Megacolon in cats is not listed in the index of probably the best book on cat health care, Cat Owner’s Home Veterinary Handbook by Drs Carlson and Giffin. That indicates that it is not a condition that is common (or the people who indexed the book missed it!).
However, searching the book manually, on page 199 (of the 1995 publication) it says that megacolon in cats can cause chronic constipation. This is because megacolon is an enlarged colon that does not work properly (contract effectively).
The book goes on to say that a cat suffering from megacolon requires a special diet and “stool softeners” under veterinary supervision.
The condition of severe of chronic constipation has been called megacolon. The colon becomes enlarged with compacted faecal matter1.
Most of the time the cause of megacolon in cats is unknown (“idiopathic” in medical jargon). Possible causes are:
Middle aged cats are more prone to suffer from this disease as are male cats. Passive, sedate and overweight cats may be more prone to contract it1.
As megacolon causes chronic constipation the symptoms of megacolon are the symptoms of constipation and we as humans probably understand those. In addition to not using the litter to defecate a chronically constipated cat might:
A vet checks for faecal compaction using their hands (“digital examination”).
Clearly a vet will have to establish the cause of constipation in the cat to then recommend treatment. The classic treatment is feeding a high-fiber diet. Commercial cat food for senior cats has a high fiber content. In addition faecal compaction is dealt with using laxatives and enemas. Sometimes fluid replacement is indicated2.
I have a page on cat constipation, which lists treatments.
2. Cat Owner’s Home Veterinary Handbook, Fully Revised and Updated
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