Do your cats go outside? They could just keep picking them up off of fleas or rats or something. I would say you need to clean the litter box several times a day, change the litter often, bleach out the box, wear gloves while going it, wash your hands often, steam clean your carpets, use very hot/boiling water on your floors to kill eggs and maybe even keep the cats separated while sorting this out. I'm not sure why you have to keep going to the vet though, you could buy stuff over the counter from a pet store or something... much cheaper if you're worried about vet bills.