I was at the vet today for Ingrid's booster. I mentioned that I really should change the blanket, but often forget and put the carrier away. The vet (a cat-only vet) said that she always recommends changing bedding and giving the carrier a good wipe over over every visit - nothing to do with hygiene, she said thatcats give off stress pheromones, so the next cat is going to react to to it.
It hadn't occurred to me before.