1.     Look for signs of injury.

A small puncture wound or even a scratch can introduce bacteria under the skin, leading to an infection that causes massive swelling. Using your fingers to part the coat, look for breaks in the skin and feel for irregularities in the surface. Report your findings to the veterinarian.

2.     Look for other swelling.

A fat leg will command your attention, but check the horse’s other legs and midline for inflammation as well. If more than one leg is swollen, or if both hind legs are thick, the horse may have an extreme case of stocking up. However, swelling that extends up through his groin and midline could be the result of a systemic illness.

3.     Take your horse’s temperature.

A swollen leg or legs plus a fever can be a sign of a localized but serious infection, such as lym­phangitis, or a systemic illness like Potomac horse fever. Record the temperature and the time you took it. This is important information that can help your veterinarian make a diagnosis.

4.     Keep the horse still

Until you know exactly what you are dealing with, it’s important to keep the horse as quiet as possible. If the leg is swollen from injury to a bone, ligament or tendon—even if the horse isn’t acutely lame—movement can make the situation worse. Stall confinement isn’t enough: Hold the horse on a halter and lead, providing a hay net or grain if that’s what’s necessary to keep him still.

5.     Cool the leg

Cold therapy will help slow the inflammatory process and provide pain relief. Very cold water from a hose is adequate, but standing the horse in a bucket of ice water is even more effective. Treat the leg for 20 minutes.