Maryellen, your Mottleds are GORGEOUS, beautiful even mottling, I'm so jealous

!
Yep, lost the two Blue Breda pullets what we surmise was leukosis. No matter what my vet could've done, the Blue's would've eventually died or had to be put down -- losing weight in spite of eating, respiratory problems that would not respond to treatment, and bloody cocci as younger juveniles, all evidences of a compromised immune system due to the leukosis. Breda owners/breeders have been wondering about the high mortality issue with Breda and chickendanz and I feel the leukosis virus passed on from mating can be the culprit.
At first I thought Marek's, but my Blue's didn't display the split-leg paralysis or nerve damage - leukosis attacks the internal organs and not the nervous system, liver I think it attacks, and there's no cure. Necropsy can mis-diagnose leukosis as Marek's because of the similar looking cancerous lesions. Some birds can survive Marek's but so far there's no hope for leukosis and euthanasia is recommended. Leukosis is passed on thru mating, fleas, lice, mites, parasites, and need a living host to continue being passed on. Marek's virus can survive in soil outside a host but leukosis needs a living host to survive. Something breeders of Breda need to know is to use only their healthy breeders that produce non-mortality chicks to stop the perpetuation of leukosis from generation to generation. I'm hoping this information is useful to help frustrated breeders of Breda who have been experiencing high mortality. Leukosis could be the culprit and eliminating mating breeders of high-mortality offspring should be culled, not to mention the obvious cleanliness of eliminating parasites.
This article explains avian leukosis much better than I can:
http://www.chickenvet.co.uk/health-and-common-diseases/avian-leukosis/index.aspx