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Until you got the new bird you're flock was fine. And I think most of us would be in the same boat. So THAT's where I get confused.
If we assume our birds have been exposed but are healthy, wouldn't it be logical to say they are Resistant? If that's true why do we see such devastation when a new bird, carrying the virus, is introduced? It just doesn't add up.
Since Marek's is endemic to Western Washington, it would probably be safe to assume our chickens have been exposed. Some breeds are resistant, and older birds (over one year old) seem to be less likely to develop severe cases of Marek's, so it would also be safe to assume they had been exposed in the past and have some immunity against it.
Marek's is mostly a disease of young birds and pullets at point of lay. In my case, I had a sickly pullet that lived in a stall in the barn. I thought the rooster had hurt her leg. My broody hen lived in the stall next door with all the dander from the sick bird flying around. When my incubator hatched chicks were big enough to go outside, they were also in the barn breathing in the dander from the sick bird day in and day out. This went on for two months before the sick bird became unable to walk and I sent her to the lab. My babies were all infected by then. They inhaled so much Marek's they really didn't have a chance. There has not been a lot of research done on the incidence of exposure vs. disease, but I can assume that if the chickens had small doses of Marek's over a long period of time, they might build some immunity. However, if they are slammed with mega-doses of virus, it is just a matter of time before it kills them.
So far the only pullets that seem to be OK are the australorps. Maybe the breed is resistant?? I don't know. My Speckled Sussex that were raised in the broody in a stall with all the Marek's dander also seem to be really healthy - knock on wood - they are only 16 weeks old.
There are also several different strains of Marek's, with some being more deadly than others. I think I have a particularly bad one.
Thank you for explaining. I have a friend dealing with Mareks, too. The more situational information I get the better chance I have to understand, so this helps.
So you raise Sp Sussex? I tried to get those this year but it didn't work out. Where did you get yours?
Best of luck in dealing with this, my heart goes out to you. Take care!