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Enlargement and dilation of the colon with feces is usually the result of rectal stricture and less frequently from colonic atony. Atresia ani leads to megacolon in pigs up to four weeks of age. Postnatal rectal strictures are a frequent sequel to rectal prolapse. Damaged rectal tissues from prolapse, whether reduced, traumatized, or chewed-off by pen mates, may heal and fibrose causing constriction of the rectum. The outcome is intermittent defecation or total obstruction of fecal flow causing the colon to dilate with accumulated feces while the pig slowly wastes away.

Colonic atony with patent rectum has been reported as a result of feeding lupine-seed meal. Neural control of tone and peristaltic movement is lost causing the colon to simply dilate with feces.