Megacolon is a genetic disease, which tends to be more common in rats with high white markings. You have a lot of high whites in your colony, mostly blazed.

Also, Ralph Davis overcrowds his rats like no other, just because he does it does not make it okay. I use a cement mixing tub rack, each tub measures 26x21x7 or so. The most crowded I have ever let a tub get is 3 adult males, or 4 adult females. This does not happen often, and usually it is no more than 2 adults per tub.