I thought about posting this in a seperate thread, but I suspect Maerk's and thought I would post it where Maerk's experts are. I hope I can be short but comprehensive.
I have had chickens about 6 years. Initially I had coyote problems which were hard to figure out. The beast would take 1 a day every day. I compensated by getting adult birds off CraigsList. Eventually put up better fencing and continued free ranging.
Then I wanted a flock of pure breed Iowa Blues. I got rid of all my stock and started over with 26 chicks in April. They were fine until about 10 weeks, then started having trouble. It would take the smaller ones, mostly hens. I had one diagnosed at Iowa State, coccidosis.
That's a little bit unusual. I have no experience with Iowa Blues but had read they're hardy birds.
What circumstances are you keeping them under? Have you used calcium carbonate lime on your soil recently? It appears to help control a surprising amount of pathogens.
It's possible the coccidiosis was a secondary symptom, due to immune suppression caused by something else.
Hatched out about 10 in August from different Iowa Blue breeder. Started showing signs of Marek's at about 6 weeks. Sometimes one leg would go, sometimes both would become week at the same time. I euthanized as needed. The last one was a rooster that made it until January, they all died.
It's possible it was a nutritional deficiency or another virus responsible. Mycoplasma synoviae, as mentioned before, is a possibility. A lot of the time respiratory disease shows no respiratory signs.
A couple of things can mimic Marek's symptoms including nutritional deficiencies. If you can, getting a proper test done would eliminate the guessing game. Nutritional deficiencies can occur despite the diet being more than adequate, it's not as simple as nutrition in = nourished bird, guaranteed, unfortunately.
I am beginning to think perhaps there is something present on your property which is damaging their immune systems till they're easy game. You're having an unusually bad run of luck there. Still, maybe your bad luck is just running into weak stock.
A hatch of 2 roosters in Oct. 31 from another stock so far seems healthy.
I believe its Marek's. I plan to vaccinate. I believe Marek's shows itself differently in different gene pools. I would like to hear other opinions. Thank you.
I've read some research that does indicate Marek's can develop different symptoms in different individuals. Immunosuppression is on symptom there is research to confirm. Different strains of Marek's cause different symptoms too and you can have multiple strains at the same time.
Best wishes.