I just got an email from the breeder that said to bring the sick bird back and she will trade it for another. Would this be worth it at this point?
You can trade her, but if she has sick chickens, chances are, you won't be getting anything better back into your flock. The new one may have already had it, got well ... then, it's a carrier, forever, and could get sick again.
Now that your birds have been near the sick one, you do need to treat them as well. I LOVE Denagard! Have to get it off the net, so will take a while to get, but they do ship quick. If, you get it pretty quick, maybe your chicks won't be showing any signs yet..that would be great, treat them, and you should be good. You can eat the eggs while treating with Denagard. But you can't with what you are treating them with now..not sure they are laying?
I personally would not keep the sick chicken,,just take it back, but honestly would not bring another one back home. from anywhere for a while. That's me.

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Good Luck, and look up the comments on Denagard on BYC here. I had a problem a couple of years ago, I keep this stuff here, and give them a preventative treatment in the spring and fall.
Some folks may ask, why treat if the aren't sick...? I just don't want to have to deal with that again..too heart breaking.