Lost one of mine today

[jrjcrf150f]

A few weeks back two of my chickens had sneezing and some nose drainage and when breathing at times a congested sound. Ive used vetrx in helping clear the airway and it has helped. Their energy level and appetite has not changed at all. my other chicken had none of the symptoms but the past few days had become really lethargic. not wanting to leave the coop. I added some probiotics to the water and aid in hydration thinking maybe she has become dehydrated. today when i went out to check on them the other two were running around but my oldest was missing. she was sitting behind the nesting box lifeless.

looking for advice in maybe what too look for for the cause. and how i can continue to treat my other chickens. one still has a wheezing sound when breathing at times but still no loss of appetite or energy.

any help is greatly appreciated. I've thought about antibiotics but have read that as a last resort.