Front Line and K9 Advantix (products in the US used to treat fleas in dogs) works well when applied to the skin below the feather, if it's being caused by a mite in the mud. If it's being caused by a fungus or bacteria in the mud, then I recommend using diluted dish soap (we use dawn) to initially wash the legs, rinse and towel dry, then I've applied an atheletes food powder to the legs, as it's medicated for fungus and bacteria, and I leave that on. Do it daily. Everyone has their own methods, but these have worked for me...
"It is true, I want a great many things I haven't got, but I don't want them enough
to be discontented and not enjoy the many blessings that are mine."
Elinore Pruitt Stewart, Letters of a Woman Homesteader