I had three mares go through what sounds exactly like this seven years ago.
I had taken one mare to a clinic at an indoor arena, something we had never done
before so she evidently didn't have any immunity as I do keep them vaccinated.
The two young ones at home got it really bad, the green snot thing. My equine
vet said he'd never seen anything like it either. We put them on Tucoprim and
they seemed to come out of it, then had a lighter recurrence about 3 weeks
later. The old cowboy who was my farrier at the time told me it was a form of
strangles but that he hadn't seen it since the '50's.
