Do you recall if the two newest chickens spent many nights piled with the earlier ones? Last year, I got four Blue Australorps and one of my older hens took them under her wing, literally. The close contact enabled her heavy virus shed to be passed to them before they could develop resistance. This is something I was completely ignorant about. I thought the virus would only pass vertically from the hen that laid the egg. The result was unbelievable heartbreak. I made a thread to document my struggles trying to save them.
https://www.backyardchickens.com/th...update-now-another-pullet-going-lame.1432738/ One chick survived and has been normal until now. Her only indication something is amiss is that she hasn't molted. LL has so many presentations of symptoms.
If you have a lab that does farm animal testing near enough that you can drive the sample to them, since mail is too slow to get a blood sample there before it degrades, the blood test is easy. You ask the lab to send you the test kit which consists of vials. You prick a foot and collect a few drops of blood and seal the vials, put them on ice, then get them to the lab within 48 hours. Beyond that, the blood is no good.
All of your chickens would carry LL. It's that contagious. The blood tests are more accurate than PCR tests, so this would confirm the diagnosis.
Chickens that have resistance to LL won't produce symptoms, but as they age or other factors lower immune response, it's always possible they can.
Your type of bedding doesn't matter, but keeping bacteria under control does help by not putting the extra strain on their immune systems. I do keep a meticulously clean coop and run, and it appears it pays off as evidenced by how long my chickens manage to live normal lives.