by: Nancy S. Loving, DVM
November 01 2008, Article # 10253
Despite good hoof hygiene and exercise, horses sometimes get this unpleasant condition; rapid recognition and proactive management can prevent deeper damage.
You've seen it many a time--a horse gallops across a field and dirt clods fly every which way. With normal activity, your horse's hooves are subject to a natural cleansing process that scours the bottom of the hoof and removes debris collected there. Any reason for inactivity, such as lameness or constraints on exercise and turnout, can influence how successful the natural cleaning action is that comes with moving across dry ground. It doesn't require a fast run to accomplish this; even just regular movement at a walk and trot will be beneficial.
Bill Moyer, DVM, department head of Veterinary Large Animal Clinical Sciences at the College of Veterinary Medicine and Biomedical Sciences at Texas A&M University, has had a special interest in equine foot health for decades. He stresses, "Most cases of thrush occur in inactive horses that live in stalls. Unfortunately, this describes a huge percentage of horses in the United States, since over the past few decades horses have become 'apartment dwellers.' As a result they may be standing in any number of different conditions, yet the foot isn't flexing and so doesn't get the opportunity to self clean."
The nature of your horse's environment impacts the health of his hooves to some degree. Certain conditions and environments predispose the frog to bacterial or fungal infections; horses live in the presence of manure and soil where potentially destructive organisms proliferate, particularly if dirt and debris remain trapped in the crevices or grooves (sulci) of the frog. Pads also tend to trap moisture in the bottom of the foot and facilitate such bacterial or fungal growth. While poor hygiene can set the stage for development of thrush, even with the best of care infection can develop in the frog or sulci of the frog if conditions are excessively moist. Horses in the western United States don't tend to have nearly as many--or as severe--thrush infections as those living in damp parts of the United States. Moyer can't overstate the importance of activity enough, saying, "I have seen thrush in some of the best-cared-for and -managed places, but the common denominator is that it develops in horses that live 'in.' "
Moyer notes, "Susceptibility to develop thrush varies with the configuration of the foot. A foot with an upright heel and deep crevices is a foot that is a real setup for thrush." When the heels are high, the frog becomes recessed below the heels of the hoof wall, debris accumulates, and disease results. Horses with a chronic lameness condition that causes heels to contract and/or limitations on exercise are also primed to develop thrush, as are horses with unproperly trimmed heels. Moyer has noticed that while some affected horses have deep crevices of the frog, in other cases there is no depth to the frog clefts at all. He says, "My impression is that the incidence may be higher in Draft breeds than in light horses, mainly due to the nature of their feet with a deeper cleft that is more likely to retain moisture, which becomes the media for bacterial growth."
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