My little welsh colt came about two weeks ago and I noticed he looked a little wormy. So I gave him quest plus gel (he’s 10 months old), and the next day he in his feces he had about 50 dead bot larvae which continued for about 4 days. Now about 2 weeks later, I’m still checking his poop and he’s got these small redish/purplish worms that are still alive! Do I need to deworm him again or do they die off? Advice? 
Extremely normal. That’s how dewormers work-- they cause the worms in the horse’s system to weaken/die/detach themselves.
Those reddish purple worms are most likely strongyles. You generally see them on the surface of the manure. It’s a good thing they are exiting his body: that means the dewormer has worked. The strongyles won’t survive outside the body. They are not of any risk in their adult form while in the pasture. It’s the eggs that they lay while inside the horse that can survive in the manure, hatch into larvae, and then re-infest the horse.
However, if you are seeing such high numbers and for so long after deworming, your horse more than likely has a very heavy parasite burden. You don’t want to deworm just yet, as deworming on too close of an interval is one of the main contributers to resistance. But you may want to ask your vet about doing a fecal egg count about in about 2.5 months. It takes about 12 weeks for the Quest to no longer be active on the parasites in their system; if you do a FEC now, you’ll have falsely low numbers because of the recent deworming.
Young horses often need to be put put on a more aggressive deworming schedule than adult horses, but you really want to confirm what type of parasite load you are dealing with when you suspect a horse may have a greater worm problem than usual. 
OK thank you! I will defiantly do that! they are at the surface! that’s how I first noticed them. gross. Now new question…what do I do about not re-infesting any of my other ponies and pasture?
Solo turnout for a week or two. Then you pick the paddock and dispose of the manure.
Ok thank you for the advice!
[QUOTE=smith_sporthorses;7960506]
OK thank you! I will defiantly do that! they are at the surface! that’s how I first noticed them. gross. Now new question…what do I do about not re-infesting any of my other ponies and pasture?[/QUOTE]
Adult worms in manure will not re-infest your other animals. They cannot live outside the body. It’s the eggs that they lay while in inside of the horse that get passed out of the body in the manure, then develop into larvae that make their way back into the horses’ mouths while grazing.
If you are in an area of the country experiencing winter where they days are below 43F, you have no worries momentarily. Eggs can’t hatch and larvae can’t readily develop below 43F. When the weather warms up, they will become active.
Picking up manure is always a good idea. But honestly, if the other animals are already out with him, I wouldn’t even bother separating him from the herd. Those horses have already been exposed to the eggs/larvae from the now-dead worms during the time prior to deworming. But again, fecal egg counts would give you some helpful insight on who has a heavy worm load and who doesn’t.
A link on the life cycle of strongyles:
I would never have given a 10 month old Quest, even though it says it’s safe to use on them after 6 months. But, I’m just conservative that way.
I would also have not used something SO comprehensive/broad spectrum on a horse - foal or adult - who looked wormy and, given, his age and looking wormy, may never have been dewormed (properly). I would have used either plain ivermectin, or a double dose of pyrantel pamoate.
That said, now that the QP is done and he’s still alive 
you should not do anything for at least 8 weeks. At that point I would double dose fenbendazole to kill ascarids.
Then until he’s well into his 2yo year, rotate between ivermectin (which you’d start 4 weeks after the double fen), and either double fen or double pyrantel pamoate. 4 week schedule.
Ivermectin will get the strongyles, the double fen/pp will get the ascarids, and double pp will get tapeworms as well.
That’s what it takes to stay on top of killing things for which different parasites have different resistances.
Thank you for saying that JB, I am of the same mind.
