I would try to get a few sick birds tested as Wyorp Rock has suggested. Your vet may be able to locate or suggest a poultry lab. If you got eggs in June, and kept them isolated in a brooder in your house until they were old enough to go outside fully feathered, you might be close to 5 months. Read a little about strict biosecurity with flocks, and take a chance on it. Consider getting new chicks vaccinated for IB at age 6 weeks. Here is some info on biosecurity:
http://extension.uga.edu/publications/detail.html?number=B1306&title=Biosecurity Basics for Poultry Growers
http://extension.uga.edu/publications/detail.html?number=B1306&title=Biosecurity Basics for Poultry Growers