The "white line" is the lamina of the horse. The lamina connects the protective outer layering of the hoof - the hoof wall - to the sensitive internal structures of the hoof - namely, the corium. A horse's lamina is naturally a thin line of velcro-like attachment that is amazingly strong...it can only give way internally. It almost literally cannot be physically separated from the hoof wall -- unless it becomes "stretched" due to metabolic insults to the horse... In other words, when the horse's hind gut bacteria are compromised due to a diet rich in carbohydrates (see nutrition page and www.safergrass.org) it causes a weakening and eventual releasing of the lamina from the wall. The exact mechanisms that cause this weakening are still being researched, but there are many processes at work.
When a horse has laminitis, this means the lamina are inflamed. "Foundering" is when the lamina gives way and the coffin bone loses attachment and appears to "rotate", or sink in the capsule (a "sinker"). If the metabolic problem has resolved, as the hoof grows you will see a tighter growth near the coronet band - this is where the lamina is once again tightly connected. (Once it has separated, it cannot grow back together, you have to start over again with new lamina)
Once compromised, a stretched white line is an inviting host to opportunistic bacteria which can continuously eat it away and prevent your horse from growing a nice "tight" hoof. This is known as white line disease or seedy toe. Once the cause of the hoof form problem has been resolved, you can treat the symptoms with a good trim and the use of a product such as Oxine, White Lightening or Clean Trax, which will safely kill not only the bacteria and fungus themselves, but also the spores that may still exist. Tough cases may require several treatments.
White Lightening: http://www.grandcircuitinc.com/proddetail.asp?prod=GOSHGCWL8
Clean Trax: http://www.centaurforge.com/products.asp?dept=69
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