Broilers are slaughtered at 8-10 weeks. Layers aren’t. The virus doesn’t really get a chance in broiler flocks. Also different breed attributes, broilers may be less suseptible.
I have a theory that the culling protocol is reversing “survival of the fittest” hardiness in the breedline since there are no flu sresitant survivors to breed from.
You might have a bit of a point on the breeding/engineering that is reducing resistance to illness... that is quite plausible. But... it doesn’t answer the SUDDEN susceptibility (claimed) of US flocks.
The reality is - the US is the only nation slaughtering millions of laying hens with almost zero foundation for such action. FACT - if a single test run shows a postiive result on a farm - 100% of the birds on the farm are killed and burned. Even if the hens are stretched across multiple laying houses with lots of space between...
that is ONE SINGLE POSITIVE test. No double-testing to confirm either. Draconian to say the least.