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Other less common factors can cause a hoof abscess, and sometimes due to the location of the abscess the cause or point of origin won't even be able to be detected.
When your horse develops an abscess what happens is purulent fluid (pus) collects as the body fights off the source of the infection. Since the hoof cannot expand to accommodate the increasing collection of pus, the increased pressure within the hoof causes a great degree of pain – so who can blame your horse for not wanting to walk on such a foot?
The pus will attempt to escape via the easiest path within the hoof, which most often is the coronary band. If a hoof wall is weakened or contains a crack it's possible for the abscess to develop there instead. Once the abscess drains the pus from the horse's hoof, the infection generally ends and your horse becomes just fine.
While an abscess can heal on its own, I do not recommend allowing it to do so as an abscess can be incredibly painful for your horse and without human intervention the healing process will take significantly longer.
Once you have determined your horse likely has a hoof abscess place a call to an expert (a veterinarian or blacksmith – just make sure they are familiar with how to handle abscesses). In most cases the expert will test the hoof to detect the point of origin, then cut a small hole to allow the fluid to drain quicker. Sometimes if the infection cannot be isolated, or is caught too deeply in the hoof, the expert will not cut the hoof since cutting away too much can do far more harm than good. Luckily most abscesses can successfully be expedited.
After your hired expert has diagnosed the problem and provided whatever remedy he could, you will need to keep the sensitive hoof as clean and protected as possible. It's recommended you soak the hoof (many use water with a touch of apple cider vinegar) and clean the infected area thoroughly before covering the small drill hole with cotton or bandages. You want to ensure dirt and manure cannot find their way into the hole and sensitive tissues.
If a drain hole was unable to be added then chances are the abscess will need to escape via the coronary band. This will be a slower and more painful process for the horse, but unfortunately there's no way around it. A poultice created from linseed mash has been said to soften the hoof/band as well as increase blood circulation, so if your horse's abscess must heal on its own give this poultice a try. It may help speed up the process and alleviate some of the pain. (A linseed poultice is created by boiling linseeds into a mash, which is then spread across the abscessed hoof and wrapped.)
Remember to soak the hoof with the water and cider vinegar solution once daily until the abscess appears to have drained completely. Since exercise increases blood circulation, taking your horse for a walk daily can also speed up the draining process… not that your horse will be thrilled with the prospect at first!
If your horse is generally shod and then is allowed to go barefoot he will have an increased chance of developing an abscess since the hoof has likely been weakened depending on how long the horse has been shoed. In addition shod horses are more likely to develop an abscess due to the nail holes, etc.
As a side note, I rarely shod my horses since I believe natural hooves are far healthier in the long run. Not once has any of my horses become afflicted with an abscess. If your horse develops multiple abscesses you may want to reevaluate whether shoes are best for your horse or whether your farrier is skilled enough to keep your horse's hooves healthy. An improperly trimmed hoof can cause significant damage beyond "simple" abscesses.
While an abscess can be painful for a horse (and thus painful for us as we empathize with him), as long as you act promptly then chances are the healing process can be expedited and resolved relatively quickly.