I've pretty much decided that this is a result of hatcheries breeding for non-stop production. Maybe some day we'll see a definitive answer, but I consulted someone very knowledgable, and barring any diseases that adversely affect the reproductive tract, he said it has to be genetic. Since I have not had any respiratory illness in this flock, I would have to go with that assessment. I also received an email from someone who has lost several of her hatchery birds(different hatchery but a big one), to the same thing at the same age. They were all non-stop layers, too. Her birds, even hatchery stock, who took breaks in laying, for molt or broodiness or just a rest, lived much longer. If it was feed, they'd all be affected and I should be seeing this issue in my younger layers, breeder stock, etc, but I'm not...at least, not at this time. Anyone who complains about their birds not laying in winter or going broody needs to think of that in a more positive light. It may save and/or extend the lives of your hens.