Roundworm after months?

I have two cats.  Both neutered males.  Both totally indoors.  We added Nico to our family in January.  He did have roundworm which we treated after a vet visit.  I followed full protocol on disinfecting litter boxes, etc.  (It was like DefCon 4:  The Worm Trials.  I examined every turd scooped, like it was my job.)  Nico had 2 rounds of the dewormer.  Bucky (our older boy) showed no signs of ever catching the worms. 

I scoop boxes at least every other day, sometimes every day.  Every other month, I dump all litter and thoroughly scrub the boxes. 

Today, I found what looked like a dead roundworm in a turd.  No idea whose poo it is. 

Is it even possible to have a worm show up after all this time?  It has been months.

This is my first visit to this site.  Be gentle.

Thanks!

E

You should probably have the vet deworm both cats again and also prescribe a monthy parasite preventative to keep the worms and other baddies away. I think sometimes worms and other parasites get immune to parasitcides so a few really though ones may still exist somewhere and return wihen conditions are just right for them.