I am a lab tech at the arkansas veterinary diagnostic lab and most of the cases we see here have been identified in goats, however we have still seen a significant increase in horses as well. Below is a good article to read if your not sure what pigeon fever is.
Also the article does not mention this but I believe everyone should know, if you and your vet suspect pigeon fever be careful what antibiotics you give the horse. Some antibiotics can actually cause complications with this particular organism. Basically they will cause it to retreat deep in the glands and hide out there indefinitly.
The Horse | Pigeon Fever in Horses Update