Fatal mystery illness, not respiratory

I've had two chickens die in two months with nothing in common but lethargy. Both were a little over 2 years old and had been steady layers.

I thought the New Hampshire Red had gleet. I brought her inside, began soaking her and treating with miconazole and giving Nutri Drench and apple cider vinegar with a dropper. She had a steady appetite but remained lethargic and the gleet never seemed to improve. She died after about 4 days.

Last week I noticed an Ameraucana acting lethargic. She didn't go eat with the rest when let out of the coop, moved slowly and tried to stay by herself. During the day she went and got back in the coop. I brought her inside and looked her over, her vent was clean and I couldn't see any issues but her crop was basically empty. I gave Nutri Drench and ACV again by dropper. The next morning was the same, she exited the coop, did not go eat...in the afternoon I found her dead on the floor of the coop.

There have been no signs of any respiratory issues. The only thing they had in common was lethargy, then death. Any ideas what this might be? Could I have caused it with the dropper? I tried very hard to be careful and just drop it in their beaks.

I will also take suggestions for any preventative treatments I can give. Should I add anything to feed and water to build immunity or prevent basic illnesses? Thanks for any advice.

Unfortunately, without necropsy, we would just be guessing. They could have died of completely unrelated things, there is no way to know. How old where they? Do you know when they last laid? For birds over the age of 2 reproductive problems are not uncommon. With illnesses they tend to hide it very well until it's pretty advanced. If you lose another, I'd recommend that you have the bird necropsied, that will tell you for sure what happened and if it could affect the flock.
Necropsy resources:
https://www.metzerfarms.com/poultry-labs.html
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf

Did they have any discoloration of the face(Purple tone), any red/purple splotches on their legs?

Are they mite infested?

Also could they be wormy. Lethargy and death with no other symptoms is often parasite related.

coach723 said:

Unfortunately, without necropsy, we would just be guessing. They could have died of completely unrelated things, there is no way to know. How old where they? Do you know when they last laid? For birds over the age of 2 reproductive problems are not uncommon. With illnesses they tend to hide it very well until it's pretty advanced. If you lose another, I'd recommend that you have the bird necropsied, that will tell you for sure what happened and if it could affect the flock.
Necropsy resources:
https://www.metzerfarms.com/poultry-labs.html
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf

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Both were about 2 years old and had been good layers.