The horse is back home and doing well. The skin is still healing, but no new lesions have come up and he is feeling and looking well.
Unfortunately they have not been able to diagnose what it was. Nothing came out of the biopsies and any of the other tests.
They sent him home with "hoping it won't come back again".
More on the mud fever front. This year seems to be a really bad time for it. Last years el nino has lasted up until this month- normally el ninos here locally are followed by drier winters. Most of the horses in the barn have had one or two mud fever episodes. This year the vets have had us wash them with a mild soap, towel them dry than apply small animal antibiotics, along with an anti fungal mixed with grease. Cover with saran wrap to act as a moisture barrier, then gauze and vet wrap. Keep them inside out of the over fetlock deep mud. the Mud Fever goes away in 4 to 10 days depending on how much help the horses immune system can give. A mare from Montana where its mostly too cold for the evil, wicked, scheming germs to thrive had a more severe outbreak that lasted longer than my gelding who has been dealing with this stuff for the last three years and has consequently been building antibodies. Recently some folks in the barn have switched to a triple antibiotic with an antifungal followed by a spray on bandage and have been able to knock it out in 4 or 5 days. So, I'm wondering what folks who are getting record rains in other areas are doing- or if they are even having mud fever issues.