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.
Warm climate produces greater danger vaccine very safe, generally little or no reaction.
Safe for foals, pregnant mares and breeding stallions.