Megacolon is a condition where the nerves to the large intestine do not work correctly and the rat has trouble moving his bowels. The colon becomes backed up with feces and the stool becomes larger than normal and also dries out, causing trouble passing stool and severe bloating.
There are different types of megacolon: genetic megacolon, late-onset genetic megacolon and injury induced megacolon. Unfortunately, rats with genetic megacolon usually die very shortly after the first signs appear.  They will begin to bloat at about 4 to 6 weeks of age, and sometimes sooner, as they are weaning. Infant males with genetic megacolon should be euthanized before their intestines rupture and they die a very painful death. Infant females can SOMETIMES be saved with a lot of help from their human. Regular enemas, a low fiber/high fluid diet, and a prescription medication called Cisapride (not available in all areas) can sometimes prolong their lives. However, if you are not willing to put in the extra time to care for a rat with megacolon, then the kindest alternative is euthanasia. Rats with late-onset megacolon may have periods of constipation followed by loose stools for many months before their intestines start to shut down completely and they start to bloat. It is also possible to prolong the lives of these rats with regular enemas, a low fiber/high fluid diet, and Cisapride. But, again, you must be willing to give these rats the extra time and care that they need.

4 week old male with
genetic megacolon

5 month old male with
late-onset megacolon

Recommended treatment:
Adults with late-onset megacolon: 
Warm water enemas given using a catheter attachment (these can be obtained from your vet) and a syringe, once per day (sometimes less often).  
A low fiber/high fluid diet. Lots of juicy fruits, such as watermelon, may entice your rat to consume extra fluids.
More dietary information:
• Cisapride - 0.1mg/kg to 0.5mg/kg once per day
Females: Euthanasia or the above recommendations
Males: Euthanasia

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