I used to include Quest in my rotation too, but my vet said he didn't use it or sell it any more and didn't recommend it. He said he'd done post mortems on several horses who had been on it (they died of other causes) and found them to FULL of worms. And it's supposed to be BETTER than ivermectin. So go figure.
I don't care for Quest either. The ranch I worked at starting useing it when it first came out. I thought the horses all showed symptoms, rough coat, no shine, of having worms even tho they wormed with it as the directions stated.
Funny, IMO, these people used it because it cost more...they're thinking, I think, is if it cost more it must be good.
Becky