Not sure that's true...
-Kathy
Talked to a Vet at the University of Alberta...Too true...They do not give wrong information....I hope the OPs birds get better...Mine were the same...One got sick and then like rungs on a ladder they all got it...They were fine for about two months....Just got it back again two days ago..This time seems worse...No deaths yet. I run a heat lamp and do let them out a few times each day in the run as not to cause them too much stress...
Bio-security is something I will never gamble with again....
FYI, UC Davis *can* test for all the respiratory diseases without doing a necropsy. Not sure why the university you work with can't, but the CAHFS labs at UC Davis can.

-Kathy