Here is what I wrote last winter about Vitamin A and skin and other problems.
Rain Rot or Rain Scald and lice are almost always nutrition related. Feeding adequate levels of Vitamin A will prevent almost all cases of rain rot and lice as well as many other conditions that occur mostly in horses with no access to green grass or in the winter and early spring when Vitamin A stores are depleted or intake is inadequate when horses are stabled. Other conditions related to a Vitamin A deficiency are:
1) Rough hair coat. Predisposition for getting skin conditions like rain rot and lice. These horses can look wormy even when they are not.
2) Goopy, runny, crusty eyes.
3) Mares failing to ovulate or conceive.
4) Mares failing to drop their afterbirth within 30 minutes of foaling.
I have recommended that several people initiate a program of supplementing vitamin A, especially during the late fall, winter and early spring months and all have reported having rain rot and lice disappear without any further medicating or treating.
There are several ways to supplement Vitamin A. If a horse is seriously deficient, I recommend using the injectable form of Vitamin A only just give it orally and not in shot form. Start with about 5 cc and just squirt it into the horse's mouth. Then follow it up with 2 - 5 cc a week. You can get it at any feed or farm store that caters to stockmen. It is a non-prescription item or you can get it shipped from any one of several catalogs like Jeffers or Valley Vet.
This should be followed up with a good supplement that has high levels of Vitamin A. Farnam has a product called "Mare Plus". It will prevent rain rot and other conditions related to Vitamin A.
Vitamin A deficiency frequently is accompanied by a mineral deficiency -- mainly a Calcium deficiency. I feed a loose mineral supplement that contains high levels of Vitamin A plus Calcium, Magnesium and zinc. Since feeding it, I have not had ONE SINGLE CASE of Rain Rot or of lice. All but 4 or 5 of our 60 horses run out with no shelters other than trees, etc on large pastures. Most are not fed grain but only get free-choice winter pasture and/or free-choice round bales of mature grass hay. They get very little Beta Carotene (the precursor of Vitamin A) or Vitamin A from their diet.
When I get in a new horse with rain rot, I do not do anything other than put it on good feed and supplement Vitamin A. I will use oral Vitamin A for 3 or 4 weeks until just keeping out our mineral can take over.