Equine Thrush is a common bacterial infection of the frog or other areas of the hoof which can cause lameness. It has a very unpleasant odor and looks black when compared to the healthy gray color of a healthy frog. While mild cases do not usually cause lameness, left untreated it can progress, migrating ever deeper into the hoof until lameness results.
It is a combination of anaerobic bacteria (bacteria that grow without oxygen) and fungal cells that begin to “eat up” the healthy tissue of the frog or other sole tissues. Thrush is frequently seen in environments that stay wet, muddy or unsanitary, but these conditions do not need to be present.
Any condition that reduces the flow of oxygen or blood circulation to the frog can facilitate Thrush infections. Horses who are shod have only about 20% of the hoof blood supply circulation that a natural hoof has because the hoof wall does not flex and the frog is frequently not in contact with the ground. Additionally, shod horses or horses that have poor trims that cause them to land toe first or flat have impaired circulation through their frog. Stalled horses (even in the cleanest and driest of stalls) frequently do not get enough exercise and have impaired circulation.
The fungus is nearly always present in a healthy horse’s hoof. However, horses with narrow heels or deep crevices between the heel bulbs are particularly susceptible because the narrowness of the clefts prevent good oxygenation of the area and allow the fungus to grow pathologically. Thrush can even settle in small crevices between the sole and the wall (commonly called white line disease or wall thrush). The horse whose hoof is pictured above has/had infections in many areas of the hoof including between the sole and the wall despite dry conditions and daily or twice daily cleaning and treatment. Left untreated this will cause more tearing of the hoof wall and more separation until it is a very serious condition indeed.
In the case of infection between the sole and the wall, the trim is critical. Any trim that leaves the toe long and pressures the wall at breakover is ill advised. That toe pressure can encourage separation of the wall. It is possible that the toe pressure, which caused the first separation, allowed the first opportunity for the fungus to get started in a hard-to-reach crevice. As you correct the condition with hoof trimming, it is imperative that you trim AHEAD of the deterioration of the wall. That might mean weekly trimming to stay ahead of the progression, giving the hoof time to grow out in a healthy way while you keep the deterioration at bay. It is labor intensive and time consuming as you wait for the new hoof to replace damaged hoof and trim ahead of the infection. A healthy hoof regenerates itself in about 6-9 months.
There may be dietary component in horses who eat a high carb diet, as sugars encourage fungus systemically. Diets rich in grains and alfalfa (or other rich or highly nitrogenous forage) can encourage Thrush. If Thrush is stubborn, change to a grass-based, low carbohydrate diet.
Several treatments seem to work against the bacteria – even pure sugar or Lysol soaks. It is of paramount importance for a Thrushy horse that you pick the feet clean every day (or more often), taking particular care to get debris out of the two collateral grooves and the central sulcus. Oxygen is to the bacteria as sunlight is to a vampire. Expose the nooks and crannies to the air and the bacteria will die. Stalls or paddocks should be kept clean and dry. Routine treatments with diluted iodine or betadine and gentian violet solutions can help prophylactically as well as after an infection has occurred. But prophylactic treatment can backfire and kill healthy yeast cells that proliferate when a frog is shed and a new frog develops. Balanced yeast and fungus are part of a healthy hoof. I would save the preventative treatments for times of prolonged rain and mud or other unhealthful situations.
Several commercial products are available to treat the hoof that is infected. See your local farm store for one of those, or for stubborn cases of hoof or mouth thrush use a systemic candida treatment such as Lufenuron. (Horses or other pets with persistent fungal problems may have compromised immune systems. A systemic approach may be best for those horses.)
Please note that this advice is neither veterinary nor prescriptive in nature but offered only as an introduction to this topic.
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