Thanks for the reply.
We probably do, but I threw them away, since I did not want to risk eating sick birds.
Someone suggested it could be due to the high stress level ISA's apparently have because they're genetically enhanced to lay as many eggs as possible. But.. I slaughtered two birds in the august and one two weeks ago and they were perfectly fine, so its definately something new.
I looked up Lymphoid Leukosis, Marek's, Pullorum, Tuberculosis. And in all cases it looks like there are other symptoms as well, which I have not noticed on my birds. But maybe they were in the early stages?
I'll slaughter one or two more tomorrow morning and hope for a different result.