Anyone have experience with Gapeworm?

Hi all,

One of my hens has started to make a strange cough/choke sound intermittently the past few days.  At first I didn't think much of it but now it seems to be getting more frequent.  Today I saw her opening her mouth wide as if yawning.  She is still laying normally and eating, drinking, and behaving normally and doesn't seem otherwise bothered.  I do let them free range around the yard when I'm home and they did eat quite a few earthworms when I was gardening a while ago. 

Looking on backyardchickens.com, looks like this could possibly be gapeworm?  Anyone have any experience with this or advice?  They get apple cider vinegar in their water already, but otherwise just grower crumble (for my other two 14 week old pullets), and free choice grit and oystershell. 

If I do need to worm them, where would I get wormer in town?  Or, are there other things I should try first?  Hope "Ginger" is ok!

Thanks all!  I appreciate your advice.

-Kristi