As you pick out your horse's hooves this spring you may notice a foul odor and an unusual black thick discharge around the frog. These are the first signs of thrush, an infection of the frog and surrounding tissues of the hoof. The bacteria (usually Fusobacterium necrophorum) infects the collateral and central sulci (creases) of the frog. The bacteria has degrading properties (called keratinolytic properties) that break down the hoof's tissues. The breakdown products and dead bacteria contribute to the accumulation of black discharge. The fissures of the sulci deepen, as do the clefts between the heel bulbs. This can be a painful problem in the heel region.
Thrush is usually encountered in the spring because of the constant wet environments in which horses stand. Unclean stables, lack of proper and timely trimming, and lack of daily hoof care also makes horses vulnerable to this condition. Unfortunately, corrective shoes and pads can occasionally contribute to thrush because they can make the hoof more difficult to clean thoroughly and can prevent aeration of the hoof. Horses with long or contracted heels with deep sulci can also be affected since such foot conformation prevents weight-bearing contact of the frog.
If left untreated, the infection may cause swelling of the lower limb and lameness. The lameness occurs when the infection spreads to the sensitive structures of the foot. It may reach deep into the central sulcus of the frog or cleft between the heel bulbs. Alternatively, the infection may extend into the dermal laminae (which connects the coffin bone to the hoof wall) and undermine the hoof wall.
Prevention of thrush can be accomplished in several ways:
If you do notice a pungent odor and distortion of the usual appearance of your horse's frog, some treatment may be necessary. Early, mild, cases of thrush can usually be treated by debriding (removing dead tissue by trimming, scraping, and vigorous scrubbing) the frog and trimming the hoof wall. More moderate cases may need to be scrubbed with an antiseptic such as betadine and then treated daily with a topical antiseptic after trimming and debridement. Examples of effective topical treatments are 10% formalin, tincture of iodine, or a commercial copper sulphate product. Some advocate soaking daily in super-saturated magnesium sulphate. Severe cases of thrush may need repeated aggressive debridement followed by sterile bandaging and topical treatment. Occasionally horses that either are resistant to treatment or have repeated bouts of thrush can be shod to raise the foot slightly off the ground to promote aeration of the bottom surface of the hoof.
If you suspect thrush and your horse is due or overdue for trimming and shoeing, make an appointment with your farrier. After debridement and trimming he or she can advise you if further treatment is necessary. If your horse has been trimmed recently and you notice signs of thrush, especially if lameness is involved, contact your veterinarian. With expedient and appropriate treatment you can prevent thrush from causing your horse discomfort, and from delaying your long-awaited spring rides.