So your horse stepped on a nail...
First of all, DO NOT PULL THE NAIL OUT. If you don't feel like reading the rest of this post, that's fine. You can stop here because the most important thing to know is that when/if your horse steps on a nail, you should not pull it out. Every part of your being is going to want to yank that nail out of your horses foot. He or she may be limping and I understand that removing the nail will provide some relief. I'm here to tell you that you should resist that urge, and leave it in place.
Step 2, call your vet. Leaving the nail in place allows us to determine how deep the nail penetrated and where it penetrated. How? By taking an x-ray. Lucky for us, nails show up like glow sticks on an x-ray. When we take an x-ray of the foot, we can see where that nail is pointed.
Why does this matter? There are MANY important structures in the foot. Some of these structures, like the deep digital flexor tendon (DDFT) and navicular bursa can become infected if a nail punctures through them. Fixing an infected DDFT or navicular bursa is a surgical emergency that is difficult and expensive to repair. It is very important that we are able to identify if these structures have been contaminated.
If the nail has not punctured these structures, you are one of the lucky ones. Usually the foot can be treated like an abscess with some oral antibiotics and daily foot soaking. So please, please, please help us help you and leave the nail in place. Once the vet has examined your horse, it can then be removed and treatment initiated.