Help! All of my chickens have started loosing weight! - Page 2
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Another thing to check for is have you sprayed your coop lately- the red mite can live in the cracks of the coop and feed at night, slowly killing the chickens and you would never know it. Always retreat in 7-10 days when killing mites. And of course check the birds too- the Northern Fowl Mite lives on the birds. If you do see bugs on the birds I have some nice links for that if you ask for help.
As far as worming goes, here is a handy chart:
If you just wormed them and they are still ill you might consider taking a fecal sample to a vet for a check for cocci/worms (false negatives are possible on the worm test).
A necropsy will tell you definitively what was wrong- that is what I would recommend. You can contact your state vet or county extension agent for information.
It is possible they are struggling with an infection or some other problem. Wormers can kill them if they are weakened and I'd not just keep throwing wormers at them until you have a diagnosis. Mites can kill a flock, making them anemic and weak/thin. I'd definitely check in that direction next.
Edited by ChickensAreSweet - 2/13/13 at 10:44am
If your coop is only a couple of months old I would not think it would already be infested with red mites but that's just me. Might want to check for bugs on the birds though.
I treat my chickens every 4 months for bugs to keep mites away (and the coops).
Sorry I don't have any more ideas for you.
If you wanted to try an antibiotic you may want to try LA200 - i believe the dosage is - .1ml per lb. or 1/2 ml for 5 lb bird. But search on the the forum to be sure.