It is always Vitamin A. We have been treating horses that have come in with rain rot by using Vitamin A for about 40 years now. It works 100% of the time.
You do not have worry about disinfecting horses or tack if it is not ringworm. Ringworm is a true pathogen. The Bacteria and Fungi that cause rain rot are normal inhabitants of soil, so every horse is exposed to it all of the time. If their immune system is healthy and their skin is healthy, you cannot give it to them if you tried.
The same thing is true of lice. You cannot keep them off of a horse with a Vitamin A deficiency and you can't give them to a horse with healthy skin and a good immune system.
When we encounter rain rot, we give a horse 2 or 3 doses of Vitamin A (usually mixed with D and E) by using the injectable form used for cattle. We just give it orally and do not inject it. I use about 5cc a week until the good feed or supplements kick in. Then, we start them on our feed which is a custom mix and has an A-D-E premix added to it at the feed mill. We also use a loose mineral that has 150,000 units of Vitamin A per pound in it. We are very picky that they get plenty in the winter and spring. Horses will never get rain rot again as long as they are healthy and getting plenty of Vitamin A.
If you do not have access to fortified feed, you can use a supplement made by Farnam called 'Mare Plus'. It has a high level of the antioxident Vitamins and does not have a lot of the ones that make horses get high and silly.
Other symptoms of a Vitamin A deficiency include:
Runny, goopy or crusty eyes with stains running down from them.
Frequent bouts of Uveitis.
Scaly skin, bad hair coats, dandruff and susceptibility to rain rot, dew poisoning and scratches. Slow shedding with no other cause (like Cushings).
Lice -- usually in late winter and early spring in the US.
Mares that do not drop their afterbirth within a few minutes of foaling.
Problems breeding mares back after foaling.
Poor hooves and slow hoof growth.
All of these things are directly influenced by a lack of Vitamin A.
Horses get Vitamin A from the Carotene in their feed. Hay and stored feed does not keep Vitamin A in it for more than a month or two, so during the winter season, horses run out of Vitamin A. Vutamin A and the other fat soluble Vitamins are stored n the liver. They liver runs out in the fall and the horse is susceptible until the green grass comes out in the spring.
Some horses evidently store Vitamin A better than others. It will be the same horses, year after year, that get lice and rain rot before the grass greens up.
I have been recommending Vitamin A for about 40 years now, and not one person that has supplemented it has not had a problem with a single healthy horse since.
I have no idea why so many Vets are clueless about this, but they are. They also are not very up on other points of nutrition. I think they make more money selling a bunch of crap you don't need. I have known several people that insisted on using baths and sprays and salves and all kinds of labor intensive and expensive junk that the Vet sold them and fought rain rot until spring and then, miraculously it went away -- the same time the grass greened up. You cannot kill it and keep it gone if the horse just keeps getting it all over again.