Thrush describes an infection that develops in the frog of one or more of the horse’s hooves, caused by a fungal or bacterial pathogen. Fusobacterium necrophorum is the bacterium most often associated with thrush infections. [1]

Hooves with frogs that have deep sulci are prone to becoming infected with pathogens that cause thrush. Wet and dirty environments contribute to the development of this infection as thrush-causing pathogens flourish under these conditions.

Typical signs of a thrush infection include hoof tenderness and a foul-smelling discharge from the hoof. The condition is often diagnosed by visually assessing the hoof and checking the health of the frog tissue.

Thrush can progress to cause permanent lameness if it results in extensive damage to hoof tissues. Prompt treatment is necessary to stop the spread of the infection.

Treatment of thrush involves removing necrotic tissue from the hoof, disinfecting it, and keeping it clean during the healing process. Management strategies to prevent thrush and support recovery aim at keeping the horse’s living environment clean.

What is Thrush in Horses?

A relatively common bacterial infection of the horse’s hoof, thrush can affect one hoof or all four at once.

Thrush typically affects the center and grooves (central and lateral sulci) of the frog – a triangular structure on the underside of the hoof. It can also occur in the heel of the hoof.

Several bacterial and/or fungal microorganisms can cause thrush, but the bacterium Fusobacterium necrophorum is most frequently involved.

Thrush infection can cause minor to significant damage to hoof tissues and requires treatment to prevent it from spreading. If left untreated, the infection can damage the sensitive structures of the hoof including the laminae and digital cushion, and result in temporary to permanent lameness.

A common symptom of thrush is a foul-smelling discharge emanating from infected hooves. Horses with thrush may exhibit tenderness when pressure is applied to their affected hooves.

Damp and dirty environmental conditions are known to promote thrush infections as they create an environment where bacterial and fungal organisms can thrive.

Prevalence

Hoof disorders are common in horses. In one study, 85% of horses observed were found to have at least one form of hoof disorder when examined during regular hoof trimming. [2]

The exact number of horses affected by thrush is unknown. Thrush affects all breeds and ages of horses.

A study conducted in the Netherlands determined that out of 942 randomly selected horses that were assessed, 45% had thrush. [2]

Signs of Thrush

Depending on the severity of the infection, thrush can cause a range of clinical signs including:

  • Hoof sensitivity or pain
  • A pungent smelling, tar-like discharge emanating from the infected hoof tissues
  • A softened frog
  • Necrotic (dead) tissue in the hoof
  • Swelling in the lower limb if the infection is severe
  • Lameness in advanced cases
  • Lethargy

Pain caused by thrush increases as the infection spreads deeper into the tissues of the hooves.

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Causes of Thrush

Thrush is caused by bacterial or fungal organisms that enter the hoof tissue. An anaerobic bacterial species, Fusobacterium necrophorum, is the bacterial pathogen most often involved with thrush infection.

The following factors can promote a thrush infection:

Hoof Conformation with a Long Heel

Horses with a long heel conformation that results in deep, narrow frog sulci may be at an increased risk for thrush infection when exposed to damp and unclean environmental conditions.

Hooves with a recessed frog are prone to collecting more mud and dirt compared to those without. Deep and narrow sulci trap moisture and bacteria-laden debris in the hooves and create an ideal environment for infection to develop.

Environmental Causes

The microorganisms that cause thrush can propagate in conditions including deeply soiled bedding, mud, or very wet pastures. Horses living in these conditions may be at risk of developing thrush.

Management Issues

Not picking your horse’s hooves out daily can increase the risk of thrush infection. Overgrown or poorly trimmed hooves can increase the amount of moisture and mud/dirt than can lodge in the sulci.

Wearing hoof pads continuously may promote thrush-causing bacteria to accumulate in the sulci because they trap moisture against the bottom of the foot.

Diagnosis

If you suspect your horse has thrush, consult with your veterinarian to confirm a diagnosis of the condition. A physical examination of the hoof is necessary to determine if the infection is present in the frog.

Your veterinarian will check for signs of infection such as discharge, odor, and necrotic tissue. They will also check for lameness and assess your horse’s response when pressure is applied to the fro