As it turns out, the suspiciously gunshot-looking head wound only extended to the bone. Pathologists tested her muscle and found tons of the Clostridium Sordellii bacteria present. They noted that she was muddy (the bacteria is often found in soil). Another common source is contaminated feed, but my horses are only on pasture and farm-raised hay, so that's unlikely. The pathologist pointed out that the actual cause of death was the head trauma, not the bacteria, so after talking with my vet it seems the most likely scenario was that she somehow came in contact with the bacteria, became septic, then injured her head during the last bit of time when her body wasn't functioning properly. She was seen healthy and normal at 3:30 pm, and found dead the following day at 9 am. It's scary thinking that these "baddies" are lurking around in the soil and there's no vaccine or anything else that can be done about it. I have 7 other horses on that farm, all Kentucky Mountain Saddle Horses. I can't help but wonder if my gypsy mare was more susceptible to the bacteria because she was purebred or because her mother and grandfather were imported from England. I wonder if the horses that have been born and bred in KY have a naturally higher resistance. My vet told me that it is transmittable to humans. I had my hands all over that mare, but he said that if I had been infected, it would have already shown itself.
If you all know anything about this bacteria, I'd love to gather as much info as possible.