Its when the horse scratches its mane and tail all the time, making the area sore and pulling great chunks of hair. It then gets scabby and can become infected, its thorght to be caused by an allergy to midge bits
And yes it is Sweet not Sweat, tho I've no idea why its called this.
I though you meant for yourself! (Humans are affected with it , too). But have a look on Google for 'Sweet Itch', there's loads. Blame the prissy Victorians for changing the spelling.
well what works for our daughter's pony that gets it is get some aloe gel mix it with citronella oil at a ratio of 20:1 and it's really helpful for the pony. We tried it on another horse though and it barely made any difference.