My horses and a few of the others at the same farm have what people are just calling "fungus." Mostly it isn't visible until brushing the horse's legs and the hair starts falling out. On the lighter horses there is a bit of black speckling that looks like dirt. There is no open skin, sores, scabs, etc. The horses don't appear to be itchy or in pain. I know it can be treated with an antifungal shampoo, but my main concerns are:
1. What is it? Rain rot? We were having a drought until a few weeks ago and it seems like one of the horses had it before we had any rain or dampness. We did have a massive heat wave though.
2. Someone mentioned that they think it is spreading from horse to horse via grooming tools, tac, and horse to horse contact. How contagious is it and how can I slow/prevent the spread?
Thank you!

Mares or geldings? Could be from pee splashing on the legs or a fungus. Just wash with an anti fungal shampoo. And you should never be sharing grooming tools anyway so that shouldn't spread it.

They make a betadine shampoo for equine that should clear it up. Dont share your grooming tools and its good to sanitize them with a little bleach water. Without a pic it's hard to say but my haflinger had what your describing and the vet called it dew poisioning. We washed with the shampoo and pulled the little scabbies off and by the end of the week you couldn't tell it had ever been there. We spritz our horses with vinegar and water which restores your horses ph levels in their coats. Also don't share your saddle pads as you can spread things that way as well, all it takes is a little break in the skin that you may not even see for horses to spread things in the herds.

Thank you!
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It seems to be just on our geldings right now. I have been using the same brushes on my horses, but I will definitely stop doing that! One of the girls who shows her horse said she thinks her horse has it on his face and body too. I have some betadine shampoo so I will try that today.
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Can you get pictures? It's not ringworm is it? That will spread through the herd.

I don't think it's ring worm. It literally doesn't look like anything until you brush it and the hair falls out. Then it just leaves a patch that is lighter than the other spots with only short bristley hair on it. There's no raised or raw skin. No circular mark or scabs. I would post a picture, but I don't know how :)
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My mare gets this on her back legs. Sometimes on her front legs. She has white legs though and she gets scratches( it has many names, dew poisoning, mud fever or sctratches are all the same thing). That is how hers starts then it turns into scabs. I use a rubber curry to get all the dead skin/hair off then wash with Microtek mediacted shampoo.

Edit: this is an ongoing battle with my mare and when she gets it, it requires daily washes.