We switched to shredded aspen about 6 months ago. Since then we've had 3 different snakes develop RI's. We've switched back to paper and will not to aspen again. Is it the primary cause? I can't say for sure. But we never had problems before the switch and it was very finely shaved and I'm convinced was a contributor to these RI's. If it is the aspen, I'm sure its a result of how this particular brand we buy is produced. I'll be even more convinced when its been a few months and we don't have any more instances of RIs (PLEASE!).