Megacolon is also a big problem in breeding high white rats as well.
An animal with megacolon has serious issues passing feces and fails to thrive and eventually dies. Anyone interested in megacolon in rats can read this article, i read it a while back and it had a ton of useful info.
http://ratguide.com/health/digestive/megacolon.php
Useful especially because a bunch of people on these forums breed their own feeders.