Anal eel

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The 34-year-old man from the northern Quang Ninh Province, Vietnam, was admitted to hospital with severe abdominal cramps after complaining of stomach aches.

[M]edics believe it entered his anus, slid up his colon and somehow entered his abdomen.

What stunned the doctors most was that the eel was still alive when they removed it.

https://www.dailymail.co.uk/news/article-13227635/surgeons-pull-live-foot-long-eel-mans-belly-slid-colon-bit-intestine-Vietnam.html