I am for having the vet look at them too, but here is my thought:
Take a wet paper towel (white) and rub each horse where the hair is falling out and also on the edge of where the hair appears to have stopped falling out. I say appear because I don't think it has stopped.
Don't be too gentle about your rubbing, but don't get too rough either.
IF you see teeny-tiny black dots on that white piece of paper towel, they have rain rot.
We have had a fair amount of rain this spring, but what we have more of is humidity (Middle Tennessee). If you are like me and just don't have the time to diligently brush each horse even three times a week, it is difficult to keep the rain rot at bay while they are shedding.
Two of my horses had small patches on both of their butt cheeks. Two had patches on their lower chests and in their armpits. The Arab had a couple spots on the cheeks of his face!
Sewwwwww, on my next day off work, no housework got done and everyone got a bath with equine iodine shampoo. You can get it at any tack shop, co-op, TSC, local feed store, etc.
I did not chintz on the shampoo either. I bought a quart bottle and used about half of it between the four horses. That was a month ago. Last Tuesday was another day off from work, so they all got another high dose iodine shampoo bath first thing in the morning. I waited until everyone was pretty much dry before turning them out because I knew two of them would find a big ole dirt hole to roll in
In between those two baths, more housework was sacrificed at the end of my work days and I spent more time taking the shedder to everyone. I also keep a 50-50 mix of listerine & water (buy WalMart's brand, it's cheaper). When I was done using the metal shedder, I would shake up the mix and spray the rain-rot areas with it.
The rain rot is gone on everyone.
Again, I would still ask the vet to be sure, but you can't hurt a thing by scrubbing them down with iodine shampoo then using the water/listerine mix on them like you would an astringent for yourself.
Also, for anyone reading this that has to live with ticks getting on their horses, the iodine shampoo will make those evil little creatures stand their hind-ends straight up in the air. It sure makes them easy to find
For those of you who don't shampoo often and just hose your horses down, add some providine or betadyne to a bucket of water and just wipe your horse down quickly. It will stand the ticks up so you can find them.
Especially in their private areas, including the anal area. Ticks also like to hide at the very tip of the bone of the tail. My Big Ole TWH kept trying to tell me something was amiss with his butt. I finally looked at the tip of the tail's bone and sure enough there was big ole nasty fat tick
My apologies for getting off the subject but it was iodine related - lol lol