I've had the opposite experience. I'm 4 generations into a AVL positive flock. I hatch my own chicks from PCR tested positive hens. I've had no losses or symptomatic birds for 15 months. All 28 hens where laying last fall and some have restarted this month. (They are not great laying breeds) . 2 years ago, I added 5-month-old pullets.... They were dead in 3 months. I don't know what to do now. I've stopped hatching because I'm inbreeding so much. Also, I can't sell these birds or get rid of extra roosters because they are contagious to other flocks and potentially wildlife.