This is a super good question. Roundworms in chickens spend their whole lives inside the gut, so when you give a dewormer, all stages present are exposed to the drug, and are susceptible to it. This is not true for many other worm species, but it is for these guys. They do, however, sometimes bury themselves in the intestinal lining, so when the adults are purged, they come out, thinking the coast is clear.
As for eggs in the environment, it takes a couple of weeks for them to develop to the infective stage. A fresh worm egg won't infect a bird, it has to go through a couple of moults before it can become infective.
Ok. I have seen too many posts with people saying "Here's my pure Americana (note the spelling) what color is it?" The pic is always of a beautiful EE.
So here goes: THESE TRAITS are what determine whether yours is the rare pure Ameraucana...or the very common EE!
(EEs are mixes that lay Blue/green and often resemble the pure breed Ameraucana, and are sold as such A LOT.)
THESE ARE VERY IMPORTANT!
Here I will show some pics of EE colors vs Ameraucana colors.
Ameraucanas come in solid colors; black, blue, silver, wheaten, and a few more. Don't be fooled by your brain going "hey, mine looks wheaten!"
Here's an EE pattern similar to wheaten vs the real color:
View attachment 2367240
View attachment 2367241
See the difference?
Here are some common EE colors. THESE ARE NOT COLORS YOU FIND IN AMERAUCANAS!
View attachment 2367243 View attachment 2367851 View attachment 2367853
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I'll give it to you straight: Ameraucanas never have green legs. Or yellow. Or whatever. They only have slate, with pink skin underneath. So if your "ameraucana" has purdy lil' green feet, rethink what you're calling her.
NEXT: COMB. Ameraucanas always have pea combs. None of those weird lopsided modified peas, or a single, or anything. Only a normal pea comb.
Modified pea: View attachment 2367246
Ameraucanas lay blue. EEs lay blue/aqua/green/sometimes pink.
If you're getting brown green or aqua, you've got yourself an EE.
Blue, likely still an EE.
NEXT: Source. Did you buy from a feed store or hacthery? Was it under 20$ per chick? If so, EE.
Hope this helps, y'all. My two EEs are some of the best things that ever happened to me, and I hope it's the same for you.
If you read through this and still aren't sure, I can tell you if you post a pic.