You mean that's still the only thing that tips you off to frog infection??
Wake up, kids. This is a freakin' epidemic. Probably one in five horses that I trim DON'T show signs of chronic thrush, and we live in the desert! Some studies indicate as many as 90% of our domestic horses suffer from some degree of bacterial/fungal hoof infection related to thrush. And it's not just a cosmetic issue, or a nuisance...it can be excruciatingly painful...painful enough to make the horse toe-walk to avoid weighting the tender, infected heel. Painful enough and so easily ignored that it may very well be one of the leading causes of heel pain. And heel pain is the first flake of snow in the monstrous snowball generally diagnosed as navicular syndrome. Are ya pulling on your muck boots to run out and check for deep fissures between the heel bulbs yet??
Thrush is an infection of the central and lateral sulci (clefts) of the frog of the horse’s foot. Although the name "Thrush" implies a fungal infection, what we call thrush in a horse's frog most often involves bacteria, although it is occasionally a fungal infection. One species of bacterium (Fusobacterium necrophorum) is particularly aggressive, invading and destroying the frog, sometimes exposing the deeper sensitive tissues.
(click image to enlarge)
If you see a deep crease between the heel bulbs, with or without heel contraction, start treating for thrush infection. You probably have seen more hooves with this type of pathology than you have seen hooves with healthy frogs. There is some debate as to whether heel contraction predisposes the hoof to thrush, or whether thrush contributes to heel contraction. I'm decidedly leaning towards the latter, because eliminating the infection causes the frog to widen, and the heel with it, providing the hoof is trimmed to allow it. But before you reach for the bleach, or the Coppertox, think about what you're about to pour those very caustic substances on: living tissue! Let's be aware that the very best way to invite thrush is to provide it with some dead tissue, or some very tender live tissue, to feed upon.
Keep in mind, too, that while it is best to keep the frog free from flaps that could harbor thrush, trimming the tough, calloused layer off of the frog reveals tender tissue that may be more susceptible to infection. So be diligent about keeping the central sulcus open, and the flaps over the collateral groove trimmed back, but leave as much of the tough stuff as you possibly can.
An effective remedy is "Pete (Ramey)'s Paste," a 50/50 mixture of 1% clotrimazole cream (labeled for athlete's foot, jock itch, or yeast infections) and triple antibiotic ointment (like Neosporin). Since we're not likely to culture every infection to determine whether we're dealing with bacteria or fungi, this may address either problem. At this point, all I can tell you is that it WORKS. I'd like to say I'd taken the time to test each ingredient seperately, but I'd rather eradicate the infection as soon as possible, so I'll stick with the combination for now. Use a 60 mL, catheter tipped syringe to penetrate deep into the groove, and use your thumb to push the goop deep up into the central sulcus daily for 4 to six weeks. I've found that with really stubborn cases, booting the hoof for at least an hour (or even leaving the boots on around the clock, but be sure to remove the boots and clean both hoof and boot thoroughly daily, checking for rubbing) after applying the paste hastens healing.
Read Pete Ramey's article for an in depth discussion on frog management.
An interesting footnote to this blog entry: after writing this post, I was really bothered by the idea that the most common underlying cause of thrush is the SAME bacterium responsible for "foot rot" in cattle and sheep. Foot Rot, or interdigital dermatitis (which, by the way, is the same technical name for equine thrush) has long been identified as a cause of severe lameness in cattle and sheep. It is also widely accepted that overtrimming the soft tissue of the feet of sheep and cattle predisposes the hoof to the disease. (Read this page on Foot Rot in cattle and sheep.) And yet most farriers still pare away frog tissue with aplomb. That thrush has not been more aggressively treated, or more widely recognized as a possible cause of foot pain in horses is more than a little baffling. I'm off to do a little more research, and to see if perhaps there is an even better over-the-counter anti-bacterial for Fusobacterium necrophorum. I'll let you know what I find out!