The internal form can be hard to determine. The horse would have to have signs of Pigeon fever but no draining abscesses. Generally needs ultrasound/blood titers for diagnosis. Luckily this kind is rare.
I think many areas are getting more of this disease. Wetter springs and dryer summers. And our flies this year were unusually persistent and biting. It was also so random who contracted it at our barn. My horse that is post EPM and has a weakened immune system did not get it .... most of the horse that go it were average age , one was older. The ones that were infected did not infect the horse next to it.
But as our weather patterns continue to change we may be seeing much more of this.