M,
How are your kitties doing?
I had something happen to my 9mo kitties a couple months back sounding just like your kitties. They would not use the litter box and preferred going on the cement floor. I ended up taking them to the vet and had all the blood work and they found nothing and no parasites. They were so close to death I ended giving them metronidazole (flagyl) and they got better. I thought they got sick on eating mice or voles but now I am not sure. Most likely bacterial/fungal related.

Do let us know how they are doing. The collidal silver sounds like a good option.