. In the short-term, topical treatments might be necessary to destroy the anaerobic bacteria. Cooper sulfate has been a go-to product for many farriers.

“The cure can be worse than the disease,” Fleming says. “I routinely use copper sulfate, but it is case-specific and you have to be careful. There is a line you cannot cross. If the thrush is invading sensitive tissue, deep sulci or heel bulb, then I opt for something that is less caustic.”

While copper sulfate is a common option to treat thrush, it can pose safety risks to farriers, horses and the environment. Copper is naturally found in the human body with acceptable levels ranging from 50-120 mg. Levels higher than that pose health risks through absorption or ingestion. In drinking water, the Environmental Protection Agency limits copper sulfate to 1 part per million.²

The National Pesticide Information Center (NPIC) cautions users about the speed at which copper sulfate is absorbed into the body and moves throughout the bloodstream once eaten or inhaled.³

The compound subsequently binds to proteins and enters different organs, according to NPIC. While the excess copper is mostly excreted from the body, deposits can be found in the liver, stomach secretions, bone, brain, hair, heart, intestine, kidneys, muscle, skin and spleen. The Center’s compilation of research also finds that it takes between 13 and 33 days for only half of a large copper dose to be eliminated from the body.²

“Be aware that in granulated form, copper sulfate is caustic to your eyes, as well as the horse’s sensitive tissue,” Fleming says. “I got it in my eyes more times than I care for. You have to be extra cautious when it is windy and take extra steps to prevent self-ingestion.”

For these safety reasons, Wildenstein advises against copper sulfate.

“Systemically in the body, copper sulfate is like the pesticides and herbicides used on crops,” he says. “It alters the animal’s biome and makes digestion more difficult because some of the biome is destroyed. The compound is used to kill all the bacteria and fungus it encounters; that’s why it’s often used to sanitize poultry and cattle barns and to control bacterial and fungal growth in water.

The 1975 study, “Copper Sulphate Poisoning in Horses,”⁴ documented horses’ sensitivity to the compound. The research showed that even a single application of 0.125 g/kg body weight in 1% concentration contributes to stomach and gut disturbances such as ulcers and colic and other poisoning symptoms.

“The horse’s sole and frog are very absorbent, so we have to be very careful in what we use on horse’s feet,” Wildenstein says. “Just think if a horse stands on black walnut shavings, they will likely become laminitic quickly; that’s how sensitive the hoof tissue is to caustic materials.”

He prefers products made from turpentine, pine tar and tea tree oil — all products derived from sap or pitch from trees. If Wildenstein knows the horse owner is hands-off in maintenance, he uses packing with pine tar under a pad.

“I find I can heal a lot of these with the application of appropriate packing and a pad,” he says. “Ichthammol is one of oldest medicines known to mankind.”

Wildenstein has also found that changing pH levels in the bedding or pastures the horses live in creates environments these infection-causing organisms need to thrive. Baking soda or vinegar are two topicals that can change the pH in the foot, he explains. Liming stalls and paddocks can also significantly influence pH levels, but it must be used with discretion as lime is caustic.

The Bottom Line

Sometimes no matter how hard the farrier and owner work to clear up a case of thrush, cases can persist until the environment changes. In Missouri, horses that develop thrush in the winter will not see a clearing until May when the ground changes.

“We have great farriers here who identify it and get the client treating it,” Rucker says. “Typically, the only time I get involved in thrush cases is when pockets in the hoof start to redevelop mid-trimming cycle. We have a shortage of farriers here, so they aren’t always available to stop in between the 6- or 8-week schedule, so I’ll clean it up in between.”

While some cases may linger, Fleming says focusing on the hoof structure is essential.

“If you can improve the function and architecture of the foot, thrush will work itself out a lot of times,” he says. “Spring shoes or finding a way to widen the heel and enhance function and action back for the foot have a way of improving chronically thrushy feet.”

References

1. Stoltz, J. Literature review of three common hoof ailments: Laminitis, thrush and navicular disease. Journal of Undergraduate Studies, 2018: 6:1.
2. Reregistration Eligibility De-cision (RED) for Coppers; U.S. En-vironmental Protection Agency, Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, U.S. Government Printing Office: Washington, D.C., 2009: 11.
3. General Fact Sheet: Copper Sulfate. National Pesticide Information Center, 2012; npic.orst.edu/fact sheets/cuso4gen.html.
4. Bauer, M. Copper sulphate poisoning in horses. Yugoslavia: N. p., 1975.