VERY SICK LETHARGIC CHICKEN, PLEASE HELP!

Best of luck, I would isolate her away from the rest. Often respiratory issues are viral.

 

As silkiechicken said, I would definitely isolate her from any other chickens, as respiratory diseases are often contagious. I don't have experience with either Doxycycline or Liquamycin LA200, but I would treat with those for maybe 3-5 days and see if she improves. If not, I would start treating her with the Tylan 50 injectable, as that antibiotic is great for treating infections.

The Tylan50 injectable dosage is 1cc for large-fowl, .5ccs for bantams, injected into one side of the breast once daily for 5 days. Alternate the side of the breast that you inject into, and use a small gauge needle, as Tylan tends to make the injection area sore. Don't feed your hen any dairy products during antibiotic treatment; I think the chick boost and ACV is fine (unless you are giving oral Tylan--then you don't want ACV in the water).

You also need to get her eating. Perhaps try feeding her some treats like scrambled/hard-boiled eggs, moistened feed, fruit, mealworms, etc. If she won't eat that, you could try mixing some feed with water to form a soup and then spoon-feeding that mixture to her. Or, you may have to begin tube-feeding. Here are some links about tube-feeding: https://www.backyardchickens.com/t/722041/how-to-t-feed-a-sick-chicken-and-give-subcutaneous-fluid
https://www.backyardchickens.com/t/...h-my-bird-pics-for-visuals-very-detailed-post

If the disease is viral, then antibiotics will have no effect, as they only work against bacteria. In that case, the only thing you can do is give supportive care (vitamins, nutritious food, warmth, etc.) and hope that her body fights off the infection. But if she doesn't improve, you might consider culling her, as some respiratory diseases never go away.

 

Yes, definitely isolate her. It sounds like she has a respiratory disease, which are usually very contagious. Some diseases are caused by bacteria, while others are viral. The Liquamycin should work if its a bacterial-caused respiratory disease. If it doesn't work, its either a viral disease, or you need a stronger antibiotic. I personally recommend giving the Tylan50 injectable, as it is a really good antibiotic for respiratory diseases. Give 1cc for large fowl, .5ccs for bantams, once daily for five days.

In the meantime, I would discontinue the apple cider vinegar. I believe that it is not a good idea to give ACV while giving antibiotics. Don't give any probiotics or yogurt, either, as they will interfere with the affect of the antibiotic.

Make sure that she eats enough by giving some scrambled egg or raw egg yolk. Most chickens also like applesauce or moistened chicken feed. If she doesn't eat enough, you'll need to think about tube feeding.

 

As a total aside thought, just remember that use of antibiotics will always increase the population of bacteria (good or bad) that are resistant to the antibiotic. These resistance plasmids that bacteria can carry, can transfer between bacteria in the bird and bacteria that live on you and the surrounding environment.