Mud fever is a disease that is most likely to affect your horse in winter. You will know if your horse has mud fever by looking at its heels. If your horse’s heels are cracked, red and sore, you know that it has mud fever. Mud fever is caused by your horse standing in wet, damp or muddy ground for long periods of time. If you see that your horse has mud fever, call your vet so that he or she can help you treat it.