Thrush in horses is a degenerative keratolytic condition of the frog.
Etiology and Pathophysiology of Thrush in Horses
Although the specific etiological agent of thrush in horses is unknown, anaerobic bacteria are thought to play a key role. Evidence of the disease commonly begins in the central sulcus of the frog and eventually spreads to the collateral sulci and main portion of the frog.
Generally, thrush in horses starts superficially, before extending deeper into the stratum corneum. Occasionally it reaches the germinal layers of the frog. Historically, thrush was associated with poor care of hooves and poor hygiene in stables, including infrequent hoof cleaning; irregular farrier care; and keeping the horse in moist, unhygienic conditions for extended periods of time. Although these are still commonly reported as predisposing factors for thrush, the disease can occur in horses receiving adequate hoof care and living in clean, dry environments.
Lack of exercise is another proposed factor in the development of thrush in horses. The theory is that adequate movement is needed for heel expansion to enable the natural “cleanout” of the frog.
Some foot conformations also tend to be predisposed to the development of thrush. Horses with deep sulci and narrow heels more commonly develop the disease, compared with horses that have wide heels and shallow sulci.
Clinical Findings of Thrush in Horses
In cases of thrush in horses, usually lameness is present only if the disease has extended to the dermal tissue. Horses may exhibit a pain reaction when the affected frog is subjected to pressure during normal hoof cleaning.
Affected hooves in cases of thrush in horses usually have a distinct foul odor and a thick black discharge emanating from on overly soft frog. The surface of the horn of the frog usually has a rough appearance, and bleeding often occurs when the frog tissue is incautiously touched with a hoof pick.
Thrush may affect a single foot or multiple feet.
Diagnosis of Thrush in Horses
The characteristic odor and appearance of the frog are usually adequate to diagnose thrush in horses. It can be differentiated from the early form of canker by its degenerative nature, confinement to the frog, and adjacent sulci. Canker is also more commonly associated with lameness than thrush is.
Treatment and Management of Thrush in Horses
With thrush in horses, debridement of abnormal horn of the frog is necessary to allow access to the deeper diseased tissue for topical treatment. A wide array of topical treatments have been used, with the main goals of disinfecting, desiccating, and hardening the frog horn. It is important to consider the depth of tissue involvement when selecting topical treatments, because many may cause further damage, owing to their caustic effects on germinal tissue.
Environmental factors should be addressed if hygiene is thought to be a contributing factor to thrush.
With adequate treatment and management, the prognosis for thrush in horses is good. Recurrence is more likely in horses with predisposing hoof conformations.
Key Points
Thrush is a degenerative keratolytic condition of the frog.
Anaerobic bacteria are thought to play a key role in the disease process.
Usually, lameness is present only if the disease has extended to the dermal tissue.
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