worms are common in chickens
and only live and eat the chickens food in the gut of the chicken
in some advanced conditions worms are emitted in to the egg tract and DO come out in the eggs also
but to worm chickens is easlily done and I will put the two kinds of wormer here
(A) Since they have not been wormed it is best to have you give the wazine first. Then 10 days later use the Ivermectin.
in between you need to give them the
(B) wet mash probiotic
1 qt of dry crumbles
1/4 cup of yoguart
1-1/2 qt of milk any kind
feed 3 tsp per chicken what they will eat in 20-30 mintutes. clean the wet feeder and restore the dry crumbles
feed this each morning for the 10 days. Then administer the ivermectin.
(C) to use the pour on Ivermectin  I will put this exerp from my friend Nathalie Ross who wrote my worming article for me
WORMING WITH IVERMECTIN POUR ON
NATHALIE ROSS
For the pour on Ivermectin (not Eprinex) the dosage I   
use is as follows:   
1 drop - OEGB sized small bantam female   
2 drops - OEGB sized small bantam male   
3 drops - average bantams   
4 drops - large bantams, small commercial fowl   
5 drops - most commercial fowl, small giant hens   
6 drops - giant breeds of chicken   
 
I always use a 3 cc syringe that I just fill to about 2 cc's with a 20 gauge needle.  The needle WON'T be injected into the chicken, but does make it easier to dispense a controlled correct sized drop.  It also is easier to get in there between the feathers.   
 
For location, you'll want to find an easy to reach   
spot with as little fluff as possible. I've had the   
best luck with the back of the neck when I am by myself.  I just pick up the chicken in my left   
hand, ruffle around the feathers with my right hand until I find a nice clear spot, then rotate the syringe around to dispense the drops exactly on the skin.   
 
If you hit the fluff, it will soak in before   
you can do anything and will be wasted.  That stuff soaks in like lightening (which I discovered to my horror when I accidently got about 1 cc of it on me from the bottle - I'm worm free now!)   
 
While you have the bird up, look them over.  This is a great opportunity to nip things in the bud!  Take advantage of it.   
 
Generally I like to recommend that first time wormers use Piperazine (Wazine being the most common brand) before using Ivermectin the first time.  This is a common practice with most livestockmen and women.   
 
You use a less effective, less broad spectrum wormer first just in case there's a high load of roundworms.  If there is a high load of roundworms and they're all killed at once, you risk either impaction or the bird having a reaction to the foreign proteins that the dead/stunned worms become.   
 
The best way in my opinion of doing this is to worm with Piperazine in the water first - full strength 24 hours, then instead of following up in 10 days with piperazine, use the Ivomec Ivermectin or better yet Ivomec eprinex (for 0 withdrawal time).   
 
Using this program, I worm once a year.  Once I have wormed with ivermectin, I don't use piperazine again unless I do a second worming during the year or have   
reason to suspect they've encountered a heavy level of parasites.  In fact, I worm once a year almost exclusively.     
GLH replied
***Also some people use DE in the feed daily at rate of 2% De to the amt of crumbles
The Iermectin kills the wworm larve in the blood stream and then the DE knicks the adult worms in the gut and they dehydrate and become protein in the birds body
thus the DE will do away with the adult worms
***** you can also use the cayanne pepper daily in the feed
1 gallon of crumbles put 2 tbsp of cayanne pepper mix good and feed daily to the chickens
this will keep the birds wormed and also keep them from coccidiosis
it is very healthy for them after the initial worming
give me a email if more questions I will try and answer you
I hope this helps you.
email me PM if there are  questions
As to eating the chickens with worms that is a personal issue. I never did but that was my preference