Chicken gasping even after 48h on antibiotics. Help?

[Clara S]

My parents left on holiday and left me to house sit for them. They warned me before leaving about a chicken that was lethargic and overall not well. The next day I called the local vet (rural area) to take a look at her, she was quite concerned, gave her some antibiotic pills (tylosin + erythromycin + neomycin) and told me to feed her scrambled eggs. The following day she was even weaker and sadly passed away that night.
Well 3 days ago I noticed another chicken that was quite lethargic and gasping for air. When I picked her up I noticed she was breathing noisily, like snoring. Scared after the first experience, I started her on antibiotics immediately and sent a message to the vet. She agreed about the antibiotic, told me to isolate her and give antibiotic to any other chickens that are sick. Well none were *as* sick, but I noticed a few that seemed a bit sleepy as well so out of anxiety I started them all on antibiotics (a total of 8 Brahma chickens in their own run).
I asked the vet to pass by yesterday and today as well but she didn't have the time to so I'm all on my own.
After 48h I was expecting to see some improvement, but she is still lethargic and still gasping for air. So at best stationary, but most likely slightly worse. She is lighter than she should be (there are a couple other chickens that are lighter as well, but they seem more energetic for now).
I did notice a lice infestation in their coop, but not a lot on the chickens themselves. I sprinkled diatomaceous earth today in the coop and i the meantime they are sleeping outside anyway (enclosed run).
I have no idea what more I can do. My parents already lost around 10 Brahma chickens a couple years ago from a similar disease I suppose (chickens ate less and became lethargic and eventually died). They did some lab tests but got no answers in the end.
Does anyone recognize these symptoms and can offer some guidance? It breaks my heart to see her struggling like this. Thank you.
Sound of her breathing -

The way she is gasping -

 

[azygous]

Respiratory viruses, once in a flock, never leave. Stress, from possible external parasites for example, can trigger the respiratory virus and the chickens in the flock will begin showing symptoms again. These symptoms are usually treated with an antibiotic such as what the vet prescribed for your parents' flock. But it won't make the virus go away. The birds carry it for life and it's transmitted to each new chicken brought into the flock.

DE powder will not make the bugs in the coop go away, either. There are two main species of parasites that attack chickens, mites and lice. The mites are blood suckers and make the chickens weak from blood loss while lice mainly live on the chickens living on dead skin and dander. Permethrin or spinosad are the two safe insecticides that will get rid of both. For coop mites, you need to treat the coop. For lice you have to treat the chicken.