There was no rabies. The vet who did the necropsy couldn't tell anything. A blood count on one of the littermates showed double the white blood cells, indicating an infection. A urinalysis indicated protein, another sign of infection.

The litter is on Clavamox as a preventative and in quarantine for 4 weeks. More advanced blood tests are in the works. They tested negative for coronavirus (FIP.) The working theory is now distemper, which is not good. But so far I've had two of the cats for 2 weeks, and one for one week.