Well, possibly, if they are dying so close to being weaned, it may be a mega colon problem. Mega colon shows up in rats who have just been weaned newly or are starting to eat normal foods. Mega colon is genetic. Sometimes mega colon shows up rather late also... google mega colon and you will find plenty of info... or maybe Becky will come along and give you plenty of info. It may not be the problem, but it is a possibility.