My chickens range from 10 weeks to 1 year. The vet was slightly confused by the older chickens contracting it but because we found a wild bird in the yard with the same symptoms he said could be the reason for them becoming infected at such an age. Symptom wise it first starts off as white diarrhea with no solid material in it, then comes "muddy butt" (poo stuck to the vent feathers), once that sets in they begin ruffling their feathers and pulling their heads close to their body (with it being 85 degrees I doubt it is because they are chilled), from their the wings and head start to droop, and within hours they are dead. If your chicks do have pullorum, I was told, that if they have been exposed but show no symptoms they can be carrier birds meaning if you add to your flock in years to come it will be spread. In addition if you hatch your eggs it will be spread to those chicks. Another thing to consider is trying to treat the with a broad spectrum antibiotic since the symptoms are just starting.
I really wish you the best of luck! Things to check is their food and water, possibly moldy food. What types of outside things are they exposed to? Have you been or do you have friends that own chickens? One of the biggest factors in spreading disease is tracking it with shoes, clothing, or handling. Does it seem to be more of a digestive thing rather than a respirtatory thing? If it has anything to do with breathing I would suggest checking into Fowl Cholera. The symptoms are slightly different but still something to check into. There was one other option that I was looking into if I come across it again I will post it for you.
We started off with chickens about 3 1/2 years ago, so I am no expert. I just hate to think when I wake up tomorrow all my "babies" will be gone.
