The problem with using a worming rotation, where you worm every 8 weeks, is that many worms have become resistant to many drugs. I get the horse.com newsletters, and they covered this recently in one of the Health newsletters. By doing the FEC and targeting the specific parasites your horse has, you will reduce the chance that your horse will have drug-resistant parasites.
The latest article I read stated that ivermectin, which had been the one drug that would still work on everything but tapeworms, is becoming less effective for small strongyles.
As far as symptoms go, it's kind of hard to say. Some horses carry a large parasite load with very few symptoms, while others can only tolerate a very light load. I've seen some horses with much longer hairs throughout their coats, weight loss, and a distended belly that had high worm loads, but there could be other causes for those things as well.
If I see symptoms, I like to do an FEC, because I want to be confident that the drugs available will keep working until new drugs come along.
ETA: The best way to be sure your dewormer worked is to do a follow-up FEC.
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