Gravel is a common cause of lameness, but is usually more an inconvenience than a problem"GRAVEL" is a condition possibly unique in equine medicine: it does not have a fancy, multi-syllable name. Textbooks say "gravel" is a lay term, but fail to furnish us with any other name, so gravel it remains. The fact that gravel has nothing to do with gravel is secondary.
For those unfamiliar with the term, a gravel is a condition of the horse's foot, manifested by varying degrees of lameness. It derives its name from the old belief (still held by some) that a piece of gravel enters the bottom of the foot and migrates upward, eventually popping out at or slightly above the coronary band.
What actually happens is an infectious microorganism enters the sole or white line through a puncture wound or crack and multiplies, bringing about the body's natural response to any foreign invader: white blood cells gather around the bacteria, and pus results. As the white blood cells battle the bacteria, the affected area within the foot enlarges as more pus is produced. The normal course is for the infected area to move to the nearest outer surface and erupt. But with this occurring within a horse's foot, the nearest outer surface is either the sole or the hoof wall, neither of which is amenable to being penetrated by an abscess, which is what this is. Therefore, the pus pocket travels in the easiest direction-up-where it eventually meets the soft tissue it seeks-the coronet-and pops.
As stated, the infection enters through the sole or white line through a puncture or crack. Although these can occur at any time, there are three conditions which predispose their occurrence: 1) extremely dry conditions; 2) extremely wet conditions; and 3) founder.
In extremely dry conditions, the horse's foot dries out. This leads to cracks in the sole and especially in the white line. Times of drought are probably when gravels are most common, as bacteria enter these cracks and infection sets in.

The communal dump theory
I am not sure why we see an increase in gravels in extremely wet times, but I have a theory. Let me preface it by saying I don't know if there is a higher incidence of gravels elsewhere in the world in wet weather, I just know I see more here in Central Kentucky. Legend has it that Kentucky was a communal Indian hunting ground hundreds of years ago. None actually lived here: they just came here to hunt. This may be true, but I think they actually used the area as a communal dump. It was the place to pitch their old prehistoric beer cans and pop bottles and broken-down cars. After a hard rain, in almost any field in this area, old pieces of glass and metal can be found.
My theory then, is this: in extremely wet weather, a horse may sink one-to-four or more inches into the mud as he moves about. By so doing, his foot encounters these buried bits of debris which, in normal or dry times, would remain unreachable. I don't know how accurate this theory is, but I do know that I see a dramatic increase in the number of gravels in really wet weather.
Gravels are also seen frequently in previously foundered animals because of the "seedy toe" associated with the condition. The white line in the toe area is never fully sealed with seedy toe, and infections enter readily.

Treatment
Lameness is the chief sign of gravel, but the lameness may vary from slight to three-legged, depending on point of entry of the offending organism, amount of pus generated, path traveled by the infection, and proximity to the coronary band. If detected early, the offending crack in the sole can be found and dug out with a hoof knife, allowing the pus to drain, but if not caught early-and most are not-opening the sole does no good because the tract of infection has proceeded too far upward.
If drainage is established through the sole, iodine and a bandage need to be applied. If it has progressed too far, soaking the foot in epsom salts will help bring the infection to the surface. In either case, a tetanus booster should be given.
Some vets recommend the administration of a pain reliever and/or antibiotics before eruption, but as these tend to slow the progression of the infection and thereby prolong the condition, I do not feel either is indicated. Also, stall rest is frequently recommended. This accomplishes two things: 1) a slowing of the upward movement of the infectious tract; and 2) the creation of more work, in the form of a dirty stall, for whomever performs the task of stall mucking. The animal is better off turned out, because the effect of moving about will cause the infection to move up faster and erupt sooner.
Except in severe cases, the prognosis of a gravel is good. The pus pocket migrates to the region of the coronet, breaks open, drains, and heals, and the horse is none the worse for the experience. Prevention is difficult, especially in horses at pasture and in previously foundered animals. Keeping the feet moist in times of drought helps greatly in reducing the incidence of the condition.