I have read through your thread because I also have several aging hens 8-10 years old. I am always looking information about caring for these older girls and problems to be aware of. There is so little information available about the older birds. I am not fortunate enough to have a vet to turn to for advise about chickens so I study here often.
As for avian influenza, as I understand it your vet may not be the one to test for this. I believe your state vet would be called. They would send a team to your property and take swabs of most or all of your birds not just your sick girl. If any swab turns up positive for avian flu, the swab is verified by a more accurate test. If this is positive
all birds on the property would be euthanized. All of this would happen very quickly within days. Your property would be quarantined for a length of time with no new birds brought in. This is to ensure there is no longer any active virus left on the premise.
Symptoms of H5n1 (the prominent strain in the world right now) include lack of energy, appetite, and coordination; purple discoloration or swelling of various body parts; diarrhea; nasal discharge; coughing; sneezing; and reduced egg production, or soft-shelled or misshapen eggs. Mortality in poultry is almost 100%.
With the death of many birds very quickly often being the first sign.
I am not a vet or medical person in any way just another concerned chicken keeper. I do not think your girl has bird flu. She could have some other infection. I would continue with the antibiotics you have and give supportive comfort care as best I could. Try to make sure she eats and drinks, offer her some scrambled eggs and maybe wet feed. I also use poultry vitamins in their water or a nutridrench drops straight to the beak. I've had a few eat canned peas and carrots for a couple days when they wouldn't touch anything else. I try to continue to offer feed though.
I do hope your girl recovers but rest easy I think she is very aware you care about her and trying hard to find answers.
Here is the process the USDA uses for handling Avian Influenza.
https://datcp.wi.gov/Documents/HPAIResponseProcess.pdf
More information from USDA about how to protect your flock and other resources about Avian Influenza. Links at bottom of page.
https://www.aphis.usda.gov/aphis/ou...nd-the-flock-program/defend-the-flock-program
I do not know if you are in the US? If so you can check for current bird flu detections in your area. These are searchable (by state) databases by USDA.
For Backyard and Commercial Flocks-
https://www.aphis.usda.gov/aphis/ou...pai-2022/2022-hpai-commercial-backyard-flocks
For Wild Birds-
https://www.aphis.usda.gov/aphis/ou...vian-influenza/hpai-2022/2022-hpai-wild-birds
They have also found it in wild mammals-
https://www.aphis.usda.gov/aphis/ou...n/avian-influenza/hpai-2022/2022-hpai-mammals
I am sorry if this is way more information than you wanted. I'm just one of those folks who reads all I can when something is amiss. These chicken keepers who post on this forum are the best and I have learned so much from them over the years. Thanks to all of them!