Brushing Up on Thrush in Horses

Good management and proper treatment are key to keeping your horses sound in the face of thrush.

Autumn is just around the corner and, for many areas of the country, that means the onset of wet weather, a reduction in riding, and more stall time for horses. For some owners, it's less time spent with their horses and more time spent snugged up on the couch with the remote. All of which could mean an uptick in the risk of your horse developing thrush--that gunky, smelly, black discharge oozing from the bottom of his hoof.

Thrush is an anaerobic bacterial infection that slowly eats away at the horse's hoof tissue. "It's characterized by black, malodorous necrotic (dead) material or exudate in the central or collateral sulci of the frog (the grooves adjacent to and in the middle of the triangle-shaped frog)," says Steve Adair, DVM, MS, Dipl. ACVS, associate professor of equine surgery at the University of Tennessee.

Early stages of thrush only involve superficial tissues and don't cause lameness. But if ignored, the infection can advance into sensitive tissues and internal structures of the foot, such as the digital cushion, hoof wall, and heel bulb, warns Ashley G. Boyle, DVM, Dipl. ACVIM, assistant professor of medicine in the section of field service at the University of Pennsylvania's New Bolton Center.


The primary factors favoring thrush are excessive hoof contact with moisture and a lack of regular foot care, either of which can occur when autumn rains and cold weather cause an owner to be less diligent about mucking the stall, picking out hooves, or turning out/exercising the horse (movement across dry ground produces a scouring action on hooves).

"Any area that stays wet, muddy, or is contaminated with urine and feces is a place that can harbor the bacteria that cause thrush," Adair says. "Swampy land, ponds, and streams provide an area that will stay wet and muddy. A common watering trough is another area. Structures that provide common shade areas or where horses can congregate will become wet and unsanitary."

Although less common, poor hoof conformation can predispose some horses to thrush, even when living in clean, dry conditions. States Boyle, "Certain breeds, such as Saddlebreds and other gaited breeds, have been found to be predisposed due to the conformation of their feet (deep grooves associated with the frog). Narrow heel conformation can result in a deep-central sulcus that extends into the heel bulbs as well." (Deeper sulci receive less air, creating a more thrush-friendly environment.)


Diagnosis is based on clinical signs (discharge, location, loss of frog) and environmental conditions. In mild to moderate cases, diagnosis and treatment can be handled by the owner, Boyle says. "More severe cases in which the central sulcus and the crack between the heel bulbs are painful, and the horse may or may not be lame, often require additional attention from a veterinarian."

Steps to resolve thrush include:

  • "First and foremost, problem environments must be cleaned up," emphasizes Adair. "Clean the stalls and change bedding regularly. Fence off ponds and streams. Scrape and regularly clean common loafing areas (beneath trees, shade structures). Construct watering facilities so there is proper drainage."
  • If you can't immediately correct the horse's environment, move the horse to a clean, dry area during the treatment period, says Adair, or protect the feet with boots.
  • Clean feet daily. "Use gauze like floss to clean out the central sulcus," says Adair.
  • The owner or veterinarian should carefully trim away any dead or loose frog with a hoof knife. This prevents trapping of the bacteria and exposes the tissues to air (and therefore oxygen, limiting bacterial growth), explains Boyle.
  • Treat the affected frog and sulci of the frog with topical agents. Adair and Boyle recommend applying dilute bleach (one part bleach to 10 parts water) or dilute chlorhexidine (eight parts chlorhexidine to two parts water) or povidone iodine (one to two parts povidone iodine to eight to nine parts water). Do this daily, then decrease to once or twice a week until the condition is eliminated. Do not use dilute formaldehyde or tincture of iodine, as they can damage healthy tissue.
  • Also apply a drying agent such as copper naphthenate (Kopertox) or an isopropanel formalin, iodine complex, and gentian-violet combination (Thrush Buster). Depending on severity, these can be used daily, every few days, or weekly.
  • "Make sure these drying agents do not touch the more sensitive tissues of the coronet band and haired skin," cautions Boyle, "as they are caustic and drying."
  • If the horse is not lame, provide exercise to clean the foot naturally.
  • "For severe cases, pack the deep sulci with cotton soaked with honey or sugar and betadine solution on a daily basis," recommends Boyle. "The high concentration of sugar acts as an antiseptic." Topical or systemic antibiotics administered under veterinary direction might be necessary for severe cases.

With appropriate treatment you should notice improvement in mild cases within a few days, and the thrush should be resolved in two or three weeks. "Horses with deep cracks between the heel bulbs may take a few months to resolve," Boyle states.

Take-Home Message

In most cases thrush is preventable by maintaining dry footing, clean hooves, and regular turnout or exercise. For horses whose underlying hoof conformation predisposes them to thrush, couple good management techniques with regular trimming of the foot and frog.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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