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This is just an informational post on a dreaded genetic disease. Some people may never have heard of it, but the ones who have experienced it will never forget it.

Genetic megacolon occurs as a result of faulty cell development in the embryo stage. The nerve cells in the wall of the colon and/or rectum are either missing or are non-functioning in affected animals. These nerves are necessary in order to move stool along and out. In rats, this means that the stool will back up, and they will bloat. This is most likely painful and eventually results in the death of the animal from malnutrition or bacteremia. Though this condition can occur in any rat, it is very often seen in rats with what are called "high-white" markings. The same relationship occurs in a certain type of Waardenburg syndrome in humans where there is either odd-eyes or a white patch of hair or pigment accompanying the digestive involvement. Lethal white foal syndrome in horses occurs in the overo types (white or mostly white) and causes death from megacolon within a few hours or days of birth.

From ratguide.com
Megacolon can be either early or late (delayed) onset.
With early onset megacolon the signs will usually begin to show as soon as the baby begins eating solid food (around 2 weeks) although sometimes it is not apparent there is a problem till they are 3 or 4 weeks old. Some babies will show a distinct failure to thrive even though they are eating well. This occurs due to the lack of proper absorption of nutrients in the damaged digestive tract. Early onset signs such as bloating, diarrhea, and severe constipation typify the quick advancement of the disease. It is recommended to consider euthanasia rather than to let the disease follow its ultimate lethal course.
Late, or delayed, onset megacolon appears to have the same genetic origin as early onset and is seen in the same lines as early onset. It may be a milder form of the disease, but unfortunately it does progress in severity until it becomes lethal. Often the first sign of delayed onset megacolon is shown by the baby rat's failure to thrive. Gastrointestinal problems may not become apparent until the rat is 2-5 months old. At that time you may see bloating, diarrhea, constipation, or alternating bouts of diarrhea and constipation. Stools passed may be hard, fibrous, foul smelling, dry, oversized, oddly shaped, blunt on the ends, and have blood or mucus in or between them.
Late onset can also be even more delayed, sometimes not becoming symptomatic until the rat has reached a later age of 4-10 months. With no early warning signs of a problem (such as failure to thrive, unusual stools, or chronic diarrhea) the very late onset type may often be too far advanced, by the time the owner notices a problem, to treat.


2 case histories of early onset megacolon

http://ratguide.com/health/figures/m..._figure_1c.php
http://ratguide.com/health/figures/m..._figure_1b.php

Late onset megacolon

http://ratguide.com/health/figures/m..._figure_2a.php