Kathy,
I used the same information and even spoke to the vet who said that fenbendazole would work. I am not refuting what others say. I am saying that I followed that advice and it did not work. One of my chooks was suffocating to death until I gave the castor oil/turpentine mixture and...
Look, it works, all of my chooks are alive (1 year later) after contracting and being treated for Gapeworm - and ingesting turpentine. I will leave you to ponder the whys as you clearly know more than I do.
The turpentine/castor oil mixture allows the chicken's airway to be cleared until you can get levamisole. I used this mixture on my flock every 12 hours until I got the med. I am not speaking philosophically, this is how I saved my flock.
All I can say is that one of my chickens could barely breathe and after I gave her the castor oil/turpentine mixture she could. Not sure how it worked, but it did. Also, the treating once and then again another 7 to 10 days doesn't work. Here is a scholarly article concerning gapeworm in...
Sorry your chook(s) are not feeling well. My flock had gapeworm last year. You need Levamisole to treat gapeworm. It goes by the name Prohibit in the US - it is labeled for cattle. Use this link, second entry down for dosing...