Photos?
Can you please post photos of your Pullet and her poop?
What do you feed, including treats?
Possible it may be Coccidiosis and/or worms. Her getting better when separated out with feed, PND, etc. then getting worse after being with the flock for a few days is a bit of concern.
Is she being picked on or kept from food/water?
Yes, you separate her out treat Just Her for Coccidiosis and worms if you wish to do that. OR if you wish to leave her with her friends, then treat all for Coccidiosis and just treat Her for worms since the worm medication is given orally daily anyway.
You can find Safeguard 10%Liquid Goat Dewormer at stores like TSC, it will be the easiest dewormer to source. Valbazen (Albendazole) is a little hard to find at the moment and it does have to be ordered.
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Liquid Corid dose is 2tsp or Powdered Corid Dose is 1 1/2tsp per gallon of water given for 5-7 days as the only source of drinking water.
Do not add any extra vitamins/electrolytes that contain B1(Thiamine) to food or water during the course of treatment.
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To treat most worms that poultry can have except for Tapeworm you can use Safeguard (Fenbendazole) or Valbazen (Albendazole). Both are given orally by weight; they do not mix well with water.
Safeguard 10% Liquid Goat Dewormer or 10% Equine Paste dose is 0.23ml per pound of weight given orally once a day for 5 days in a row.
Here's how to give oral medications:
https://www.backyar