So, we’ll have to do the best we can with what we have. This is what I’ve learned through a lot of trial and error over the years:
The terms mud fever and scratches are more or less interchangeable. Older books will refer to it as dew poisoning, greasy heel or cracked heels. You could compare them to rain rot or rain scald, as it is basically the same condition occurring on the legs. The skin is designed to have a healthy layer of natural oils, grease and dead skin to protect it from the elements and infectious agents. But, overexposure of the legs to wet conditions, too much washing and scrubbing, brushing with a coarse brush, etc. and even sunburn can break down the skin’s natural defense against the bacteria, fungus and even mites which can cause scratches by softening the skin, opening the pores, removing the grease and abrading the protective layer of dead skin cells. This opens up the skin to infectious agents and you get “scratches.”
It can start as just redness, inflammation and hair clumping together and falling out, but if left untreated can become a full-blown infection where the skin become severely inflamed and can crack, allowing outside infectious agents to get into the wounds and fester there. These cracks will exude serum (which is part of that crusty stuff you see on the pasterns and heels) and because the skin in this area stretches a lot, they will continue to open up and get even worse. Sometimes the horse will go lame due to them, either because of the painful cracks or the uncomfortable swelling.
The best way to treat this persistent condition is to clip away the hair from the area, both so you can dry the area quickly and so you can get the medication to the source! Then, for a...
Mild Case (red or chapped skin): Give a good wash with mild soap and water, dry thoroughly and apply Desitin.
Moderate Case (inflammation, but no cracking): Wash (avoid scrubbing) with Betadine or Nolvosan scrub, dry thoroughly and apply a cortisone ointment to help relieve inflammation and protect the skin from moisture.
Severe Case (lesions leaking serum, crusty build-up, infection): Soak ONCE in a warm, MILD saline (I like to use sea salt) and Betadine solution for 15 minutes, rinse, towel dry and apply a warm, moist Animalintex poultice pad to the area, bandage with gauze or Vetwrap, and apply standing bandages as necessary (do not apply Vetwrap to a bare leg, but pad with sheet cotton, gamgee or other bandaging material to avoid cording/a bandage bow.) Leave this on for the recommended 12 hours and let the legs air for the other 12 hours in a dry pasture or clean shavings. Reapply Animalintex. Do not wash off the medication while the legs air (though if the treatment lasts more than a day or two, washing may be needed every other day.) Reapply Animalintex. In the case of severe or persistent infection, 24 hour bandaging may be in order. Avoid muddy turnout or work during this stage. The medication in the poultice will work on the exudate and infection, while soothing the skin and providing a good, protective layer against further infection. Resist the temptation to pick at the crusty scabs; using the bandage wet helps soften the crusty build-up and remove it gently on its own.
Severe Infection: In a very severe case where there is a lot of inflammation, infection, or the horse is lame, it is best to call a vet, as oral or injectable antibiotics and/or anti-inflammatories may be needed in addition to topical treatment; only your vet can advise on this. While you should always follow your vet’s advice on such matters, I personally think Panalog ointment is a waste of time on crusty scratches, and you may want to talk to your vet about trying the Animalintex for a few days first, and allowing it to gently remove the crusts before applying the Panalog - you may even find you don't need it at all.
Once the skin heals, it is important to keep the legs clean and dry, and to avoid washing with soap in order to allow the protective oils/grease to accumulate on the skin again. If you know your horse is prone to the condition, you have to be especially careful. If the legs are muddy, wait until they are DRY to brush them, and avoid using too coarse a brush, as it may abrade the skin and open it up to new infections (I like to use a soft rubber curry or mitt and then a medium-soft bristle brush.) You can rinse the mud off if you dry the legs afterwards. If you have to wash them, use a mild soap, dry thoroughly and apply something protective like Desitin, Corona, or Horseman’s Dream, etc..
While treating to prevent infection, be careful not to over-do with the Betadine, as it not only strips the skin of its protective oils, but it can dry the skin out and lead to cracking and... scratches!