Hi Lill, I am having exactly same symtoms you have had, lifting of rump and coughing once or twice, he fine in walk but as soon as I ask for trot it happens. Have found if I lounge first it can help a bit. Do you or anyone else have any ideas why it might be happening and what can do to help...
Stinging nettles lose there sting once they have been chopped down and left to wilt for a few hours. I am led to believe feeding them is good for horses with sweet itch, when I can find access to some I plan to give it a go as my pony suffers badly in the summer.