Regular changing of the nest bedding can work wonders. -have raised many chickens.
That's true. Even then, every once in a while you end up with a blood covered one.
The whole point is to preserve the protective coating and I don't see how this can happen when you clean the eggs. I'm not a chicken farmer though and maybe the dirt
is removed somehow.
They aren't washing them. More commercial operations might brush them with a dry brush (to remove dirt and chicken poop), while others are just collecting them more immediately, but it's illegal in Europe to wash eggs -- which is required by law in America. Washing eggs removes the bloom, which coats them and protects the permable egg from infection. The trade-off is that European eggs should be washed just before use.
In America, we mandate that eggs are washed in a chemical bath. This diminishes the bloom and replaces it with a chemical wash.
Both mechanisms are ostensibly intended to prevent Salmonella, but I don't have data on which is more effective.
As this piqued my interest I tried to find data but it is complicated mainly by the fact that vaccination rates are vastly different. If we go by raw, reported Salmonella cases the American approach seems to be significantly safer, but then again we seem not to have a good idea about the unreported cases. It probably doesn't help that it is a politically charged topic too - see
"Eggwina" in the UK [1].
[1] https://en.m.wikipedia.org/wiki/Salmonella-in-eggs_controver...
Maybe they're getting a thorough wipe? That's about the only way I could imagine this happening, unless you have 24/7 surveillance to clean as soon as they poop and snatch up the eggs. Even then, both sometimes happen simultaneously.