I was wondering that too, if Cocci could be a secondary infection. In my older chicks...it showed up as lethargy and they died within hours.OMG! I'm so sorry!
Possible mycoplasma synoviae? What other illness can cause respiratory and neurological? And that kills so fast? I wonder if you have more than one thing going on? Have you considered cocci as a secondary infection?
I'm not that familiar with Newcastle. I'm off to read about it too.
I'm so sorry you're experiencing this magdelaine.
