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Yep. Wild birds spread carry these diseases everywhere.
And to add gasoline to a fire- I am not sure that culling a chicken that has survived a disease is doing Ckickendom any favors. In other words- How can we produce chickens resistance to these diseases if we cull them?
Dont jump all over me. This is JMHO.
I'm not going to "jump all over you". I am going to explain why this theory does not work biologically lest people are reading and thinking "Gee! We can keep sickly birds because they'll pass on RESISTANCE!"
Ummmmmm, no.
That would be great if that's how it worked, I suppose. It'd certainly be a whole lot more feel-good for those who like to save everything, if nothing else. Unfortunately, it just does not.
The first thing we have to understand to know why not is how exactly being sick results in immunity or "resistance" to that illness. When a chicken -- or any animal -- is ill the body creates certain antibodies to fight that disease off. In some cases these antibodies remain in the body for a long time after the illness has passed, ready and waiting, to attack should that particular nasty rear its head again. In other cases the body simply "remembers" how to quickly and efficiently manufacture these antibodies; it "learns" how to fight off the disease so next time it's faced with the disease it's able to fend it off quickly and easily. This is an exposure related resistance. It's resistance, but it's not a natural lack of sensitivity such as the resistance that you'll often hear people talking about breeding in.
There are a couple of reasons for this. The first and foremost is that exposure resistance cannot be passed on from chicken to chicken to chicken. It is due to personal exposure and that is the only way it can be gained. Each chicken must develop this immunity on its own. There is evidence that antibodies can be passed trans-ovarially, that is from hen to chick via the yolk in the egg, but these antibodies -- like the antibodies passed to other species in the same way -- are short lived. Most are completely gone by three weeks of life at which point the chick then has zero immunity to those diseases. Another important thing to know with this is that even when antibodies are passed on they're usually passed on in much smaller quantities than they are found in the hen herself which means the chick isn't even wholly protected on day one. He simply has a small amount of antibodies present that may or may not afford him a little bit of immunity. It's also important to know that not all antibodies are passed on to begin with. Antibodies against Salmonella, Infectious Bronchitis, Fowl Plague and a few others have been observed but there have been plenty of trials where none at all were observed as well.
The second is that exposure resistance is expensive. To have every chicken needing to get ill and then better again in order to capitalize on "resistance" is not an economic way to raise birds. In many cases carriers are the exception to the rule, the survivors and the mortality rate of the disease may be as high as 80-90%. Losses are expensive, treatments are expensive, ill chickens are time consuming. Aiming to create birds that will result in time and money down the drain is not effective. That's why we have vaccines, to artificially give exposure immunity to chickens who cannot be reasonably expected to have natural resistance. It's cheaper, quicker and more effective to administer faux exposure immunity than to wait for the real exposure immunity to take its course with one's birds.
Now, the type of resistance you would want to (and can) breed for is natural resistance; a genetic propensity to lack sensitivity to pathogens and a strong immune system to fend them off without clinical affliction. These are the chickens who never fall ill to begin with. Some call them hardy, some call it vigor. Whatever you call it they have a demonstrated ability to not just survive, but thrive. This is "survival of the fittest" because chances are your chicken with exposure immunity wouldn't have survived the disease without your intervention. These are the chickens that can pass their genetics on to offspring and be reasonably expected to create more of the same. Because their resistance is genetic, not created out of previous illness, it can be passed on to future generations.
This isn't to completely discount environment. There is strong evidence across species boundaries that indicates that an overly-sterilized environment leads to a compromised immune system. But there is also strong evidence that a genetic component for reacting in such a way that the immune system is boosted when exposed to pathogens, rather than afflicted with illness, plays a large role in the overall hardiness of an animal -- which includes humans.