It is still possible to be Potomac.It has nothing to do with mosquitos, it normally pops up in pastures that have creeks or streams nearby. Snails carry it onto shore and on the grass, which the horse eats.
We had a stallion with PHF last summer. If a horse hasn't been vaccinated for it, it will most likely die within a week.Our stallion WAS vaccinated and he also had Lyme disease, the vet drew blood to be tested, and immediately put him on meds. Tests both came back positive 4 days later.
If this is the case for you, treatment needs to start now.
Our guy's symtoms(remember thogh that he had both diseases): lethargic,head hung low,won't eat hay, but will grass and grain,walks slowly and painfully,high temp.
Good luck to you, I hope you get a diagnosis soon.