My third is likely dying, with similar symptoms, but I think I know what it is--internal fungal condition. I believe they got it from eating some corn on the cob I fed them that I'd trimmed some black mold off of, plus possible mold on a dirty, pressboard coop floor.
The fungus uses up calories and nutrition, plus causes damage in guts causing poor digestion, and perforations in the walls of guts so contents are leaked out into the rest of the chicken's body. The fungus also produces bad toxins.
My hens have been emaciated despite the fact they usually had free access to food and ate well early on. They had bad diarrhea that went through different phases. They became increasingly lethargic.
I did not spot problems with my first hen until it was too late to help.
With 2nd, I wasn't sure what the condition was.
With the current one, I didn't know what I needed to do to treat early on, and it may be too late now.
If it can help anyone else's chickens who have a fungal condition, though, here are treatment guidelines based on my research on the internet:
Diet is critical!
No grain, or less that 15% of diet at the very least. Oat bran or other low-carb grains are preferable. (This means you have to restrict using lay mash, grower feed, etc. and give no scratch grain.)
Hard-boiled egg yolks.
Green vegetables. Cauliflower. Pumpkin and pumpkin seeds (mash up).
No carrots or other sweet vegetables.
No cooked or canned tomatoes. Unsure about fresh tomatoes?
Flax and sunflower seeds.
Freshly hulled nuts (except don't feed walnuts)--Nuts that aren't freshly shelled are more likely to have fungus in them (especially peanuts).
No fruit except maybe a little unsweetened applesauce.
No sugars--This rules out Poly-Visol, Nutri-Drench, and other supplements that have sugar ingredients (dextrose, sucrose, molasses, cherry flavoring, etc.)
Xylitol sweetener is actually helpful, however.
Vi-Tal (vitamins, minerals and electrolytes) powder in water is good.
Conflicting info on apple cider vinegar--It seems to be good if it is raw, unfiltered and given "not at same time" as electrolytes (Does this just mean same time of day? Or different days? or ??)
Natural unflavored yogurt with probiotics. (Be a bit conservative--excessive yogurt gives stomach-ache and extreme diarrhea). Chickens with a fungal condition have a hard time processing fat, so I'm not sure which "fat" is best. Probably "lowfat" is the safest bet.
No other dairy products, except maybe a little non-aged cheese.
Pure meat or fish (not processed meats like ham, bacon, sausage, etc.). Cut into tiny pieces so it's easy to digest.
Insects
Raw garlic
It is SO IMPORTANT the chickens have constant access to water.
Chickens do not eat as much and don't feel as well when they can't drink.
Water will help prevent dehydration, which is a big danger with diarrhea.
Water will help with digestion, plus help toxins to get flushed out of the chicken's system.
Dipping your chicken's beak into water can "remind" it to take a drink on its own.
DO NOT GIVE ANTIBIOTICS! They kill the good bacteria, and then the fungus flourishes even more.
Copper sulfate is useful. Be careful with dosing. Giving every other day instead of every day may be best for weak chickens. (I can give more details if you PM me.)
Deworming/de-miting
I found out my chickens were anemic, as well, initially from a Northern Fowl mite problem I hadn't spotted. I used an internal dewormer (moxidectin + praziquantel) and external powder (Sevin dust) to treat. From what I've observed, the m+p dewormer didn't help much with this kind of mites; but the Sevin dust was effective.
Do not use diamataceous earth while a chicken has this problem. Their guts are sore and DE would potentially cause more soreness.
Tips to help get your chicken to eat better:
* Chickens eat better where they can see well and there is good lighting but not too dazzling of lights or too dark contrast from shadows.
* If your chicken's eyes are sunken in, it may help to CAREFULLY trim the feathers right in front of its eyes so it can see better.
* Picking up and dribbling food from your fingers (and making rooster "meal call" clucks occasionally) can help a chicken take interest in food.
* Some chickens don't like some food combinations. However, they may eat the foods either separately or combined with different foods.
* Force-feeding some food into an undernourished but reluctant-to-eat chicken may help it get feeling better enough to eat some more on its own after a few minutes.
* Try to give a fairly balanced diet, but not too much variety in each meal. The chicken's system can digest individual foods more easily, plus if your chicken eats something while it has a stomach-ache, it may associate that food with feeling unwell--so it's better for fewer foods at a time to get crossed off its "I-will-eat" list.
* You can try a suppository (Mix some warm water with chopped, garlic, xylitol, Vi-Tal) to get some nutrients absorbed into the chicken's system if needed.
To conserve your chicken's strength
Keep your chicken in a warm area or put it in a cage with a heat lamp over it, so it doesn't have to burn calories trying to keep its body temperature up. This will also keep the chicken's drinking water from freezing, which will be a big help.
This condition can stretch on for weeks or months, and can be fatal. Correct treatment may not yield good enough results, or not for quite a while. I have 2 other hens and a roo in my same coop who might have had the condition to some degree--I'm not sure whether they got much of the corn--but, if so recovered pretty much without special care, though.
Best wishes.