what do you do to help control horses from re-infesting themselves with worms in the pasture? Seems we might be having a problem with this, this year due to it being hot and super moist here with all the excess rain. Pasture rotation is unfortunately not feasible for us. This may sound crazy but is there anything that can be sprayed on pastures to kill larvae/worms that horses pass? Any other suggestions? 

You need to have your horses on a deworming schedule/program. Deworm all your horses at the same time and rotate dewormers and dont over do deworming. If you do this you will not have to worry about your horses getting reinfested.

We have been doing this.....that is the problem....they are picking them back up in the pasture

also   get a pasture drag  break up manure   so eggs don/t hatch

How big is your pasture and how many horses do you have on it? 

The biology teacher across the hall from me, who had her masters degree in equine parasitology,  told me that worms eggs do not survive I. Hot temperatures and it really was not even necessary to worm during the summer. I still do because it messes up my schedule. Worms go through an encysted stage and wormer my not get those. Be sure to use a wormer that does get those encysted worms. I think that is where your problem is. My friend also said that the only two wormers that worms do not build up a resistance to are ivermectin and moxidectin which are in the same family. Ivermectin is all I worm with. Also,  my vet, who was equine only, said give them two ivermectin at once every once in a while. I take ivermectin to prevent COVID so oink oink to you. 

We have about 10 acres with 9 horses on it. 

Do you harrow your pastures?  We call it brushing around here, and its done every spring on the meadows where we winter the cow herd.  I do the horse paddocks and pastures once in the spring and once in the fall.   When you break up the poop piles and spread things out the sun will kill the parasite eggs.