It's my understanding even if only applied topically, Ivermectin is absorbed into the bloodstream.I think the topical absorbs into the blood stream as well. I just treated my chickens two days ago with the pour on and saw a worm in their poo this morning. Before that, I didn't see any worms. I was treating due to one having leg mites and their egg production is down now due to molting anyway.
My question is now that I have seen an actual worm (round worm, but only about a 1/2" long not as big as shown in this thread) should I treat with something else? Or just let the ivermectin do it's thing for now?
Since you are seeing roundworms, you can use something like Wazine that treats only roundworms. Valbazen and Fenbendazole can also be used to treat worms as well.