Squirmy Wormies!!!

So. . . Super quick post. The amazing Gus Gus had his first official vet check today. Yes, we have had him coming up to 7 months now, and no. . . We did not have him vet checked at purchase. He is totally and completely healthy! His weight is good and he was such a good boy through the whole thing. No fussing, no resistance, nothing! He just stood there and did what was asked of him, including letting the vet pull his tongue out! It turns out he is actually 7-9 years old, according to his teeth. He even stood perfectly still for his vaccines and temperature to be taken. Yeah, I know. Other horses stand proper, too. However, for Gus, this is a first! Now here is where I found my interest drawn. The vet is running a fecal test to see if worming is necessary. In the beginning, I was told to worm him every 3-4 months, and to use a different wormer every time. Apparently this is wrong. The vet said that worming was only to be done yearly. She said the worms are starting to become immune to the dewormers and surviving. Wtf!?!?! Has anyone else heard this yet???