UPDATE ON MY ISBAR SAGA:
Some of you were interested in the eventual outcome of my situation. I will make a quick recap for those not familiar.
I've had a breeding trio of Isbars since spring. All chicks hatch fine, and at about 2-3 weeks old stop eating and drinking, and then die. Probably 90% death rate. Very frustrating. The adult pen is moved in the yard once-twice per week, brooder and incubator properly disinfected, tried both regular and medicated feeds, treating for cocci, disinfecting eggs, all with no improvement. I was puzzled as the only conclusions I had were 1. genetic problems (but my rooster is from a different person than my hens so I didn't think this was likely) 2. some kind of infection that was vertically transmitted. From my research most yolk infections lead to death in less than a week.
I took 3 chicks an hour away to the state university testing lab. While waiting for the results, I began treating all the pens with Tetroxy HCA (tetracycline). By the time the results were back, my chicks were surviving again- about 80% survivability. OSU was very surprised that tetracycline worked as it is such a low level general antibiotic but they sent me the results and wished me luck. I finished treating the adults and all chicks that were hatched before the eggs were "clean" and then stopped treatment. Well in the last few weeks again my survivability rate has dropped, to about 40%. So I took the next step suggested by OSU and took a chicken and the lab results to a vet this morning. Finding a vet to see a chicken was a ride in itself....
OSU paperwork shows positive cultures for a strain of e. coli, with susceptibility to Baytril and several other antibiotics (OSU had recommended trying Baytril first). The vet said US vets are no longer allowed to treat poultry with Baytril, and the FDA is going after vets who prescribe it for poultry. Instead we are treating the adults with the other OSU recommendation, I can't remember the name but they listed it as TRIMETH/SULFA. In the vet's opinion, there is no question in his mind that my original hatching eggs carried this when I hatched them and it came from one of the Isbar breeders I got stock from. It is not a strain typically found in the soil which I had wondered about. He said the Tetroxy HCA only knocks bugs down so the bird's immune system can fight it, it doesn't usually kill something like this. He asked me to keep him updated in a few months as he is interested to know if this does the job. My only other option is to cull this batch of Isbars and start over.