Thrush is a anearobic/fungal infection of the frog in the hoof. It usually presents with a characteristic foul odor and black, tar-like discharge, or may simply leave the frogs puny and tattered looking with with powdery funk under shedding flaps. It can also simply look like an otherwise normal frog, but has a pinched "butt crack" appearance to the middle part. This is often in conjunction with contracted heels (the pinched version).  Either way, the frog is compromised and weak, and can be very sensitive. Many horses with the stinky version aren't as tender as the "butt crack" version. Even farriers miss the non-smelly version quite frequently. Some horses may be sound for the most part, but take a really gimpy step here and there on rocks, and often it's the latter form of thrush. instead of sole sensitivity. I like to call it a dry fungal infection, or deep sulcus thrush.

  Thrush is often associated with wet, dirty conditions. Stalls that are full of manure not only breed fungus (think of how they raise mushrooms in the dark on manure) but the ammonia vapors can weaken the hoof's structures and make them more vulnerable to infections as well. Mud in itself doesn't make for thrush, necessarily, but a waterlogged hoof is also less resistant not only to thrush, but also to bruising and puncture wounds from being soft. Sometimes it is just impossible to provide a perfectly clean and dry environment at all times. However, simply turning your horse out into pasture can get him off the manure pile and into cleaner moisture.

 Picking the feet out daily will also help to expose the frog to air to combat infection. Even if the foot just goes right back into the mud, daily picking goes a long way to helping to prevent or clear up a thrush infection. Picking also allows you to notice when infections start, so treatment can be started sooner, saving your horse a lot of tenderness and halting further damage. I find, too, that this keeps them in good manners if done frequently. Pea gravel, limestone screenings, or other such rock put in common loafing areas can go a long way to preventing thrush by providing better drainage as well as helping to ex-foliate the soles and frogs which minimizes flaps and tatters hanging around that could hide germs.

Increased exercise helps eliminate thrush. A healthy frog is one that gets used. Grazing, walking, longing, riding, ponying, etc; any form of movement can provide good stimulation in a well trimmed hoof. The cycle of weight bearing in stride, then release as the hoof is lifted, provide a sort of massaging action to the frog, which helps to increase circulation as well as self cleaning the hooves at times (flinging the dirt out as they move). Rigid support provided by metal shoes with plastic pads can't provide a release of pressure, and actually slow circulation even more. If you have ever sat for too long and your foot fell asleep, you understand the principle I'm talking about. Constant pressure slows blood flow, while alternating pressure and release increase it. Therefore, bar shoes and wedges simply do not support frog health. Even regular metal shoes create a rim that holds dirt and manure in the hoof, increasing the need for daily or even twice daily picking.

Good trimming is vital as well. Trimming must allow the frog adequate, but not excessive amounts of weight bearing in motion, depending on the surface. Horses in soft footing  may need longer heels, as the ground is pressed into the hoof. Horses on hard terrain may need less heel to allow more frog contact and shock absorption. Excess trimming of the frog itself can remove the calloused outer layer and actually invite thrush infections, just as not trimming the exfoliating flaps can leave hiding places that thrush thrives in and can eat its way through to healthy tissues (I've seen horses come out of shoes whose entire frog had sloughed off, save for the thin skin covering the corium, and these frogs can become abscessed by the thrush eating its way into the hoof). If the heels are left too long, the frog is suspended above the ground, where it can't do it's job. If too much frog pressure is allowed, it can cause sensitivy in the frog, and the horse walks tippy-toe, the oppostie of what you want. There is an art to finding the right balance for each horse and his unique situation and thrush can be caused by poor trimming just as poor hoof mechanics can cause thrush. Tippy toe walking allows the frog to atrophy, and...you guessed it, become infected from lack of use. With horses, the principle of "Use it or loose it" really does apply to their hooves.

 Just an interesting side note, horses can function without adequate frog contact. I believe that is why so many shod horses don't seem lame and can run, jump, etc. However, eventually the horse suffers long term effects from poor frog stimulation (whether from shoes/poor trimming or from tenderness relating to thrush) and the horse breaks down elsewhere in his body. Landing toe first leads to navicular syndrome, back problems, etc from the strain of compensation for poor hoof mechanics. This works both ways, however, as compensating for a bad saddle fit can create the toe walking, which leads to thrush, etc.  The frog has what is called "proprioceptors" in it. These are nerves that detect sensation- or basically, the frog is what feels where the foot is landing better than the rest of the foot. Taking the frog off the ground and allowing it to wither or become sore with infection can make your horse stumble footed, and less confident when moving out.  Sure, they can get by with a compromised frog, but it is still better to have healthy, functioning frog. I can walk with my foot half numb from sitting too long, but I can't win a race that way, I would have to be more careful about how I'm placing my foot instead of concentrating on winning.

Above are pictures of various forms of thrush. The white powdery kind, that doesn't have a smell, is the first pictured. It's usually a very mild infection and is found under flaps that are about to shed. The yeast help to detach shedding material. The organism is opportunistic, however, and can begin to eat into healthy tissues. The black discharge starts along the side of the frog, in the parallel grooves (collateral grooves) and appears to be black mud or tar. This is from the sweat glands in the hoof, and when not exposed to the air, the bacteria and fungus thrives and the black goo appears. It does stink, as it contains dead hoof material that begins to decay. In severe cases, black pus comes from within the hoof and literally oozes out. The same pus that comes with abscessing. This kind of thrush generally leads to the whole frog sloughing off, leaving a yellowish, waxy looking miniature frog exposed. This is a thin layer covering the very core of the frog, and will be very sensitive and can cause lameness until the frog grows new material.

Next, you have the "deep sulcus thrush" that by most standards, doesn't appear to be thrush. The frog looks dry and there is no odor and no black exudate exists. The frog WILL appear pinched up, the heels contracted to some degree, and seem to have a butt crack instead of a wide groove in the middle toward the heels. The sulcus is the name of that groove or valley. This kind of thrush does cause sensitivity in the heel area, and can mimic navicular disease's clinical symptoms. A hoof pick pressed into the groove will cause a flinch or other reaction. It is very painful. This causes horses to tippy toe, and this can create a lot of other soundness issues. Please look at the pictures to get an idea of what thrush can look like. The pictures I used are of horses fresh out of shoes, with the exception of the first and the example of healthy frog.