This a very common problem where ever there are wet-dry cycles. Hooves naturally have porous areas on the bottom of their feet, usually where the 'white line' is growing out of the bottom of it. It is even more porous when the horse has had laminitis ( or Foundered ) so that during dry weather, the hoof acts like a piece of wood and opens up cracks as it dries out. When the wet weather comes and the horse stands in puddles, that contaminated water soaks up into those cracks and porous areas and can work its way up to sensitive tissue and start an infection or localize into an abscess. Some are under the sole and some go up under the hoof wall. Occasionally they turn inward and can invade deeper into the foot and even reach the coffin bone.
There are a few things that you might do if you anticipate wet weather such as putting him into the barn for a while. Not a practical solution but in some cases it does work. I usually advise owners to seal up the porous areas whenever the hoof is rasped, and do it with something tarry such as Ichthammol, or even forschners hoof packing. If you put the tar like sealers into those cracks and holes, then the water has a harder time seeping up into the hoof and causing an infection. Every time you see the farrier, watch when he/she cleans the bottom of the hoof and before the shoe goes on or the foot goes back onto the ground, push that tar up into every crack or hole you can find. Hoof dressings all around the hoof wall will help to limit the drying of the hoof too. Don't be afraid to put those hoof dressings on the bottom of the hoof too and the whole foot may not dessicate as much. Whereever the hoof is rasped or nipped, it will lose moisture quickly, so be there to seal things up and keep it from cracking and opening holes where moisture can enter. Bob Saunders DVM