IF A HORSE"S NUTRITION IS ADEQUATE from a caloric standpoint, Vitamin A along with D and E are the big factors. This is why almost all rescues and malnourished horses have a myriad of skin conditions. They are both malnourished and lacking in Vitamins. That s not the case with most horse owners unless they are dealing with rescues or newly acquired horses.
When I first got onto the vitamin A connection it was through the Moorman Mineral Representative with a PhD in livestock nutrition and working from a very advanced lab for the Moorman Mineral Company.
We did several semi-scientific trials when I got in many broodmares in the early spring. Almost all of them came in with rain rot and/or lice. Some were thin and had wintered poorly, (especially the dry TB mares) but others were otherwise in good condition -- just had horrible rain rot and lice.
He could not convince me at first to NOT treat them topically. I also had always used Chlorhexidine or some other very strong solution and rubber gloves for both the horses and every piece of equipment that touched them. [Looking back on it -- what a waste of time.] Since I neck-banded in-coming broodmares, over 100 each year, to be bred to my AAA son of Go Man Go and my leased TB stallion (first 'Classic' winner to stand in Colorado), I could easily treat half the way I always treated them and half by ONLY giving them the Moorman Mineral that was very high in Vitamins, particularly Vitamin A.
Well, the ones getting the Moorman Minerals (made for Dairy Cattle) not only cleared up much quicker than the ones I treated the way Vets had always had me treat them (and most still do), they got into breeding condition much earlier and had about a dozen other things that improved in them and not in the mares that received good nutrition but only the regular Vitamin Premix (including A) that was added to all of my feed.
I was the one that asked him if I could get them more Vitamin A more quickly by using the injectable. At that time there was an injectable horse Vitamin A-D-E on the market called 'Injacom'. He told me to try it and they responded even more quickly. Not only did they clear up their skin conditions, the ones with goopy, crusty eyes cleared up within a day or two. He told me that I would never have to 'clean' another mare if I gave them the mineral throughout the winter leading up to foaling. I haven't. They also bred back much easier and I saw far fewer uterine infections after foaling.
The difference between the mares getting the extra Vitamin A and the others was like night and day. Even the mares that came in thin and in poor condition got good hair coats and were 'bright eyed' long before they got fat.
It took me a while to give up the disinfecting of brushes and blankets, etc. He had been soooo right about everything else, that I finally decided that he was right about that. He firmly believed that you could not give rain rot to a healthy horse with a healthy immune system and that you could not do anything to keep one cleared up if they were not getting adequate A. You could not give them lice either, no matter how much contact they had with lousy horses and cattle.
The same was true for weaned calves. If you added the Vitamins to their feed, they did not get lice in the winter and spring. All classes of cattle will breed back much more quickly after calving if they are given added Vitamin A, even when they are on green grass or alfalfa, but especially in the winter on hay or dry, cured grasses.
So yes! There are other factors, especially adequate caloric intake. Obviously, thin debilitated parasite infested horses are going to have compromised immune systems. Horses that are otherwise healthy, usually need ONLY vitamin A (preferably with D and E) to completely lose the rain rot with no other treatments or disinfectants and they will clear up much more quickly with just the Vitamin A than any other treatment.