Highly contagious, YES. Miserable, YES. Pain in the ass, Yess. Fatal, usually not.
Honey had it when I bought her. Had to quarantine her, give her injections, and drain her entire jaw area EVERY DAY!
I had to lance the walnut to softball(there were lots of them) sized abcesses a few times a day and drain the contents into a bucket(about a half cup to a pint would come out each time).
She could not eat for a few days and had me really worried when she seemed to have trouble breathing(hence the name "strangles"), I had to use a cooking baster to force her to drink... You could not see her jawline, her jowls were swollen... happy honey.JPG
During healing, you can see the funny angular look to her jaw? That was an abcess I had just drained, I had to bribe her to eat with carrots, though she was only lipping them even at this point. You can see the swelling, her face had no distinction of any bony processes.... sierrapics 121.JPG
This was the day before she started showing symptoms.
It was miserable.
My advice is to move your horse ASAP, until the whole facility is sanitized and all the horses in residence given the all clear.
OR (if you cannot move her)
Clean anything the infected horses could have come in contact with with a strong bleach solution. Get your horse vetted if it has not had its distemper shot. Make sure the sick animals are quarntined away from your horse. Do not turn your horse out in any paddocks recently used by the infected animals.