She is a little better today (a very little) but I think culling is on the cards tomorrow if there is no improvement.
Reading the link I posted, it would seem that Meraks is everywhere, you can't really stop it, you can't cure it and, even if you don't think your birds have it, they probably do but are not showing symptoms or it is latent.
I'm not stressing about it. I do my best for my birds: they get good food and clean water, clean dry well ventilated housing, have 24 hours access to outside, are free range all day and are not over crowded. What more can I do.