I lost my obvious runt in one of my orphaned litters.
Recent rescue Shadow had just kittened in my bathroom. I had been trying to trap another pregnant female before she kittened, but the evening I finally got the trap from Animal Control, she showed up skinny and I knew I had missed my oportunity.
I returned the trap and bought my own.
Anyway, when outside mama's kittens were 9 days old, she was hit/killed in the road in front of my house and we buried her under our Maple/cat-garden tree. I searched for her kittens and finally found them (5) on the evening of the second day. Shadow would not accept them, but she would let me put them on her to nurse, while I would take them off her one at a time to bottle nurse and stimulate for pottying. Shadow let me do this for about 5 days, long enough for me to get them transferred completely to the bottle. I couldn't leave them for her to nurse as her own litter of 5 were already dragging her condition down.
Here is a pic of the just-rescued litter nursing on Shadow. The center one is the runt.
The runt had continuous diarrhea and cried a lot. Albon didn't work. When I took them for another visit to the vet, I was advised to step up the weaning and stop bottle feeding. The next evening CryBaby was in dire straights, crying repeatedly and steadily, and was unresponsive to outside stimuli. I rushed him to the Emergency Vet, speeding and running one interminably long red light. (no traffic) ERvet was the same Vet who had seen him the day before and advised stopping the bottle and he felt the kitten was too far gone to try rehydrating and recommended euthanasia. He did this at no charge, as he knew I was fostering for the cat adoption program. I told him I would pay for treatments and keep him, to save him, and he asured me that he felt the kitten really was too far gone.
CryBaby is buried next to his mother.
In telling this sad story, I would say to watch your runt closely. Perhaps try to 'force' more human contact with her and gently handle her. If she is not exhibiting any signs of distress (crying) or health problems (diarrhea) then I would not be too concerned about her not eating kitten food, yet. Some kittens take up to 5-6 weeks before they feel like trying the canned food, or kitten food. I have found with other litters, once the runt figured out eating on its' own it usually managed to catch up to its' littermates because it no longer had to fight for a teat, and didn't show a visible difference in growth after a few weeks.