My chicken has been sneezing like this for over a month now. I tried antibiotics to no avail. She has also lost her voice, but all other behavior seems perfectly normal. Any suggestions would be greatly appreciated.
I'm thinking gapeworm from what i've researched and looked at recently. That mouth gaping is as far as I understand a big tell tale.
Currently treating our chickens for roundworm.
Hope your chicken feels better soon!
Typically my understanding is that when one chicken has worms, you treat them all. Though i'm not familiar with the thoughts on gapeworm and if it's as spreading as other worms are like roundworms. Since most worms are pretty common to chickens especially free-ranging ones where they get into a lot of bugs that can carry the eggs as well as when they scratch in the dirt where they poop (and potential eggs being in there).
Here are some helpful links:
Maybe someone with better experience in gapeworm can help out too...
Hope these help you out and your chicken feels better soon!