Is it croup??

One of our chickens makes a wheezing noise at night. We had another chicken making a similar noise a couple of months ago. She gradually became poorly and in the end we had to put her down.
We think it may be croup but can find nothing on the internet to give her - or put in the water - or whatever - to help with condition. There is one "cure" from an American company, but what would it cost to get it over here?

The chickens were bought as point of lay and we were told they had been injected. They are free range by day ( in a huge garden) and in at night. They have plenty of layers pellets, grain and clean water. They are now about 6 months old and have been laying well.
Can anyone advise, please?

Could only find historical references to croup in chickens.

An old homeopathic remedy suggests boiling rue with treacle to cure croup in chickens.  :shock:
LINK

Think I'd rather try antibiotics first.

Rob