If I were you I wouldn't worry way too much. Last year we had a yearling and a pregnant mare get it. Albeit we didn't know the mare was pregnant at the time, but her foal is healthy as can be.
The yearling got better super fast. From my experience pigeon fever isn't going to kill the horse. Whenever they got the swollen bump we would slice it open, drain it and then flush/treat it every day.
I just googled pigeon fever and those pictures are way more intense than the normal case, and our vet only made a 1/4 inch or so incision to drain the pus.