Tends to pop up in clusters. Is mosquito born, so getting rid of those pesky buggers should be a top priority. Fever, lethargy and extreme watery, shooting diarrhea, the kind that paints the stall walls.
Major complication is probably laminitis.
All depends on how quickly you can catch it and institute proper measures. The treatment is expensive and can require hospitalization. It is a very debilitating illness and requires time to come back from. I just found my notes on a seminar for Potomac Fever that I took several years ago and they tend to support Jo Crain's information. Davis does 1 blood test initially, 2nd test 10 days later.