You need to test your chickens poo..every environment is different. I test twice a year since I eat my chickens and they free range. I have never added chemical wormers to my chickens. I change the environment and added natural herbs, seeds, and garlic. Retest to make sure they are clear.
Every animal has worms. We do too. Finding a balance that is healthy should be the goal. Worming your animals with chemicals makes super worms. It removes the weak ones and leaves the strong ones. They build a resistance and you end up changing worming chemicals, worming more often, never getting a healthy balance, and struggling to maintain healthy flocks.
Some offer Food Grade DE to chickens daily to keep parasites to a minimum. I have not use this, however if my methods did not work it ..it is something I would try. From what I have read, you offer it daily and in a few month, no more parasites.
One article for you to read..there are thousands to read
http://wolfcreekranch1.tripod.com/diatomaceous_earth.html