Thrush and White Line Disease in Horses


There has been a vast amount of information published in the scientific horse industry literature concerning the causes, prevention, and treatment of thrush. Although often overlooked, proper nutrition can promote the development of hoof tissue with better characteristics to help prevent or control thrush. 

How Does Thrush Develop in Hooves?

Thrush develops in the equine foot when bacterial and fungal organisms begin populating the frog’s dark and moist crevices, or sulci. These organisms thrive and divide in oxygen-poor (anaerobic) environments often contaminated with wet organic material. 

The organisms causing thrush are opportunistic and typical in the soil and environment. All horses are exposed. High humidity or wet environments predispose horses to thrush. The opportunity for a progressive invasion and subsequent infection of the frog tissue can occur once the organisms begin dividing in the frog sulci. Many of the same microbes linked with white line disease are associated with thrush.

The Importance of Oxygen in Thrush Prevention

The blocking of oxygen to the tissue of the frog and surrounding areas will predispose a horse to develop thrush. Debris, footpads, boots, or the application of grease and oils to the foot can block oxygen. 

The material associated with thrush is usually black and characteristically has a highly unpleasant odor. Infection of the frog and surrounding tissues often leads to lameness. 

Hoof Blocked of Oxygen
Hoof blocked of oxygen due to packed debris

Important Nutrients for a Strong Hoof

In Thrush, the first line of defense from environmental microbes invading the hoof capsule is a “sound” hoof wall structure. The same nutrients necessary for a sound hoof wall are also essential for dense frog and sole tissue

  • Calcium is necessary for the proper bonding of cells to each other.
  • Zinc is important for healthy keratin, the tough pigmented material found in the outer layers of the frog and sole. 
  • Methionine is a sulfur-containing amino acid necessary to form the healthy “cross-links” of collagen that add strength and elasticity to tissue. 
  • Phospholipids are needed to form healthy cell membranes that give the cells the ability to maintain proper moisture and oxygen balance and repel excess environmental moisture. 
  • Copper and ascorbic acid (Vitamin C) are also necessary, serving as catalysts in the formation of strong and healthy connective tissue. 

Effective Thrush Prevention Involves:

  • A combination of maintaining a clean, dry (but not too dry) environment.
  • Cleaning the feet on a routine basis.
  • Adequate exercise.
  • Proper preventative nutrition.

Proper nutrition involves supplying nutrients in the correct amounts and ratios to each other without over supplementing.

Important Tips for Thrush Treatment

If the horse develops thrush, provide the horse with a clean and dry environment on which to stand. Clean the bottom of the foot and frog area by removing any debris and washing the area thoroughly. 

Treating Hoof with Thrush
Farrier Applying Life Data® Hoof Clay® into Frog infected with Thrush

Hoof disinfectants containing tamed iodine are ideal for treating or preventing thrush. Products containing tea tree oil and low levels of iodine packed into the frog sulci can also be effective. Surgical debridement of affected tissue may be necessary. 

Learn More About Our Recommended Treatment for Thrush

White Line Disease

To address the relationship between nutrition and white line disease, we must not overlook the important observations regarding the disease and the predisposing factors over the years.

White Line Disease
Hoof with White Line Disease

Burney Chapman

Burney Chapman, a world-renowned farrier from Lubbock, Texas, USA, became one of the foremost authorities on the condition back in the late eighties and early nineties. At that time, he began to see an alarming increase in the number of white line cases he encountered in his farrier practice both in the U.S. and U.K. 

Burney determined that it was not a disease of the white line but the result of a fungal invasion of the middle hoof wall. The disease is known as stall rot, seedy toe, hollow foot, wall thrush, and white line disease.

Burney’s Observations

At first, Burney thought white line disease was found in environments that were poorly maintained. However, the more he encountered it, he began to realize the disease occurred more often in clean, well-managed stables and barns. He also observed no correlation to breed, color, or front vs. back feet and that the initial stages were non-painful and usually detected by the farrier during routine hoof care.

What Causes White Line Disease?

Today, we know more about white line disease and recognize that all horses are exposed. The medial (middle) hoof wall is affected by white line disease. Both bacterial and fungal organisms commonly found in the environment cause the damage. These organisms require a nutrient-rich environment that lacks oxygen to flourish. 

White Line Disease and the Hoof

The outer hoof wall is more resistant to invasion. This is due to its higher density and exposure to environmental oxygen compared to the lower density and lack of oxygen in the middle hoof wall. 

The outer and medial hoof walls are derived primarily from the coronary band. The coronary band provides the pigment to the outer and medial hoof wall. 

The third section of the hoof wall, the inner hoof wall, is derived from the laminae and is therefore non-pigmented and glistening white. It is more resistant to microbial invasion due to the proximity of live tissue in this area. The live tissue is not only oxygen-rich, thereby inhibiting these opportunist anaerobic organisms, but also has infection-fighting abilities. 

We know that often trauma to the hoof capsule creates bruising and bleeding. The damaged and leaking blood vessels create a good food source for the “hoof-eating” microbes.  

Other predisposing factors include a prior occurrence of an abscess or laminitis in which the hoof wall becomes full of holes and crevices. Nail holes or hoof cracks allow organisms to gain access. Also, high moisture environments tend to soften the foot and allow the bacterial and fungal organisms a more accessible entrance into the hoof.

Nutrition and White Line Disease

Properly balanced nutrition and quality hoof supplements will strengthen and increase the density of the hoof wall, reducing the likelihood or severity of white line disease

No one has proven this fact more sufficiently than Dr. Susan Kempson. Dr. Kempson is a noted equine nutrition researcher in the Dept of Preclinical Veterinary Sciences, Royal School of Veterinary Studies, at the University of Edinburgh in Scotland. Dr. Kempson has conducted numerous electron microscope studies of the equine hoof wall. Electron microscopes use electrons rather than light to capture images and magnify them up to two million times. 

Dr. Kempson’s Research

Dr. Kempson demonstrated with scanning electron microscope photos that a horse supplemented with excessive selenium developed a lack of structure in the hoof horn. Sulfur is required to build the strong cross links necessary for a healthy hoof horn. However, excess selenium substitutes in place of sulfur, creating a weak hoof structure. In addition, the soil where the horse lived was deficient in copper. Copper will help protect the horse from the detrimental effects of small amounts of excess dietary selenium.

The electron microscope photos also showed that bacteria had invaded the space between the cells. Interestingly, Dr. Kempson found that bacteria were the front-line invaders of the hoof wall, setting up the environment for the secondary fungal invaders. The several different species of organisms that have been isolated from infected hooves all have one thing in common – they thrive on the sulfur-containing collagen of the hoof wall. 

In the above case, after three months of discontinuing the selenium and supplementing copper, the horse returned to his normal activities.

Calcium Deficiencies

Hoof wall quality also deteriorates with calcium deficiency. Calcium deficiency is often a result of diets high in bran and creates weak, crumbly hoof horn. The poor hoof wall quality develops holes and defects. This allows the bacteria and fungi associated with white line disease to penetrate.

Final Observations

The above information explains many circumstances associated with white line disease. However, it does not explain the puzzle of why horses in the well-managed facility and the overfed horse often have some of the most challenging white line problems. In agreement with Burney’s Chapman’s observations, it is possible that the horse consuming a high carbohydrate diet, especially a sweet feed, is predisposed to white line disease due to changes in the hoof wall environment that are favorable for these hoof-eating organisms.