Oh wow that’s so interesting!! My roosters seem to be most affected too and I too thought it was a fight at first with my first one. I’ve had to put down 3 now. Two last year about a month apart and then just last month another one. Of those 3, 2 of them were boys and the two last year were also both Welsummers oddly.
Also I had a cockerel drop dead last summer at around I think 8-9 months? I thought it was heat stroke as it had been unusually hot for a few days and he was huge and fluffy but complications from Mareks?
His son was the one I lost last month. His other son (the other bird’s half brother) and daughter are still going strong so far.

I lost the other daughter to a hawk so won’t get to find out if she’s resistant unfortunately. These birds are around 7 months old now so
Mine have all gotten the classic leg paralysis, droopy wings, one had the gaping, etc. although I do also have one hen with ocular Mareks.
And oh yes, chicken math has me firmly in her grip

hence not adding any more till some of these ones die or something.