This is what I have successfully combated mud fever with ;- Make sure that the legs are clean,I prefer not to wash the legs so wait until the mud is dry and then rub it off gently with your fingers,I try not to brush the legs if possible because brushing can cause the already weak skin to break and then allow more bacteria to enter and spread making the condition worse, then apply a generous amount of suda cream which is nappy rash cream that is used for babies and it is very cheap and you can buy it from most places,then if the condition is not too bad and the mud is not knee high then just turn out and then just repeat the process daily,if the condition is bad and/or the mud is knee high get a clean piece of gamgee or something and put it onto the area then bandage over with exercise bandages,they will get muddy and wet so be sure to have enough bandages so that you can change them to fresh ones daily.
To prevent mud fever I apply a good layer of suda cream as above about twice or three times a week depending on whether the mud is knee high or just hoof high.Good luck and remember that the mud fever bug is still around at summer so always be on guard and once your horse has had mud fever it is more likely to get it again.I hope this helps.