How can you tell if your horse, who has suddenly stopped bearing weight on one leg for no apparent reason, has a hoof abscess or something more serious? Other possibilities include a fracture, tendon/ligament tear, bruise, foreign body (such as stepping on a nail), cellulitis, laceration, puncture wound, "hot" nail, and laminitis. Most of the conditions listed above (including an abscess) may cause increased heat, bounding digital pulses, and swelling proximal to (above) the foot due to inflammation. Blunt trauma, especially of the hoof, may not cause any swelling/laceration that is visible from the outside, so radiographs (x-rays) are needed to diagnose a fracture. Ultrasound and/or MRI is most useful for diagnosing soft tissue injuries of the hoof/pastern. The first thing to do is check the bottom of the hoof carefully for any foreign objects- use a hoofpick with a stiff brush to clean all debris out of the hoof, and if you see a foreign body do NOT pull it out! Veterinarians need to examine(usually with radiographs) how deep the object was embedded to determine if any critical structures (such as a joint) were affected to determine the appropriate treatment. Hoof testers can be useful to determine if the horse has a focal injury (such as a bruise/well-formed abscess), or is painful at the toes, which is most common with laminitis.
Your veterinarian may also block the nerves going to the foot with a local anesthetic to localize the source of pain. If cellulitis (diffuse infection of the skin and associated tissues) is suspected, it is also important to take the horse's temperature to help determine if the infection is localized or systemic. Because the prognosis is improved with early, accurate diagnosis and appropriate treatment, it is best to call your veterinarian as soon as you notice the lameness.