Never heard of mud fever. My horse stays out 24/7 and it has been really rainy here all year as well. My horse looks like he stocks up in the hind legs sometimes but the BO never mentioned mud fever. After being ridden the swelling goes down. Can anyone elaborate on mud fever?
It's a bacterial infection that gets in the skin and/or hair follicals. You will see itchy, irritated, red skin when it gets bad. The skin can also look dry and "cracked".
Google it and you will see a lot of good photos and treatments.